Python Cheat Sheet

Python is a powerful, easy-to-learn programming language that has gained popularity due to its simplicity and versatility. This comprehensive guide will walk you through the fundamentals of Python, from installing it to using advanced features like classes, functions, and file handling.

What is Python?

Python is a high-level, general-purpose programming language that emphasizes code readability and simplicity. Created by Guido van Rossum in the late 1980s, Python is widely used for web development, data science, automation, and more. Its syntax is easy to learn, making it a great choice for beginners and professionals alike.

What Can Python Do?

Python can be used for a variety of tasks, such as:

• Web development: Using frameworks like Django or Flask.
• Data analysis and machine learning: Using libraries like pandas and scikit-learn.
• Automation: Writing scripts to automate tasks.
• Game development: Using libraries like Pygame.
• Desktop applications: Building graphical interfaces with libraries like Tkinter.

Why Python?

Python stands out because:

• Easy to read: Python's syntax is straightforward and looks similar to English.
• Versatile: Python can be used across many domains, from web development to machine learning.
• Large community: Python has a huge user base, offering plenty of tutorials, documentation, and libraries.

Python Syntax

Python syntax is simpler and more readable compared to languages like C++ or Java. It uses indentation instead of braces for defining code blocks, making the code clean and easy to follow.

Python Install

To get started with Python, you can download it from the official website python.org. Follow the installation instructions for your operating system (Windows, macOS, or Linux).

Python Version

After installation, you can check the installed Python version by running:

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python --version

The Python Command Line

You can run Python code directly in the command line or shell by typing python in your terminal or command prompt. This will open an interactive session where you can type Python commands and see immediate results.

Python Syntax

Python syntax is designed to be clean and easy to understand. For example, here’s a simple Python code to print "Hello, World!":

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print("Hello, World!")

Python Indentation

Python uses indentation to define blocks of code, rather than curly braces {} like other languages. Indentation must be consistent.

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if 5 > 2:
    print("Five is greater than two")

Python Comments

You can add comments in Python using the # symbol. Comments are ignored by the interpreter and are used to explain code:

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# This is a comment
print("Hello, World!")  # This will print a message

Python Variables

Variables in Python are used to store data values. You don’t need to declare the type of a variable; it is inferred from the value assigned to it.

Creating Variables

To create a variable, just assign a value to it:

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x = 5
y = "Hello"

Python Casting

You can explicitly convert (cast) variables from one type to another:

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x = int(5.5)  # Casts float to integer

Get the Type

You can check the type of a variable using the type() function:

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x = 5
print(type(x))  # Output: <class 'int'>

Single or Double Quotes?

Python allows both single (') and double (") quotes to define strings:

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x = 'Hello'
y = "World"

Case-Sensitive

Python is case-sensitive, which means myVar and myvar are considered different variables.

Variable Names

Variable names must start with a letter or an underscore, and can contain letters, digits, and underscores.

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my_var = 5

Multi-Word Variable Names

Camel Case

Each word after the first starts with a capital letter:

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myVariableName = "Camel Case"

Pascal Case

Each word starts with a capital letter:

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MyVariableName = "Pascal Case"

Snake Case

Words are separated by underscores:

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my_variable_name = "Snake Case"

Python Variables - Assign Multiple Values

You can assign multiple values to multiple variables in one line:

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x, y, z = 1, 2, 3

Print Output Variables

You can print variables by using the print() function:

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x = 5
print("The value of x is", x)

Global Variables

A global variable is declared outside of any function and can be accessed inside functions as well.

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x = "Global"

def my_function():
    print(x)

my_function()  # Output: Global

Python Data Types

Python has several built-in data types like integers, floats, strings, and more.

Built-in Data Types

Common Python data types include:

• int for integers
• float for floating-point numbers
• str for strings
• list for lists
• dict for dictionaries

Getting the Data Type

Use type() to find the data type of any variable:

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x = 10
print(type(x))  # Output: <class 'int'>

Setting the Data Type

You can explicitly set data types using casting:

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x = str(10)  # Converts integer to string

Python Numbers

Python supports three types of numbers: integers (int), floating-point numbers (float), and complex numbers (complex).

