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Just Enough Python: Cheat Sheet

A reference for all the Python syntax and language features you need for Python Self-Study 1.

Comments

Notes written in your code for developers to read. The computer will ignore these when executing your code.

 # single-line comment

'''
  multiple line comment
  (actually a string, but don't worry about that just yet)
'''

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Data Types

The smallest pieces of data in a Python program. There are many data types but you only need to know these for now:

# Booleans
True
False

# Strings
''  # empty string
'hello'
'"hello"'  # quotes in a string (1)
"'hello'"  # quotes in a string (2)

# Integers
0
1
-100

# Floats
0.0
1.5
-3.14

# isinstance(): checks if a value is a certain type
isinstance("a string", str)  # True
isinstance(True, bool)  # True
isinstance(1, int)  # True
isinstance(1.0, float)  # True

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Operators

Ways to transform data. An operator takes in 1 or more values and evaluates to a new value.

Operators in Python are a huge topic, for now this should be enough:

# -- string concatenation --
"hello" + " " + "world"  # "hello world"

# -- logical (or boolean) operators --
True and False  # False
True or False  # True
not True  # False

# -- arithmetic: int int --
# addition
4 + 2  # 6
# subtraction
4 - 2  # 2

# -- arithmetic: float float --
# addition
4.0 + 2.0  # 6.0
# subtraction
4.0 - 2.0  # 2.0

# -- arithmetic: mixed --
# addition
4 + 2.0  # 6.0
# subtraction
4.0 - 2  # 2.0

# -- comparisons --
# equality
4 == "4"  # False
# inequality
4 != "4"  # True
# greater than
4 > 3  # True
4 > 4  # False
# less than
4 < 4  # False
4 < 5  # True
# greater than or equal to
4 >= 3  # True
4 >= 4  # True
4 >= 5  # False
# less than or equal to
4 <= 3  # False
4 <= 4  # True
4 <= 5  # True

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Assertions

You can test if something is True or False using assert. If the assertion If the assertion fails, there will be an error and the program stops. If the assertion passes, nothing happens and the program continues. This is a useful tool for studying, you can use assertions like a small quiz to test your understanding of the program.

Adding a description to your assertion will help you read the error messages in your console.

# a passing assertion without a description
assert 4 > 3

# a passing assertion with a description
assert 4 > 3, 'four is greater than 3'

# a failing assertion without a description
assert 4 == 3 # AssertionError

# a failing assertion with a description
assert 4 == 3, 'four is equal to three' # AssertionError: four is equal to three

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String Manipulation

The data type used for storing and manipulating text data. Strings will be the main type of data used in the first Python workshops.

# string length
len("")  # 0
len("a")  # 1
len("ab")  # 2

# indexed access
"abc"[-1]  # 'c'
"abc"[0]  # 'a'
"abc"[1]  # 'b'
"abc"[2]  # 'c'

# --- string methods ---

"HeLlO".lower()  # 'hello'
"HeLlO".upper()  # 'HELLO'

"b" in "abc"  # True

"+a+b+c+".replace("+", "")  # 'abc'

"  abc    ".strip()  # 'abc'

"abc".find("a")  # 0
"abc".find("")  # 0
"abc".find("b")  # 1
"abc".find("bc")  # 1
"abc".find("x")  # -1

# getting a substring by index
"abc"[0:]  # 'abc'
"abc"[1:]  # 'bc'
"abc"[2:]  # 'c'

"abc"[:0]  # ''
"abc"[:1]  # 'a'
"abc"[:2]  # 'ab'

"abc"[0:0]  # ''
"abc"[0:1]  # 'a'
"abc"[0:2]  # 'ab'
"abc"[1:1]  # ''
"abc"[1:2]  # 'b'
"abc"[2:2]  # ''

"abc"[-2:-1]  # 'b'

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Printing

A simple way to print data to the console while the program is running. This is helpful for knowing what data is stored in your program at different points in execution.

print("hello");

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Variables

Variables allow you to save values to use again later in your program.They're kind of like a box that can only hold one thing at a time.

Variables are also an important tool for writing code that is clear for other developers to read and understand. Using helpful names can make your code read (sort of) like a story.

# assign: name
name = "Python"

# read: name
print(name) # "Python"

# assign: exclaim
exclaim = "!"

# read: name, exclaim
# assign: name
name = name + exclaim

# read: name
print(name)  # "Python!"

# cannot read a variable before assigning it
print(noop) # NameError: name 'noop' is not defined

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Lists

Lists store multiple values in one data structure.

letters = ['b', 'c']

# add an item to the end of the list
letters.append('d')
print(letters) # ['b', 'c', 'd']

# insert an item at a specific index in the list
letters.insert(0,'a')
print(letters) # ['a', 'b', 'c', 'd']

# get the length of a list
len(letters) # 4

# get a specific letter by it's index
letters[0] # 'a'
letters[1] # 'b'
letters[2] # 'c'
letters[3] # 'a'

# slice a part of the list to a new list
letters[1:3] # ['b', 'c']

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Input

Programmers can pass string data into your programs using input.

# --- input ---

# allows the user to enter a string value for the program to use
string_from_user = input("enter some text:\n")

# --- output ---

appreciation = 'thank you for this text: ' + string_from_user

# prints a message, but does not accept new input from the user
print(appreciation)

Explanation of str(input()), int(input()), and float(input())

This document explains how to use str(input()), int(input()), and float(input()) in Python with simple examples.

1. str(input())

  • Explanation:
    • input() collects data from the user as a string by default.
    • Wrapping it in str() ensures the input is treated as a string explicitly.
    • Useful when working with text.

Example:

name = str(input("Enter your name: "))  # User enters: Obay
print("Hello, " + name)                # Output: Hello, Obay

2. int(input())

  • Explanation:
    • input() collects data as a string, but int() converts it into an integer.
    • Use this for numbers that don’t have decimals (whole numbers).
    • If the user enters a non-numeric value (like "abc"), it will throw an error.

Example:

age = int(input("Enter your age: "))  # User enters: 25
print("You are " + str(age) + " years old.")  # Output: You are 25 years old.

3. float(input())

  • Explanation:
    • input() collects data as a string, but float() converts it into a decimal (floating-point number).
    • Use this for numbers that might include decimals.

Example:

price = float(input("Enter the price: "))  # User enters: 12.99
print("The price is $" + str(price))       # Output: The price is $12.99

Key Notes

  1. Always use the correct type (int for whole numbers, float for decimals, and str for text).
  2. If the input can't be converted (e.g., entering text when int() is expected), Python will show an error.
  3. You can also use type() to verify the data type of user input.

Example:

value = int(input("Enter a number: "))
print("Type:", type(value))  # Output: <class 'int'>

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Conditionals

Execute different blocks of code depending on whether an expression evaluates to True or to False:

# -- if statements conditionally execute one block of code based on one expression --
if an_expression:
    print('if') # executed if the expression is true

# -- if/else statements conditionally execute two blocks of based on one expression --
if an_expression
    print('if') # executed if the expression is true
else:
    print('else') # executed if the expression is false

# -- if/elif/else statements execute blocks of code based on multiple expressions --
#      the else block is executed by default if no expression if no condition is met
if expression_1:
    print('if') # executed if the first expression is true
elif expression_2:
    print('elif') # executed if the second expression is true
else:
    print('else') # executed if neither expression is true


# you can use if/elif without else when you do not want a default behavior
if expression_1:
    print('if') # executed if the first expression is true
elif expression_2:
    print('elif') # executed if the second expression is true

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While Loops

Repeat a block of code as long as an expression evaluates to true.

  1. Evaluate the expression
  2. Check if the expression is True or False
    1. if it is True, execute the block
    2. return to step 2
  3. Move on to the next line after the loop
while an_expression:
   print('still looping') # executed each time the expression is evaluated to True

# next line after the loop

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For-In Loops

Iterate over a string or list, executing the loop body once for each character or item.

text = 'bye'
for char in text:
  print (char) # 'b' -> 'y' -> 'e'

# next line after first loop

numbers = [1, 2, 3]
for number in numbers:
  print(number) # 1 -> 2 -> 3

# next line after second loop

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Functions

Wrap useful lines of code in a function so you can write them in one place and reuse as many times as you want.

# --- function declaration ---
def add(x, y):
    sum = x + y
    return sum


# --- call the function and save the result to a variable ---

seven = add(3, 4)
print(seven) # 7

eight = add(2, 6)
print(eight) # 8

# --- test the function with assertions ---

assert add(3, 4) == 7, '3 + 4 = 7'
assert add(2, 6) == 7, '2 + 6 = 8'

zero = add(-1, 1)
assert zero == 0, '-1 + 1 = 0'

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Pass

pass allows you to write the control flow structure of a program before writing all of the logic. Without using pass, Python throws a syntax error if a block is empty.

if an_expression:
  pass

if expression_1:
  pass
elif expression_2:
  pass
else:
  pass

while an_expression:
  pass

for char in 'a string':
  pass

for item in []:
  pass

def funky():
  pass

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