Week 2 Notes

Python,
Week 2

Meaningful names, string methods, conditionals and your first modules: where Python code starts to look like real programs.

7 chapters41 worked examples~44 min read

Stringsif / elif / elseCaesar cipherModules

Chapters

01Variables: Names, Comments and TypingPart 1
02Operators and Strings: Membership, Chaining, Escapes, QuotesPart 2
03String Methods and the Caesar CipherPart 3
04Conditional StatementsPart 4
05Practice ZonePart 5
06Importing Modules: math, random, calendarPart 6
07Week 2 Summary SheetAll parts

Basics

Variables: Names, Comments and Typing

How to name variables, document code, assign several names at once and update values with short-hand operators.

1.1Meaningful variable names

Programmers write variables all the time, so a name that describes the stored information makes a program easier to read. Names such as a, b or x are fine for a tiny calculation, but they hide what the value means.

Hard to read

Python

a = "Ayush"
b = 18
print(a)
print(b)

Output

Ayush
18

Easy to read

Python

name = "Ayush"
age = 18
print(name)
print(age)

Output

Ayush
18

Exam idea

Meaningful names improve readability and make the purpose of a variable clear. The output is the same; only the human reading it benefits.

1.2Comments

Comments explain code to other people. A # starts a single-line comment; Python ignores everything after it on that line.

Python

name = "Ayush"
# This variable stores my name
age = 18
# This variable stores my age
print(name)
print(age)

Output

Ayush
18

# starts a single-line comment.
Comments are ignored during execution.
Their purpose is explanation and understanding.

Python

x = 5  # a comment can follow code on the same line
# print("this line never runs")
print(x)

Output

1.3Dynamic typing

Python is a dynamically typed language: a variable can refer to values of different types at different points of the run. You never declare a type before assigning.

Python

thing = "Hello World"
print(type(thing))
thing = 12
print(type(thing))

Output

<class 'str'>
<class 'int'>

Figure 1. The name thing first points to a string, later to an integer

Key point. The type belongs to the value, not to the name. The name is just a label you can move to another value.

1.4Rules for variable names

Start with a letter or underscore (_); never with a digit.
Names are case-sensitive: age and Age are different.
Only letters, digits and underscores are allowed.
It must not be a keyword (and, or, not, for, if, else, while, ...).

Name	Valid?	Why
`student_name`	valid	letters and underscore
`_age`	valid	may start with an underscore
`marks2`	valid	digit is fine after the first character
`2nd_place`	invalid	starts with a digit
`first-name`	invalid	`-` is not allowed
`for`	invalid	reserved keyword

Python

student_name = "A"
_age = 20
marks2 = 95
print(student_name, _age, marks2)

Output

A 20 95

Python

age = 18
print(Age)

Error

NameError: name 'Age' is not defined

The last block shows case-sensitivity: Age was never created, so Python raises a NameError.

1.5Multiple assignment and unpacking

Python

x, y = 1, 2
print(x, y)

Output

1 2

Python

x = y = z = 8
print(x, y, z)

Output

8 8 8

x, y = 1, 2 gives each name its own value; x = y = z = 8 gives all the names the same value. Values taken from a collection can be unpacked into several names:

Python

a, b, c = (100, 200, 300)
print(a)
print(b)
print(c)
print(type(a), type(b))

Output

100
200
300
<class 'int'> <class 'int'>

Figure 2. Unpacking needs as many names as values

Python

a, b = 1, 2, 3

Error

ValueError: too many values to unpack (expected 2, got 3)

Important. The number of names and values must match, otherwise Python raises ValueError.

1.6Deleting a variable

Python

x = 3
print(x)
del x
print(x)

Error

3
NameError: name 'x' is not defined

After del x the name no longer exists, so using it raises a NameError. (The source shows the last print as a comment for this reason.)

1.7Short-hand assignment operators

Operator	Example	Equivalent
`+=`	`x += 3`	`x = x + 3`
`-=`	`x -= 3`	`x = x - 3`
`*=`	`x *= 3`	`x = x * 3`
`/=`	`x /= 3`	`x = x / 3`
`%=`	`x %= 3`	`x = x % 3`
`//=`	`x //= 3`	`x = x // 3`
`**=`	`x **= 3`	`x = x ** 3`

Python

x = 10
x += 3
print("+= ", x)
x -= 2
print("-= ", x)
x *= 2
print("*= ", x)
x /= 2
print("/= ", x)
x %= 3
print("%= ", x)
x //= 2
print("//= ", x)
x **= 3
print("**= ", x)

Output

+=  13
-=  11
*=  22
/=  11.0
%=  2.0
//=  1.0
**=  1.0

Watch the type

After x /= 2 the value becomes a float (11.0), so every later step stays a float: 2.0, 1.0, 1.0.

1.8Going deeper

Naming style (extra). Python programmers usually write multi-word names in snake_case such as student_name.
The same operator works on strings (extra). s += "x" means s = s + "x".
Swapping (extra). The tuple form makes a swap one line: x, y = y, x.

Python

s = "Py"
s += "thon"
print(s)
x, y = 1, 2
x, y = y, x
print(x, y)

Output

Python
2 1

1.9Exam tips and common mistakes

Exam tips

For "is this a valid name?" check the four rules in order: first character, allowed characters, keyword, then case only matters when comparing two names.
Trace short-hand operators line by line and write the type (int or float) next to each value.

Common mistakes

Writing first-name or 2nd_place as a name.
Using a variable after del.
Forgetting that /= turns an int into a float.
Unpacking three values into two names.

Practice questions (graded-assignment style)

Q1A valid name

Multiple Choice Question (MCQ): choose the one correct option.

Which of these is a valid variable name?

2nd_place

first-name

_total

for

Solution

Idea

Check the first character, the allowed characters and the keyword list.

_total starts with an underscore and has only allowed characters.
2nd_place starts with a digit, first-name has a hyphen, and for is a keyword.
Answer: option 3.

Q2Short-hand chain

Numerical Answer Type (NAT).

What does this program print?

Python

x = 10
x += 3
x -= 2
x *= 2
x /= 2
x %= 3
x //= 2
x **= 3
print(x)

Solution

Idea

Trace x line by line; after /= it is a float.

Python

x = 10
x += 3
x -= 2
x *= 2
x /= 2
x %= 3
x //= 2
x **= 3
print(x)

Output

1.0

10 → 13 → 11 → 22 → 11.0 → 2.0 → 1.0 → 1.0.
Answer: 1.0

Q3Delete and print

Multiple Choice Question (MCQ): choose the one correct option.

What does the program do?

Python

x = y = z = 8
print(x, z)
del y
print(y)

Prints 8 8 only

Prints 8 8, then raises a NameError for y

Prints None

Raises a SyntaxError before running

Solution

Idea

del y removes only the name y.

Python

x = y = z = 8
print(x, z)
del y
print(y)

Error

8 8
NameError: name 'y' is not defined

x = y = z = 8 gives all three names the value 8; print(x, z) prints 8 8.
After del y, print(y) raises NameError.
Answer: option 2.

Q4Unpacking

Multiple Choice Question (MCQ): choose the one correct option.

What happens when this runs?

Python

a, b = 1, 2, 3

It prints 1 2

It prints 1 2 3

It raises a ValueError

It raises a NameError

Solution

Idea

Count the names and the values.

Python

a, b = 1, 2, 3

Error

ValueError: too many values to unpack (expected 2, got 3)

Two names but three values: the numbers do not match, so Python raises ValueError.
Answer: option 3.

Q5Dynamic typing facts

Multiple Select Question (MSQ): select all that apply.

Choose the correct statements.

A variable can refer to an int and later to a string.

The type must be declared before the first assignment.

The type belongs to the value, not the name.

Age and age are the same variable.

Solution

Idea

Recall dynamic typing and case-sensitivity.