Python Casting

Casting allows you to change the type of a variable:

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x = int("10")  # Converts string to integer

Python Strings

Strings in Python are sequences of characters enclosed in single or double quotes:

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x = "Hello"

Python - Slicing Strings

You can slice strings to extract parts of them:

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x = "Hello"
print(x[1:4])  # Output: ell

Python - Modify Strings

Strings can be modified using various methods:

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x = "hello"
print(x.upper())  # Output: HELLO

Python - String Concatenation

You can concatenate strings using the + operator:

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x = "Hello"
y = "World"
print(x + " " + y)  # Output: Hello World

Python - Format Strings

You can format strings using the format() method:

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age = 30
txt = "I am {} years old"
print(txt.format(age))  # Output: I am 30 years old

Python - Escape Characters

Escape characters like \n (new line) and \\ (backslash) are used to include special characters in strings:

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txt = "Hello\nWorld"
print(txt)

Python - String Methods

Python offers various string methods, such as strip(), replace(), and split():

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txt = " Hello "
print(txt.strip())  # Output: "Hello"

Python Booleans

Booleans represent one of two values: True or False. They are often used in conditional statements.

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x = True

Python Operators

Operators in Python are used to perform operations on variables and values, such as arithmetic (+, -, *, /), comparison (==, >, <), and logical (and, or, not) operators.

Python Lists

Lists are used to store multiple items in a single variable. Lists are ordered, changeable, and allow duplicate values.

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my_list = [1, 2, 3]

Python - Access List Items

You can access list items using their index:

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print(my_list[0])  # Output: 1

Python - Change List Items

Lists are mutable, meaning their items can be changed:

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my_list[1] = 10

Python - Add List Items

You can add items to a list using append() or insert():

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my_list.append(4)

Python - Remove List Items

You can remove items from a list using remove() or pop():

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my_list.remove(10)

Python - Loop Lists

You can loop through a list using a for loop:

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for item in my_list:
    print(item)

Python - List Comprehension

List comprehension is a concise way to create lists:

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squares = [x**2 for x in range(10)]

Python - Sort Lists

You can sort a list using sort():

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my_list.sort()

Python - Copy Lists

Lists can be copied using the copy() method or slicing:

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new_list = my_list.copy()

Python - Join Lists

You can join lists using the + operator:

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list1 = [1, 2]
list2 = [3, 4]
result = list1 + list2

Python - List Methods

Python lists have many built-in methods like append(), extend(), index(), and count().

Python Tuples

Tuples are similar to lists but are immutable (unchangeable). They are defined by placing elements inside parentheses ():

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my_tuple = (1, 2, 3)

Python - Access Tuple Items

You can access tuple items by their index:

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print(my_tuple[0])

Python - Update Tuples

Tuples are immutable, but you can update them by converting them into a list and then back to a tuple.

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my_tuple = list(my_tuple)
my_tuple[0] = 4
my_tuple = tuple(my_tuple)

Python - Unpack Tuples

You can unpack a tuple into variables:

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x, y, z = my_tuple

Python - Loop Tuples

You can loop through tuples using a for loop:

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for item in my_tuple:
    print(item)

Python - Join Tuples

You can concatenate two tuples using the + operator:

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tuple1 = (1, 2)
tuple2 = (3, 4)
result = tuple1 + tuple2

Python - Tuple Methods

Tuples only have two built-in methods: count() and index().

Python Sets

Sets are unordered collections of unique elements. They are defined using curly braces {}:

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my_set = {1, 2, 3}

Python - Access Set Items

You cannot access set items by index because sets are unordered. Instead, you can loop through the set:

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for item in my_set:
    print(item)

Python - Add Set Items

You can add items to a set using add():

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my_set.add(4)

Python - Remove Set Items

Items can be removed using remove() or discard():

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my_set.remove(2)

Python - Loop Sets

You can loop through sets using a for loop:

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for item in my_set:
    print(item)

Python Dictionaries

Dictionaries store data in key-value pairs and are defined using curly braces {}:

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my_dict = {"name": "John", "age": 30}

Python - Access Dictionary Items

You can access dictionary items by referring to their key:

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print(my_dict["name"])

Python - Change Dictionary Items

Dictionary values can be updated by referencing their key:

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my_dict["age"] = 31

Python - Add Dictionary Items

You can add new key-value pairs to a dictionary:

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my_dict["address"] = "New York"

Python - Remove Dictionary Items

Remove an item from a dictionary using pop() or del:

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my_dict.pop("age")

Python - Loop Dictionaries

You can loop through dictionary keys, values, or both:

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for key, value in my_dict.items():
    print(key, value)

Python - Copy Dictionaries

Dictionaries can be copied using the copy() method:

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new_dict = my_dict.copy()

Python - