Python 3 Cheat Sheet: A Beginner's Guide to Fundamentals, Cheat Sheet of Programming Languages

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Real Python: Python 3 Cheat Sheet

Contents

• 1 Introduction
• 2 Primitives
  • Numbers
  • Strings
  • Booleans
• 3 Collections
  • Lists
  • Dictionaries
• 4 Control Statements
  • IF Statements
  • Loops
• 5 Functions

Chapter 2

Primitives

Numbers

Python has integers and floats. Integers are simply whole numbers, like 314, 500, and 716. Floats, meanwhile, are fractional numbers like 3.14, 2.867, 76.88887. You can use the type method to check the value of an object. 1 >>> type (3) 2 <class 'int'> 3 >>> type (3.14) 4 <class 'float'> 5 >>> pi = 3. 6 >>> type (pi) 7 <class 'float'> In the last example, pi is the variable name, while 3.14 is the value.

You can use the basic mathematical operators: 1 >>> 3 + 3 2 6 3 >>> 3 - 3 4 0 5 >>> 3 / 3 6 1. 7 >>> 3 / 2

13 >>> num = 3 14 >>> num = num - 1 15 >>> print(num) 16 2 17 >>> num = num + 10 18 >>> print(num) 19 12 20 >>> num += 10 21 >>> print(num) 22 22 23 >>> num -= 12 24 >>> print(num) 25 10 26 >>> num *= 10 27 >>> num 28 100 There’s also a special operator called modulus, % , that returns the remainder after integer division. 1 >>> 10 % 3 2 1 One common use of modulus is determining if a number is divisible by another number. For example, we know that a number is even if it’s divided by 2 and the remainder is 0. 1 >>> 10 % 2 2 0 3 >>> 12 % 2 4 0 Finally, make sure to use parentheses to enforce precedence. 1 >>> (2 + 3) * 5 2 25 3 >>> 2 + 3 * 5 4 17

1 >>> str = 'WOAH!' 2 >>> str.lower() 3 'woah!' replace() allows you to replace any character with another character. 1 >>> str = 'rule' 2 >>> str.replace('r', 'm') 3 'mule' Finally, count() lets you know how many times a certain character appears in the string. 1 >>> number_list =['one', 'two', 'one', 'two', 'two'] 2 >>> number_list.count('two') 3 3 You can also format/create strings with the format () method. 1 >>> "{0} is a lot of {1}".format("Python", "fun!") 2 'Python is a lot of fun!'

Booleans

Boolean values are simply True or False. Check to see if a value is equal to another value with two equal signs. 1 >>> 10 == 10 2 True 3 >>> 10 == 11 4 False 5 >>> "jack" == "jack" 6 True 7 >>> "jack" == "jake" 8 False To check for inequality use !=. 1 >>> 10 != 10 2 False 3 >>> 10 != 11 4 True 5 >>> "jack" != "jack" 6 False 7 >>> "jack" != "jake" 8 True You can also test for > , < , >= , and <=. 1 >>> 10 > 10 2 False 3 >>> 10 < 11 4 True 5 >>> 10 >= 10 6 True 7 >>> 10 <= 11 8 True 9 >>> 10 <= 10 < 0 10 False 11 >>> 10 <= 10 < 11 12 True 13 >>> "jack" > "jack" 14 False

Chapter 3

Collections

Lists

Lists are containers for holding values. 1 >>> fruits = ['apple','lemon','orange','grape'] 2 >>> fruits 3 ['apple', 'lemon', 'orange', 'grape'] To access the elements in the list you can use their associated index value. Just remember that the list starts with 0 , not 1. 1 >>> fruits[2]‘’ 2 orange If the list is long and you need to count from the end you can do that as well. 1 >>> fruits[-2]‘’ 2 orange Now, sometimes lists can get long and you want to keep track of how many elements you have in your list. To find this, use the len () function. 1 >>> len(fruits) 2 4 Use append() to add a new element to the end of the list and pop() to remove an element from the end.

1 >>> fruits.append('blueberry') 2 >>> fruits 3 ['apple', 'lemon', 'orange', 'grape', 'blueberry'] 4 >>> fruits.append('tomato') 5 >>> fruits 6 ['apple', 'lemon', 'orange', 'grape', 'blueberry', 'tomato'] 7 >>> fruits.pop() 8 'tomato' 9 >>> fruits 10 ['apple', 'lemon', 'orange', 'grape', 'blueberry'] Check to see if a value exists using in the list. 1 >>> 'apple' in fruits 2 True 3 >>> 'tomato' in fruits 4 False

Chapter 4

Control Statements

IF Statements

The IF statement is used to check if a condition is true. Essentially, if the condition is true, the Python interpreter runs a block of statements called the if-block. If the statement is false, the interpreter skips the if block and processes another block of statements called the else-block. The else clause is optional. Let’s look at two quick examples. 1 >>> num = 20 2 >>> if num == 20: 3 ... print('the number is 20') 4 ... else : 5 ... print('the number is not 20') 6 ... 7 the number is 20 8 >>> num = 21 9 >>> if num == 20: 10 ... print('the number is 20') 11 ... else : 12 ... print('the number is not 20') 13 ... 14 the number is not 20 You can also add an elif clause to add another condition to check for. 1 >>> num = 21

2 >>> if num == 20: 3 ... print('the number is 20') 4 ... elif num > 20: 5 ... print('the number is greater than 20') 6 ... else : 7 ... print('the number is less than 20') 8 ... 9 the number is greater than 20

Chapter 5

Functions

Functions are blocks of reusable code that perform a single task. You use def to define (or create) a new function then you call a function by adding parameters to the function name. 1 >> def multiply(num1, num2): 2 ... return num1 * num 3 ... 4 >>> multiply(2, 2) 5 4 You can also set default values for parameters. 1 >>> def multiply(num1, num2=10): 2 ... return num1 * num 3 ... 4 >>> multiply(2) 5 20 Ready to learn more? Visit Real Python to learn Python and web development. Cheers!