Statements 1 and 3 are true.
Statement 2 is false (no declaration), and statement 4 is false (names are case-sensitive).
Answer: options 1 and 3.

Q6Swap

Numerical Answer Type (NAT).

What is printed? (Give the two numbers separated by a space.)

Python

x, y = 1, 2
x, y = y, x
print(x, y)

Solution

Idea

The right side is evaluated first.

Python

x, y = 1, 2
x, y = y, x
print(x, y)

Output

2 1

x, y = y, x builds (2, 1) first and then assigns, so the values swap.
Answer: 2 1

Revision overview

NamesMeaningful, readable
snake_case (extra)

RulesLetter or _ first
No -, no keywords
Case-sensitive

TypingDynamic: the value has
the type

VariablesWeek 2 · Part 1

Assignmentx, y = 1, 2
x = y = z = 8
unpacking: counts match

Removaldel x
then x is a NameError

Short-hand+= -= *= /= %= //= **=
/= gives a float

Revision: how to name, assign and update variables

Next · BasicsOperators and Strings: Membership, Chaining, Escapes, Quotes

Basics

Operators and Strings: Membership, Chaining, Escapes, Quotes

Testing membership, chaining comparisons, and writing special characters and multi-line text in strings.

2.1Membership operators

Operator	Meaning	Example
`in`	value is present	`x in y`
`not in`	value is not present	`x not in y`

Python

sentence = "I am Ayush"
print("Ayush" in sentence)
print("boy" not in sentence)

Output

True
True

A membership test always gives True or False. For strings it checks for a substring, not just a whole word.

Python

sentence = "I am Ayush"
print("Ay" in sentence)
print("ayush" in sentence)
print(" " in sentence)
print(3 in [1, 2, 3])

Output

True
False
True
True

Extra. Case matters: "ayush" in sentence is False. For a list, in needs a whole element.

2.2Chaining comparison operators

Chained comparison

x < y < z is equivalent to x < y and y < z. The middle expression is evaluated only once. Similarly a < b > c means (a < b) and (b > c).

Figure 3. 1 < x < 10 holds for x = 5

Python

x = 5
print(1 < x < 10)
print(10 < x < 20)
print(x < 10 < x * 10 < 100)

Output

True
False
True

1 < 5 < 10: both 1 < 5 and 5 < 10 are true, so True.
10 < 5 < 20: the first test 10 < 5 is false, so False.
5 < 10 < 50 < 100: three tests, all true, so True.

Python

a, b, c = 1, 5, 3
print(a < b > c)
print((a < b) and (b > c))

Output

True
True

2.3Escape characters

Escape	Result
`\'`	single quote
`\"`	double quote
`\n`	new line
`\t`	tab

Python

print("It's Python")
print("He said \"Hello\"")
print("First line\nSecond line")
print("Name:\tEngiKode")

Output

It's Python
He said "Hello"
First line
Second line
Name:	EngiKode

Python

text = 'We are the so called \'Viking\' from north.'
print(text)

Output

We are the so called 'Viking' from north.

Remember. An escape such as \n is written with two characters but is stored as one: len("a\tb") is 3.

2.4Different types of quotes

Single quotes '...' and double quotes "..." make ordinary strings; they behave the same.
Triple quotes ('''...''' or """...""") can span multiple lines.

Python

single = 'Python'
double = "Python"
triple_single = '''Python
Week 2'''
triple_double = """EngiKode
Python"""
print(single)
print(double)
print(triple_single)
print(triple_double)

Output

Python
Python
Python
Week 2
EngiKode
Python

Pick the quote that avoids escapes

"It's Python" needs no backslash because the outer quotes are double. Use whichever quote type does not appear inside the text.

2.5Exam tips and common mistakes

Exam tips

Expand a chain into and parts and test them left to right; stop at the first false part.
Read the escape sequences first, then count what is printed on each line.

Common mistakes

Treating \n as two printed characters.
Thinking a < b < c compares a with c only.
Forgetting that in is case-sensitive.

Practice questions (graded-assignment style)

Q1Chained output

Multiple Choice Question (MCQ): choose the one correct option.

What does the program print?

Python

x = 5
print(1 < x < 10)
print(10 < x < 20)
print(x < 10 < x * 10 < 100)

True False True on three lines

True True True on three lines

False False True on three lines

It raises an error

Solution

Idea

Expand each chain into and parts.

Python

x = 5
print(1 < x < 10)
print(10 < x < 20)
print(x < 10 < x * 10 < 100)

Output

True
False
True

1 < 5 < 10: True. 10 < 5 < 20: False. 5 < 10 < 50 < 100: True.
Answer: option 1.

Q2Membership

Multiple Select Question (MSQ): select all that apply.

With s = "I am Ayush", which expressions are True?

Python

s = "I am Ayush"
print("Ayush" in s)
print("boy" not in s)
print("ayush" in s)
print("Ay" in s)

"Ayush" in s

"boy" not in s

"ayush" in s

"Ay" in s

Solution

Idea

Strings are searched for substrings, with case.

Python

s = "I am Ayush"
print("Ayush" in s)
print("boy" not in s)
print("ayush" in s)
print("Ay" in s)

Output

True
True
False
True

Options 1, 2 and 4 are True. Option 3 is False because of the lower-case a.
Answer: options 1, 2 and 4.

Q3Escapes

Multiple Choice Question (MCQ): choose the one correct option.

How many lines does this print, and what does the second one show?

Python

print("First\n\tEngiKode")

2 lines; second is Second line shown with a tab

2 lines; the second starts with a tab then EngiKode

1 line only

2 lines; the second is \tEngiKode as text

Solution

Idea

\n breaks the line, \t inserts a tab.

Python

print("First\n\tEngiKode")

Output

First
	EngiKode

\n ends the first line, so there are two lines.
\t is a tab character, so the second line starts with a tab space before EngiKode.
Answer: option 2.

Q4Length of an escaped string

Numerical Answer Type (NAT).

What does the program print?

Python

print(len("a\tb\n"))

Solution

Idea

Each escape is a single character.

Python

print(len("a\tb\n"))

Output

"a\tb\n" has the characters a, tab, b, newline: 4.
Answer: 4

Q5a < b > c

Multiple Choice Question (MCQ): choose the one correct option.

With a = 1, b = 5, c = 9, what is a < b > c?

Python

a, b, c = 1, 5, 9
print(a < b > c)

True

False

5

An error

Solution

Idea

It means (a < b) and (b > c).

Python

a, b, c = 1, 5, 9
print(a < b > c)

Output

False

1 < 5 is True but 5 > 9 is False, so the whole chain is False.
Answer: option 2.

Revision overview

Membershipin / not in
True or False
substring for strings

Chainingx < y < z means
x < y and y < z

Escapes\' \" \n \t
one character each

Operators and stringsWeek 2 · Part 2

QuotesSingle, double, triple
triple spans lines

Pitfallin is case-sensitive

TipChoose quotes to avoid escapes

Revision: testing membership, chaining comparisons and writing special characters

Next · StringsString Methods and the Caesar Cipher

Strings

String Methods and the Caesar Cipher

The Week 2 string methods, and a cipher that shifts letters with % 26.

Week 2 has a large set of string methods. For each one know what it does and whether it transforms the string (returns a new string) or checks a condition (returns True/False).

3.1Case methods: lower, upper, capitalize, title, swapcase

Method	What it does	Result for the sample
`lower()`	all characters lowercase	`python`
`upper()`	all characters uppercase	`PYTHON`
`capitalize()`	first character uppercase	`I love python`
`title()`	first character of each word uppercase	`I Love Python`
`swapcase()`	lower ↔ upper	`pYthON`

Python

x = "PyTHon"
print(x.lower())
print(x.upper())
print(x.swapcase())
y = "i love python"
print(y.capitalize())
print(y.title())

Output

python
PYTHON
pYthON
I love python
I Love Python

3.2Check methods: islower, isupper, istitle, isdigit, isalpha, isalnum

These return True or False.

Method	True when	Example
`islower()`	all (cased) characters are lowercase	`"python".islower()` → True, `"Python".islower()` → False
`isupper()`	all (cased) characters are uppercase	`"PYTHON".isupper()` → True, `"Python".isupper()` → False
`istitle()`	title-case rules hold	`"Python String Methods".istitle()` → True
`isdigit()`	all characters are digits	`"123".isdigit()` → True, `"123abc".isdigit()` → False
`isalpha()`	all characters are letters	`"abc".isalpha()` → True, `"abc123".isalpha()` → False
`isalnum()`	all characters are letters or digits	`"abc123".isalnum()` → True, `"abc123#".isalnum()` → False

Python

print("python".islower())
print("Python".islower())
print("PYTHON".isupper())
print("Python".isupper())
print("Python String Methods".istitle())
print("Python string methods".istitle())
print("123".isdigit())
print("123abc".isdigit())
print("abc".isalpha())
print("abc123".isalpha())
print("abc123".isalnum())
print("abc123#".isalnum())

Output

True
False
True
False
True
False
True
False
True
False
True
False

Extra. "".isdigit() (an empty string) is False, and a space or # is neither a letter nor a digit, which is why "abc123#".isalnum() is False.

3.3Trimming: strip, lstrip, rstrip

Method	Purpose
`strip()`	removes specified characters from both ends
`lstrip()`	removes them from the left
`rstrip()`	removes them from the right

Python

x = "---Python---"
print(x.strip("-"))
print(x.lstrip("-"))
print(x.rstrip("-"))

Output

Python
Python---
---Python

Figure 4. Only the ends are trimmed, and only the given characters

Extra. With no argument, strip() removes whitespace. Characters in the middle are never removed.

Python

print("  hi  ".strip() + "|")
print("--a-b--".strip("-"))

Output

hi|
a-b

3.4Searching and replacing

Method	Returns
`startswith(v)`	True if the string starts with v
`endswith(v)`	True if it ends with v
`count(v)`	how many times v occurs
`index(v)`	position of v; ValueError if absent
`replace(old, new)`	a string with old replaced by new

Python

x = "Python"
print(x.startswith("P"))
print(x.startswith("p"))
print(x.endswith("n"))
print(x.endswith("N"))

Output

True
False
True
False

Python

x = "python string methods"
print(x.count("t"))
print(x.count("s"))
print("python".index("t"))

Output

3
2
2

Python

x = "python string methods"
x = x.replace("python", "Python")
x = x.replace("methods", "Methods")
print(x)

Output

Python string Methods

Python

print("python".index("z"))

Error

ValueError: substring not found

Important. index() raises ValueError when the value is not found. (Extra: find() returns -1 instead.)

3.5Methods return new strings

Strings cannot be changed in place. A method such as upper() or replace() returns a new string and leaves the original untouched. That is why the replace example assigns back: x = x.replace(...).

Python

x = "python"
x.upper()
print(x)
x = x.upper()
print(x)

Output

python
PYTHON

3.6Quick table

Method	Main use
`lower() upper()`	lowercase / uppercase
`capitalize() title()`	first character / each word
`swapcase()`	swap letter case
`islower() isupper() istitle()`	case checks
`isdigit() isalpha() isalnum()`	content checks
`strip() lstrip() rstrip()`	trim both / left / right
`startswith() endswith()`	start / end checks
`count() index() replace()`	count, find position, replace

3.7The Caesar cipher

A Caesar cipher is a substitution technique: each character is replaced by the character a fixed number of places further along the alphabet. The fixed number is the shift. With a shift of 1: A → B, B → C, C → D, ...

The alphabet has 26 letters, so % 26 wraps an index that goes beyond 25 back to the start:

Python

alpha = "abcdefghijklmnopqrstuvwxyz"
i = 25
print(alpha[i % 26])
j = 28
print(alpha[j % 26])
k = 26
print(alpha[k % 26])

Output

z
c
a

Figure 5. s → t is a plain shift; z → a needs the % 26 wrap

Core formula

alpha[(alpha.index(character) + shift) % 26]

Find the letter's position, add the shift, wrap with % 26, and read the letter at the new position.

Python

alpha = "abcdefghijklmnopqrstuvwxyz"
name = "sky"
copy = ""
k = 1
copy = copy + alpha[(alpha.index(name[0]) + k) % 26]
copy = copy + alpha[(alpha.index(name[1]) + k) % 26]
copy = copy + alpha[(alpha.index(name[2]) + k) % 26]
print(copy)

Output

tlz

Letter	Index	+ shift 1	% 26	New letter
s	18	19	19	t
k	10	11	11	l
y	24	25	25	z

3.8Going deeper

Wrap-around with y (extra). With shift 3, y (index 24) goes to 27 % 26 = 1, which is b.
Decoding (extra). Subtract the shift instead: (index - k) % 26. Python's % never gives a negative result here, so (0 - 1) % 26 is 25.
Typing less (extra). The three repeated lines can be written as a loop over the letters (loops come later in the course).

Python

alpha = "abcdefghijklmnopqrstuvwxyz"
name = "sky"
copy = ""
k = 3
for ch in name:
    copy = copy + alpha[(alpha.index(ch) + k) % 26]
print(copy)
print((0 - 1) % 26)

Output

vnb
25

3.9find, split and join: the string tools that programs use most

Section 3.4 introduced index(), count() and replace(). Three more tools finish the toolkit: find() (a safer index()), split() and join(). The last two are how a program turns one line of typed input into several values, and back.

A. find, rfind, index and count in detail

s.find(v) returns the position of the first match, or -1 if there is none. s.index(v) does the same but raises ValueError when there is none. s.rfind(v) searches from the right and gives the position of the last match.
All of them accept optional start and end positions: s.find(v, 3) starts looking at index 3.
s.count(v) counts non-overlapping matches, scanning left to right.
s.replace(old, new, k) replaces only the first k matches when you give k.
v in s answers only "is it there?" (True/False); find also says where.

Python

s = "banana bandana"
print(s.find("ana"))        # first match
print(s.find("ana", 3))     # start looking at index 3
print(s.rfind("ana"))       # last match
print(s.find("xyz"))        # not found: -1, no error
print(s.count("ana"))       # non-overlapping matches
print("aaaa".count("aa"))   # 2, not 3: matches do not overlap
print(s.replace("ana", "ANA", 1))   # only the first match

Output

1
3
11
-1
2
2
bANAna bandana

Python

print("banana".index("xyz"))

Output

ValueError: substring not found

find versus index. Use find when "not present" is a normal outcome you will test for (if s.find(v) == -1). Use index when absence would be a bug. A classic slip: if s.find("a"): is wrong, because -1 is truthy and a match at position 0 is falsy. Write if s.find("a") != -1: or if "a" in s:.

B. split and join

s.split() with no argument splits on any run of whitespace and drops empty pieces. s.split(sep) splits at every sep and keeps empty pieces. s.split(sep, 1) splits at most once. The result is always a list of strings.
sep.join(list_of_strings) glues the pieces together with sep between them. The separator is the string you call join on; the list is the argument.
Reading numbers: input() returns one string, so a, b = input().split() splits a line like "3 4" into two strings, and each still needs int().

Python

print("a,b,,c".split(","))        # empty piece is kept
print("  a   b ".split())          # whitespace: no empty pieces
print("a-b-c".split("-", 1))       # at most one split
print("-".join(["x", "y", "z"]))
print("".join(reversed("abc")))    # reverse a string
line = "3 4"                       # what input() would return
a, b = line.split()
print(a, b, "sum", int(a) + int(b))

Output

['a', 'b', '', 'c']
['a', 'b']
['a', 'b-c']
x-y-z
cba
3 4 sum 7

Types after split. "3 4".split() gives ["3", "4"], two strings. Adding them directly gives "34", not 7. Convert each piece with int() first.

C. A few more checks and a palindrome test

startswith and endswith accept a tuple of alternatives: name.endswith((".csv", ".txt")).
isalnum() is True when every character is a letter or digit (and the string is non-empty); isspace() when every character is whitespace; isdigit() only for the digits 0 to 9, so "-12" and "3.5" are False.
A word is a palindrome when it equals its reverse: s == s[::-1]. For a test that ignores case, lower both sides first.

Python

print("file.csv".endswith((".csv", ".txt")))
print("abc 123".isalnum())
print("  ".isspace())
print("-12".isdigit())
print("3.5".isdigit())
print("level" == "level"[::-1])
print("Level".lower() == "Level".lower()[::-1])

Output

True
False
True
False
False
True
True

3.10Exam tips and common mistakes

Exam tips

Label each method as a transform or a check before predicting output.
For the cipher, write the index of every letter first, then add the shift, then % 26.

Common mistakes

Expecting x.upper() to change x without assigning the result.
Using index() for a value that may be absent: it raises ValueError.
Forgetting % 26, which fails for letters near the end such as z.
Confusing strip("-") (removes dashes at the ends) with replace("-", "") (removes all dashes).

Practice questions (graded-assignment style)

Q1Title and swapcase

Multiple Choice Question (MCQ): choose the one correct option.

What does this print?

Python

print("i love python".title())
print("PyTHon".swapcase())

I Love Python then pYthON

I love python then pYthON

I Love Python then PYTHON

i love python then pythOn

Solution

Idea

title() capitalises every word; swapcase() flips every letter.

Python

print("i love python".title())
print("PyTHon".swapcase())

Output

I Love Python
pYthON

"i love python".title() is I Love Python.
"PyTHon".swapcase() is pYthON.
Answer: option 1.

Q2Counting

Numerical Answer Type (NAT).

What does the program print?

Python

x = "python string methods"
print(x.count("t") + x.count("s"))

Solution

Idea

Add the two counts.

Python

x = "python string methods"
print(x.count("t") + x.count("s"))

Output

"t" occurs 3 times (python, string, methods) and "s" occurs 2 times (string, methods): 3 + 2 = 5.
Answer: 5

Q3Trim

Multiple Choice Question (MCQ): choose the one correct option.

What does the program print?

Python

x = "---Python---"
print(x.rstrip("-"))

Python---

---Python

Python

---Python---

Solution

Idea

rstrip trims the right end only.

Python

x = "---Python---"
print(x.rstrip("-"))

Output

---Python

rstrip("-") removes the three dashes at the right, leaving ---Python.
Answer: option 2.

Q4Index of a missing letter

Multiple Choice Question (MCQ): choose the one correct option.

What happens?

Python

print("python".index("z"))

It prints -1

It prints None

It raises a ValueError

It prints 0

Solution

Idea

index() raises an error when the value is absent.

Python

print("python".index("z"))

Error

ValueError: substring not found

"python" has no z, so index("z") raises ValueError.
Answer: option 3.

Q5Caesar shift

Numerical Answer Type (NAT).

What does this print?

Python

alpha = "abcdefghijklmnopqrstuvwxyz"
print(alpha[(alpha.index("s") + 3) % 26] + alpha[(alpha.index("k") + 3) % 26] + alpha[(alpha.index("y") + 3) % 26])

Solution

Idea

Add 3 to each index and wrap with % 26.

Python

alpha = "abcdefghijklmnopqrstuvwxyz"
print(alpha[(alpha.index("s") + 3) % 26] + alpha[(alpha.index("k") + 3) % 26] + alpha[(alpha.index("y") + 3) % 26])

Output

vnb

s(18)+3 = 21 → v; k(10)+3 = 13 → n; y(24)+3 = 27 → 27 % 26 = 1 → b.
Answer: vnb

Q6Transform or check?

Multiple Select Question (MSQ): select all that apply.

Which of these return True/False (checks)?

isdigit()

upper()

startswith()

replace()

Solution

Idea

A check answers a yes/no question.

isdigit() and startswith() return booleans.
upper() and replace() return new strings.
Answer: options 1 and 3.

Q7Wrap-around index

Numerical Answer Type (NAT).

What does this print?

Python

alpha = "abcdefghijklmnopqrstuvwxyz"
j = 28
print(alpha[j % 26])

Solution

Idea

Compute 28 % 26 first.

Python

alpha = "abcdefghijklmnopqrstuvwxyz"
j = 28
print(alpha[j % 26])

Output

28 % 26 = 2, and alpha[2] is c.
Answer: c

Q1find versus index

Multiple Select Question (MSQ): select all that apply.

Consider s = "Mississippi". Select all the true statements.

s.find("ss") is 2

s.find("ss", 3) is 5

s.count("ss") is 2

s.find("z") raises a ValueError

s.rfind("ss") is 5

Solution

Idea

Index the letters: M0 i1 s2 s3 i4 s5 s6 i7 p8 p9 i10. Run each call.

Python

s = "Mississippi"
print(s.find("ss"))
print(s.find("ss", 3))
print(s.count("ss"))
print(s.find("z"))
print(s.rfind("ss"))

Output

1 ✓ first ss starts at index 2. 2 ✓ searching from 3 skips the first and finds index 5. 3 ✓ two non-overlapping matches.
4 ✗ find returns -1; it is index that raises ValueError. 5 ✓ the last match starts at index 5.
Answer: options 1, 2, 3 and 5.

Q2Overlapping matches

Numerical Answer Type (NAT).

What does the program print?

Python

print("aaaaa".count("aa"))

Solution

Idea

count scans left to right and never reuses characters.

Python

print("aaaaa".count("aa"))

Output

Match at positions 0 to 1, then the scan resumes at 2 and matches 2 to 3; the final a is left over. Two matches.
Counting overlapping matches (positions 0, 1, 2, 3) would give 4. That is not what count does.
Answer: 2

Q3Reading what split returns

Multiple Choice Question (MCQ): choose the one correct option.

What does the program print?

Python

line = "10 20 30"
parts = line.split()
print(parts[0] + parts[1])

30

1020

10 20

an error

Solution

Idea

split() returns a list of strings; + on strings joins them.

Python

line = "10 20 30"
parts = line.split()
print(parts[0] + parts[1])

Output

parts is ['10', '20', '30']. "10" + "20" is "1020". To add the numbers write int(parts[0]) + int(parts[1]), which is 30.
Answer: option 2.

Q4Replace only the first match

Multiple Choice Question (MCQ): choose the one correct option.

What is printed by the program?

Python

s = "banana"
print(s.replace("a", "o", 2))

bonono

bonona

bononb

bonana

Solution

Idea

The third argument is the maximum number of replacements, counted from the left.

Python

s = "banana"
print(s.replace("a", "o", 2))

Output

bonona

The first two as (positions 1 and 3) become o; the last stays. Result bonona.
Answer: option 2.

Q5Joining and reversing

Multiple Select Question (MSQ): select all that apply.

Which of the following print yhp?

print("php"[::-1])

print("".join(reversed("phy")))

print("phy"[::-1])

print("-".join("phy")[::-1])

Solution

Idea

Reversing "phy" gives "yhp". Check each option.

Python

print("php"[::-1])
print("".join(reversed("phy")))
print("phy"[::-1])
print("-".join("phy")[::-1])

Output

php
yhp
yhp
y-h-p

1 ✗ reverses php (a palindrome), giving php. 2 ✓. 3 ✓.
4 ✗ "-".join("phy") is p-h-y, and its reverse is y-h-p with the hyphens.
Answer: options 2 and 3.

Revision overview

Caselower upper capitalize
title swapcase

Checksislower isupper istitle
isdigit isalpha isalnum
→ True / False

Trimstrip lstrip rstrip

String methodsWeek 2 · Part 3

Searchstartswith endswith
count index replace

RememberMethods return new strings
index raises ValueError

Caesaralpha[(alpha.index(c) + k) % 26]

Revision: what each string method does and how the cipher wraps

Next · DecisionsConditional Statements

Decisions

Conditional Statements

if, else, elif, logical operators, nested if and the conditional expression.

Decision-making lets a program run different statements depending on a condition. This part covers if, if...else, if...elif...else, logical operators in conditions, nested if and the conditional (ternary) expression.

4.1The if statement

Syntax

if expression: followed by an indented block of statement(s).

The block runs only when the expression is True. Statements of the block must be indented consistently.

Python

x = 10
y = 5
if x > y:
    text = "Hello World"
print(text)

Output

Hello World

Python

x = 3
y = 5
if x > y:
    text = "Hello World"
print(text)

Error

NameError: name 'text' is not defined

The second program shows what happens when the condition is false: the block is skipped, text is never created, and print(text) raises a NameError. (Extra.)

4.2if ... else

Python

age = 15
if age >= 18:
    print("User is old enough")
else:
    print("User is not old enough")

Output

User is not old enough

else has no condition; it runs when the if condition is false.

4.3if ... elif ... else

Figure 6. Conditions are tested top to bottom; the first true block runs

Python

x = 5
if x > 10:
    print("Yes")
elif x < 10:
    print("No")
else:
    print("x is 10")

Output

No

Python

x = 10
if x > 10:
    print("x is more than 10")
elif x < 10:
    print("x is less than 10")
else:
    print("x is 10 because it is neither more nor less")

Output

x is 10 because it is neither more nor less

First true wins

Once a branch runs, the remaining elif and else parts are skipped, even if they would also be true. So the order of branches matters.

Python

x = 95
if x > 50:
    print("above 50")
elif x > 90:
    print("above 90")

Output

above 50

4.4Logical operators in conditions

Use and (both must be true) and or (at least one must be true) to combine tests.

Python

x = 10
y = 5
if (x == 10) and (y == 5):
    text = "Hello World"
print(text)
if (x == 10) or (y == 10):
    print("At least one condition is true")

Output

Hello World
At least one condition is true

Expression	x = 10, y = 5
`(x == 10) and (y == 5)`	True
`(x == 10) and (y == 10)`	False
`(x == 10) or (y == 10)`	True
`(x == 0) or (y == 0)`	False

4.5Nested if

An if can sit inside another if (or else). The inner test runs only if the outer one let the program in.

Python

x = 15
if x > 5:
    print("x is more than 5")
    if x > 10:
        print("x is more than 10")
    else:
        print("x is between 5 and 10")
else:
    print("x is 5 or below")

Output

x is more than 5
x is more than 10

x	Output
15	`more than 10`
8	`between 5 and 10`
3	`5 or below`

4.6Conditional (ternary) expression

Form

value_if_true if condition else value_if_false

A one-line choice between two values, handy for simple assignments.

Figure 7. The three parts of a conditional expression

Python

x = 10
text = "x is 10" if x == 10 else "x is not 10"
print(text)

Output

x is 10

Python

x = 7
print("even" if x % 2 == 0 else "odd")

Output

odd

4.7Going deeper

Indentation is the syntax. A body that is not indented is an IndentationError (extra).
Use == to compare. A single = assigns and is a syntax error inside an if (extra).
elif versus separate ifs (extra). Separate if statements are all tested; an elif chain stops at the first true branch.

Python

x = 95
if x > 50:
    print("above 50")
if x > 90:
    print("above 90")

Output

above 50
above 90

Python

x = 5
if x > 3:
print("big")

Error

IndentationError: expected an indented block after 'if' statement on line 2

4.8Exam tips and common mistakes

Exam tips

Trace from the top: test the first condition, run its block if true and skip everything else in the chain.
For nested ifs, indent level tells you which else belongs to which if.

Common mistakes

Writing = instead of == in a condition.
Forgetting the colon after if, elif or else.
Putting a broader test before a narrower one in an elif chain.
Mixing indentation levels.

Practice questions (graded-assignment style)

Q1elif chain

Multiple Choice Question (MCQ): choose the one correct option.

What does the program print?

Python

x = 5
if x > 10:
    print("Yes")
elif x < 10:
    print("No")
else:
    print("x is 10")

Yes

No

x is 10

Nothing

Solution

Idea

Test the conditions top to bottom.

Python

x = 5
if x > 10:
    print("Yes")
elif x < 10:
    print("No")
else:
    print("x is 10")

Output

No

5 > 10 is false, 5 < 10 is true, so No is printed and the rest is skipped.
Answer: option 2.

Q2Order of branches

Multiple Choice Question (MCQ): choose the one correct option.

What is printed?

Python

x = 95
if x > 50:
    print("above 50")
elif x > 90:
    print("above 90")

above 90

above 50

Both lines

Nothing

Solution

Idea

The first true branch wins.

Python

x = 95
if x > 50:
    print("above 50")
elif x > 90:
    print("above 90")

Output

above 50

95 > 50 is true, so the first block runs and the elif is never tested.
Answer: option 2.

Q3Nested if

Multiple Choice Question (MCQ): choose the one correct option.

What is printed for x = 8?

Python

x = 8
if x > 5:
    if x > 10:
        print("x is more than 10")
    else:
        print("x is between 5 and 10")
else:
    print("x is 5 or below")

x is more than 10

x is between 5 and 10

x is 5 or below

Nothing

Solution

Idea

The outer test passes; then decide the inner one.

Python

x = 8
if x > 5:
    if x > 10:
        print("x is more than 10")
    else:
        print("x is between 5 and 10")
else:
    print("x is 5 or below")

Output

x is between 5 and 10

8 > 5 is true, so the outer block runs; 8 > 10 is false, so the inner else prints.
Answer: option 2.

Q4Ternary

Numerical Answer Type (NAT).

What does the program print?

Python

x = 11
text = "x is 10" if x == 10 else "x is not 10"
print(text)

Solution

Idea

The condition decides which value is chosen.

Python

x = 11
text = "x is 10" if x == 10 else "x is not 10"
print(text)

Output

x is not 10

x == 10 is false for x = 11, so the value after else is used.
Answer: x is not 10

Q5Logical operators

Multiple Select Question (MSQ): select all that apply.

For x = 10, y = 5, which conditions are true?

Python

x = 10
y = 5
print((x == 10) and (y == 5))
print((x == 10) and (y == 10))
print((x == 10) or (y == 10))
print((x == 0) or (y == 0))

(x == 10) and (y == 5)

(x == 10) and (y == 10)

(x == 10) or (y == 10)

(x == 0) or (y == 0)

Solution

Idea

and needs both; or needs one.

Python

x = 10
y = 5
print((x == 10) and (y == 5))
print((x == 10) and (y == 10))
print((x == 10) or (y == 10))
print((x == 0) or (y == 0))

Output

True
False
True
False

Options 1 and 3 are true.
Option 2 fails on y == 10; option 4 fails on both parts.
Answer: options 1 and 3.

Q6Separate ifs

Multiple Choice Question (MCQ): choose the one correct option.

What does this print?

Python

x = 95
if x > 50:
    print("above 50")
if x > 90:
    print("above 90")

Only above 50

Only above 90

above 50 then above 90

Nothing

Solution

Idea

Two separate if statements are both tested.

Python

x = 95
if x > 50:
    print("above 50")
if x > 90:
    print("above 90")

Output

above 50
above 90

There is no elif, so each if is tested on its own and both are true.
Answer: option 3.

Revision overview

ifBlock runs when the
expression is True

else / elifelse: otherwise
elif: more conditions
first true wins

Combineand: both
or: at least one

Conditional statementsWeek 2 · Part 4

Nestedif inside if
indent shows ownership

Ternarya if condition else b

Watch== not =
colon
branch order

Revision: making the program choose between blocks

Next · DecisionsPractice Zone

Decisions

Practice Zone

Four programs that turn decision rules into code, traced with real inputs.

Try each problem yourself before reading the solution. Every program below was run for real; the sample runs show what it prints.

5.1Problem 1: even or odd

Take an integer as input and decide whether it is even or odd. A number is even when dividing by 2 leaves remainder 0.

Python

x = int(input("Enter a number: "))
if x % 2 == 0:
    print("Even")
else:
    print("Odd")

Output

Enter a number: 7
Odd

Input	Output
7	`Odd`
10	`Even`
0	`Even`
-3	`Odd`

Extra. -3 % 2 is 1 in Python, so negative odd numbers are still reported as Odd.

5.2Problem 2: the last digit

Decide whether the last digit of a number is 0, 5 or something else. A last digit of 0 or 5 means the number is divisible by 5; among those, divisible by 10 means the last digit is 0. That is a nested if.

Python

x = int(input("Enter a number: "))
if x % 5 == 0:
    if x % 10 == 0:
        print("0")
    else:
        print("5")
else:
    print("Other")

Output

Enter a number: 25
5

Input	% 5 == 0 ?	% 10 == 0 ?	Output
40	yes	yes	`0`
25	yes	no	`5`
7	no	—	`Other`

5.3Problem 3: student grade

Grade	Marks (as given)
A	$> 90$ and $\leq 100$
B	$> 80$
C	$\geq 70$
D	$\geq 60$
E	$\leq 60$ and $> 0$

Python

x = int(input("Enter your marks: "))

if x > 90 and x <= 100:
    print("A")
elif x > 80:
    print("B")
elif x >= 70:
    print("C")
elif x >= 60:
    print("D")
elif x <= 60 and x > 0:
    print("E")
else:
    print("Not Valid")

Output

Enter your marks: 72
C

Figure 8. How the elif chain actually grades each mark

Marks	Output	Why
95	`A`	first test: 95 > 90 and ≤ 100
85	`B`	not A; 85 > 80
80	`C`	80 > 80 is false, so it falls to ≥ 70
72	`C`	≥ 70
60	`D`	≥ 60 is tested before the E branch
30	`E`	only the last branch fits
0	`Not Valid`	fails every test

Order does the work

Each elif only runs if every test above it was false, so x > 80 really means "above 80 and not A".
The table says E is "≤ 60", but 60 is also "≥ 60" (D). The code resolves the overlap in favour of D, because D comes first.

Correction (extra). The source program has a gap: marks above 100 are not rejected, they print B (the output for 105 above). A safer version checks the valid range first.

Python

x = int(input("Enter your marks: "))

if x <= 0 or x > 100:
    print("Not Valid")
elif x > 90:
    print("A")
elif x > 80:
    print("B")
elif x >= 70:
    print("C")
elif x >= 60:
    print("D")
else:
    print("E")

Output

Enter your marks: 105
Not Valid

Marks	Output (safer version)
105	`Not Valid`
100	`A`
91	`A`
60	`D`
59	`E`
0	`Not Valid`

5.4Problem 4: travel decision

A flowchart problem: travel from City A to City B. First ask whether the trip time is longer or shorter than a threshold, then compare the price with a threshold to pick coach/train (long trip) or red-eye/daytime flight (short trip).

Figure 9. The decision tree behind the program

Python

print("Travel from City A to City B")

time = int(input("Enter time: "))
longer = int(input("Define longer: "))

if time >= longer:
    price = int(input("Enter price: "))
    higher = int(input("Define higher: "))

    if price >= higher:
        print("Train")
    else:
        print("Coach")
else:
    price = int(input("Enter price: "))
    higher = int(input("Define higher: "))

    if price >= higher:
        print("Daytime Flight")
    else:
        print("Red Eye Flight")

print("Arrive City B")

Output

Travel from City A to City B
Enter time: 5
Define longer: 3
Enter price: 200
Define higher: 100
Train
Arrive City B

time	longer	price	higher	Result
5	3	200	100	`Train`
5	3	50	100	`Coach`
1	3	200	100	`Daytime Flight`
1	3	50	100	`Red Eye Flight`

Notice that higher and longer are values you define, so the program is really two nested decisions. The last line, print("Arrive City B"), is outside both blocks and runs every time.

5.5Exam tips and common mistakes

Exam tips

Make a small table of inputs and trace each one through the branches.
Test boundary values (60, 70, 80, 90, 100) because that is where branch order matters.

Common mistakes

Using > where the boundary should be included (>=).
Forgetting int(...) around input, which leaves a string and breaks the comparison.
Assuming the code rejects invalid marks without checking the range.

Practice questions (graded-assignment style)

Q1Grade for 80

Multiple Choice Question (MCQ): choose the one correct option.

For the input 80, what does the grade program print?

Python

x = int(input("Enter your marks: "))

if x > 90 and x <= 100:
    print("A")
elif x > 80:
    print("B")
elif x >= 70:
    print("C")
elif x >= 60:
    print("D")
elif x <= 60 and x > 0:
    print("E")
else:
    print("Not Valid")

A

B

C

D

Solution

Idea

Is 80 > 80 true?

Python

x = int(input("Enter your marks: "))

if x > 90 and x <= 100:
    print("A")
elif x > 80:
    print("B")
elif x >= 70:
    print("C")
elif x >= 60:
    print("D")
elif x <= 60 and x > 0:
    print("E")
else:
    print("Not Valid")

Output

Enter your marks: 80
C

80 > 90 is false, 80 > 80 is false, 80 >= 70 is true: C.
Answer: option 3.

Q2Grade for 60

Multiple Choice Question (MCQ): choose the one correct option.

For the input 60, what is printed?

Python

x = int(input("Enter your marks: "))

if x > 90 and x <= 100:
    print("A")
elif x > 80:
    print("B")
elif x >= 70:
    print("C")
elif x >= 60:
    print("D")
elif x <= 60 and x > 0:
    print("E")
else:
    print("Not Valid")

C

D

E

Not Valid

Solution

Idea

Which of >= 60 and <= 60 is tested first?

Python

x = int(input("Enter your marks: "))

if x > 90 and x <= 100:
    print("A")
elif x > 80:
    print("B")
elif x >= 70:
    print("C")
elif x >= 60:
    print("D")
elif x <= 60 and x > 0:
    print("E")
else:
    print("Not Valid")

Output

Enter your marks: 60
D

>= 60 comes before <= 60 and > 0, so D is printed.
Answer: option 2.

Q3A bug

Multiple Choice Question (MCQ): choose the one correct option.

For the input 105, what does the original grade program print?

Python

x = int(input("Enter your marks: "))

if x > 90 and x <= 100:
    print("A")
elif x > 80:
    print("B")
elif x >= 70:
    print("C")
elif x >= 60:
    print("D")
elif x <= 60 and x > 0:
    print("E")
else:
    print("Not Valid")

A

B

Not Valid

An error

Solution

Idea

Follow the chain: the first test needs x <= 100.

Python

x = int(input("Enter your marks: "))

if x > 90 and x <= 100:
    print("A")
elif x > 80:
    print("B")
elif x >= 70:
    print("C")
elif x >= 60:
    print("D")
elif x <= 60 and x > 0:
    print("E")
else:
    print("Not Valid")

Output

Enter your marks: 105
B

The first test fails (105 is above 100) but 105 > 80 is true, so B is printed.
Answer: option 2.

Q4Last digit

Numerical Answer Type (NAT).

What does the program print for the input 25?

Python

x = int(input("Enter a number: "))
if x % 5 == 0:
    if x % 10 == 0:
        print("0")
    else:
        print("5")
else:
    print("Other")

Solution

Idea

25 is divisible by 5 but not by 10.

Python

x = int(input("Enter a number: "))
if x % 5 == 0:
    if x % 10 == 0:
        print("0")
    else:
        print("5")
else:
    print("Other")

Output

Enter a number: 25
5

25 % 5 == 0 is true; 25 % 10 == 0 is false, so the inner else prints 5.
Answer: 5

Q5Travel

Multiple Choice Question (MCQ): choose the one correct option.

For the inputs 1, 3, 50, 100 (in that order), what is the result line before "Arrive City B"?

Python

print("Travel from City A to City B")

time = int(input("Enter time: "))
longer = int(input("Define longer: "))

if time >= longer:
    price = int(input("Enter price: "))
    higher = int(input("Define higher: "))

    if price >= higher:
        print("Train")
    else:
        print("Coach")
else:
    price = int(input("Enter price: "))
    higher = int(input("Define higher: "))

    if price >= higher:
        print("Daytime Flight")
    else:
        print("Red Eye Flight")

print("Arrive City B")

Train

Coach

Daytime Flight

Red Eye Flight

Solution

Idea

1 >= 3 is false, so use the else branch.

Python

print("Travel from City A to City B")

time = int(input("Enter time: "))
longer = int(input("Define longer: "))

if time >= longer:
    price = int(input("Enter price: "))
    higher = int(input("Define higher: "))

    if price >= higher:
        print("Train")
    else:
        print("Coach")
else:
    price = int(input("Enter price: "))
    higher = int(input("Define higher: "))

    if price >= higher:
        print("Daytime Flight")
    else:
        print("Red Eye Flight")

print("Arrive City B")

Output

Travel from City A to City B
Enter time: 1
Define longer: 3
Enter price: 50
Define higher: 100
Red Eye Flight
Arrive City B

time >= longer is false, so the program goes to the else block; 50 >= 100 is false, so Red Eye Flight is printed.
Answer: option 4.

Q6Even or odd

Numerical Answer Type (NAT).

What does the even/odd program print for the input 10?

Python

x = int(input("Enter a number: "))
if x % 2 == 0:
    print("Even")
else:
    print("Odd")

Solution

Idea

Check the remainder on division by 2.

Python

x = int(input("Enter a number: "))
if x % 2 == 0:
    print("Even")
else:
    print("Odd")

Output

Enter a number: 10
Even

10 % 2 is 0, so Even.
Answer: Even

Revision overview

Problem 1x % 2 == 0
Even else Odd

Problem 2Nested: % 5 then % 10
0, 5 or Other

Problem 3elif chain, order matters
60 → D, 105 → B (bug)

Practice zoneWeek 2 · Part 5

Problem 4Two nested decisions
time then price

MethodTrace a table of inputs

TestBoundary values

Revision: turning a decision rule into if/elif/else code

Next · ModulesImporting Modules: math, random, calendar

Modules

Importing Modules: math, random, calendar

Reusing code with import, and the four ways of importing.

A module is reusable code that you can bring into your program with import. It saves you from rewriting common tools.

6.1Importing modules

Python

import math
print(math.pow(10, 3))

Output

1000.0

Use from to import a particular part of a module; then you use the name directly, without the math. prefix.

Python

from math import sqrt
print(sqrt(9))

Output

3.0

6.2The math module

Name	What it gives	Sample
`math.sqrt(x)`	square root of x	`3.0`
`math.pow(x, y)`	x raised to the power y (float)	`8.0`
`math.ceil(x)`	rounds up to an integer	`3`
`math.floor(x)`	rounds down to an integer	`2`
`math.pi`	the constant π	`3.141592653589793`
`math.e`	the constant e	`2.718281828459045`
`math.cos(x) sin(x) tan(x)`	trigonometric functions (x in radians)	`1.0 0.0 0.0`

Python

import math
print(math.sqrt(9))
print(math.pow(2, 3))
x = math.ceil(2.6)
y = math.floor(2.6)
print(x)
print(y)

Output

Python

import math
print(math.pi)
print(math.e)
print(math.cos(0))
print(math.sin(0))
print(math.tan(0))

Output

3.141592653589793
2.718281828459045
1.0
0.0
0.0

Watch the types. sqrt and pow return floats (3.0, 8.0), while ceil and floor return integers.

Extra: for a negative number, floor goes to the smaller integer, which is not the same as cutting off the decimal part.

Python

import math
print(math.floor(-2.6))
print(math.ceil(-2.6))
print(int(-2.6))

Output

-3
-2
-2

6.3The random module

random.random() gives a random float from 0.0 up to (but not including) 1.0.
random.randint(x, y) gives a random integer in the range x to y; both ends can occur.

Python

import random
x = random.random()
print(x)
y = random.randint(1, 10)
print(y)

The printed values change every run, so the exact output cannot be predicted. What you can predict are the ranges:

Python

import random
x = random.random()
y = random.randint(1, 10)
print(0 <= x < 1)
print(1 <= y <= 10)
print(type(x), type(y))

Output

True
True
<class 'float'> <class 'int'>

6.4The calendar module

Python

import calendar
print(calendar.month(2020, 10))

Output

    October 2020
Mo Tu We Th Fr Sa Su
          1  2  3  4
 5  6  7  8  9 10 11
12 13 14 15 16 17 18
19 20 21 22 23 24 25
26 27 28 29 30 31

calendar.month(year, month) returns the text of one month; the week starts on Monday. calendar.calendar(year) returns the whole year. Here are the first lines of calendar.calendar(2018):

Python

import calendar
lines = calendar.calendar(2018).splitlines()
print("\n".join(lines[:6]))

Output

                                  2018

      January                   February                   March
Mo Tu We Th Fr Sa Su      Mo Tu We Th Fr Sa Su      Mo Tu We Th Fr Sa Su
 1  2  3  4  5  6  7                1  2  3  4                1  2  3  4
 8  9 10 11 12 13 14       5  6  7  8  9 10 11       5  6  7  8  9 10 11

6.5Different ways of importing modules

Figure 10. Four import forms and how to call sqrt after each

Python

# 1. Whole module
import math
print(math.sqrt(4))

# 2. Specific item
from math import sqrt
print(sqrt(4))

# 3. Alias
import math as m
print(m.sqrt(4))

# 4. Everything
from math import *
print(sqrt(4))

Output

2.0
2.0
2.0
2.0

Form	Use	Note
`import math`	`math.sqrt(4)`	safest; the module name shows where a function comes from
`from math import sqrt`	`sqrt(4)`	only the named item is brought in
`import math as m`	`m.sqrt(4)`	short alias for a long name
`from math import *`	`sqrt(4)`	brings in everything; can overwrite your own names

Python

import math
print(sqrt(4))

Error

NameError: name 'sqrt' is not defined

Python

from math import sqrt
print(math.sqrt(4))

Error

NameError: name 'math' is not defined

A name only exists in the form you imported it. After import math you must write math.sqrt; after from math import sqrt the name math itself is not defined. Both mistakes give a NameError.

6.6Going deeper

Imports usually go at the top (extra). It makes clear what a program depends on.
pow versus ** (extra). math.pow(2, 3) returns a float 8.0, while 2 ** 3 returns the integer 8.
Why avoid import * (extra). If two modules define the same name, the later one silently wins.

Python

import math
print(math.pow(2, 3))
print(2 ** 3)

Output

8.0
8

6.7Exam tips and common mistakes

Exam tips

Match the way you call a function to the way you imported it.
For random functions, reason about the range, never about the exact value.

Common mistakes

Calling sqrt(4) after only import math.
Calling math.sqrt after from math import sqrt.
Thinking random.randint(1, 10) never returns 10.
Passing degrees to sin and cos; they expect radians.

Practice questions (graded-assignment style)

Q1pow

Numerical Answer Type (NAT).

What does the program print?

Python

import math
print(math.pow(2, 3))

Solution

Idea

math.pow returns a float.

Python

import math
print(math.pow(2, 3))

Output

8.0

2 raised to 3 is 8, and pow returns the float 8.0.
Answer: 8.0

Q2ceil and floor

Numerical Answer Type (NAT).

What does the program print?

Python

import math
print(math.ceil(2.6) + math.floor(2.6))

Solution

Idea

Round 2.6 up and down, then add.

Python

import math
print(math.ceil(2.6) + math.floor(2.6))

Output

ceil(2.6) is 3 and floor(2.6) is 2: 3 + 2 = 5.
Answer: 5

Q3Which import?

Multiple Choice Question (MCQ): choose the one correct option.

What happens when this runs?

Python

import math
print(sqrt(4))

It prints 2.0

It prints 4

It raises a NameError

It raises a SyntaxError

Solution

Idea

Which name did the import create?

Python

import math
print(sqrt(4))

Error

NameError: name 'sqrt' is not defined

import math creates the name math, not sqrt, so sqrt(4) is a NameError.
Answer: option 3.

Q4Imports that work

Multiple Select Question (MSQ): select all that apply.

For which setups does print(sqrt(4)) print 2.0?

import math

from math import sqrt

import math as m

from math import *

Solution

Idea

Which setups bring the bare name sqrt in?

from math import sqrt and from math import * bring in sqrt itself.
import math needs math.sqrt, and the alias form needs m.sqrt.
Answer: options 2 and 4.

Q5Random ranges

Multiple Choice Question (MCQ): choose the one correct option.

Which statement about the program is always true?

Python

import random
x = random.random()
y = random.randint(1, 10)
print(0 <= x < 1)
print(1 <= y <= 10)

The first line printed is always 0.5

Both printed lines are True

The second line can be False when the integer is 10

The program raises an error

Solution

Idea

Think about the ranges, not the values.

Python

import random
x = random.random()
y = random.randint(1, 10)
print(0 <= x < 1)
print(1 <= y <= 10)

Output

True
True

random.random() is at least 0 and below 1; randint(1, 10) is between 1 and 10 including both ends.
Answer: option 2.

Q6Lines in a month

Numerical Answer Type (NAT).

How many lines does calendar.month(2020, 10) print?

Python

import calendar
print(len(calendar.month(2020, 10).splitlines()))

Solution

Idea

Title line, weekday line, then the weeks.

Python

import calendar
print(len(calendar.month(2020, 10).splitlines()))

Output

One title line, one weekday-header line and five week lines make 7.
Answer: 7

Revision overview

importimport math
math.sqrt(...)

mathsqrt pow ceil floor
pi e cos sin tan

randomrandom(): [0, 1)
randint(x, y): x to y

ModulesWeek 2 · Part 6

calendarmonth(y, m)
calendar(y)

Import styleswhole, from, alias, *

CarefulCall matches import
avoid import *

Revision: reusing code with modules

Next · All partsWeek 2 Summary Sheet

All parts

Week 2 Summary Sheet

One page to revise all of Week 2: topics, tools, the exam checklist and the mistakes to avoid.

The big picture

Part 1 · Variables

Meaningful names; # comments.
Dynamic typing; naming rules; del; short-hand operators.

Part 2 · Operators and strings

in / not in; chained comparisons.
Escapes \n \t; single, double, triple quotes.

Part 3 · String methods

Case, checks, trim, search, replace.
Caesar cipher: alpha[(index + k) % 26].

Part 4 · Conditions

if, else, elif, nested, ternary.
and / or combine tests.

Part 5 · Practice

Even/odd, last digit, grades, travel.
Order of elif decides edge cases.

Part 6 · Modules

import, math, random, calendar.
Four import styles.

Revision sheet

Topic	Key point
Meaningful variables	names should communicate purpose
Comments	# ; not executed
Dynamic typing	a variable can refer to different types
Variable rules	letter/underscore start; case-sensitive; no keywords
Multiple assignment	`x, y = 1, 2`
Same-value assignment	`x = y = z = 8`
Delete	`del x`
Short-hand	x += 3 etc.
Membership	in, not in
Chained comparisons	`a < b < c`
Escape characters	\n, \t, escaped quotes
Quotes	single, double, triple
String methods	case, checks, trimming, searching, replacing
Caesar Cipher	shift letters; % 26 wraps
if / else / elif	run a block / alternative / more conditions
Nested if	if inside if
Conditional expression	`a if condition else b`
import	bring a module into the program
math	mathematical functions/constants
random	random values
calendar	calendar output

Which tool when?

You want to…	Use
test if a word is inside a text	`in`
check a value lies between two numbers	`a < x < b`
print a quote or a tab inside a string	`\" \' \t`
make a string lowercase / title-case	`lower() title()`
test if a string is all digits	`isdigit()`
shift a letter and wrap past z	`% 26`
choose between two values on one line	`a if cond else b`
use square root, ceiling, pi	`math`

Exam checklist (from the source)

Explain dynamic typing.
Apply variable naming rules.
Perform multiple assignment and unpacking.
Delete a variable using del.
Use short-hand assignment operators.
Use in and not in.
Read chained comparisons.
Use \n, \t and escaped quotes.
Distinguish single, double and triple-quoted strings.
Know the Week 2 string methods.
Explain the Caesar Cipher formula.
Write if, if-else and if-elif-else.
Combine conditions with and and or.
Write a nested if and the conditional expression.
Import and use modules; know the different import styles.
Use math.sqrt(), pow(), ceil(), floor(), random.random(), random.randint() and the calendar module.

Common mistakes

Invalid names

2nd_place, first-name and keywords are not allowed.

Methods change nothing

String methods return a new string; assign it back.

Wrong branch order

A broad elif before a narrow one hides the narrow one.

= versus ==

Compare with ==; = assigns.

Import mismatch

Call a function the way you imported it.

Unvalidated input

The grade program prints B for 105; check the range first.

Python,Week 2

Chapters

Variables: Names, Comments and Typing

1.1Meaningful variable names

1.2Comments

1.3Dynamic typing

1.4Rules for variable names

1.5Multiple assignment and unpacking

1.6Deleting a variable

1.7Short-hand assignment operators

1.8Going deeper

1.9Exam tips and common mistakes

Practice questions (graded-assignment style)

Operators and Strings: Membership, Chaining, Escapes, Quotes

2.1Membership operators

2.2Chaining comparison operators

2.3Escape characters

2.4Different types of quotes

2.5Exam tips and common mistakes

Practice questions (graded-assignment style)

String Methods and the Caesar Cipher

3.1Case methods: lower, upper, capitalize, title, swapcase

3.2Check methods: islower, isupper, istitle, isdigit, isalpha, isalnum

3.3Trimming: strip, lstrip, rstrip

3.4Searching and replacing

3.5Methods return new strings

3.6Quick table

3.7The Caesar cipher

3.8Going deeper

3.9find, split and join: the string tools that programs use most

A. find, rfind, index and count in detail

B. split and join

C. A few more checks and a palindrome test

3.10Exam tips and common mistakes

Practice questions (graded-assignment style)

Conditional Statements

4.1The if statement

4.2if ... else

4.3if ... elif ... else

4.4Logical operators in conditions

4.5Nested if

4.6Conditional (ternary) expression

4.7Going deeper

4.8Exam tips and common mistakes

Practice questions (graded-assignment style)

Practice Zone

5.1Problem 1: even or odd

5.2Problem 2: the last digit

5.3Problem 3: student grade

5.4Problem 4: travel decision

5.5Exam tips and common mistakes

Practice questions (graded-assignment style)

Importing Modules: math, random, calendar

6.1Importing modules

6.2The math module

6.3The random module

6.4The calendar module

6.5Different ways of importing modules

6.6Going deeper

6.7Exam tips and common mistakes

Practice questions (graded-assignment style)

Week 2 Summary Sheet

The big picture

Revision sheet

Which tool when?

Exam checklist (from the source)

Common mistakes

Python,
Week 2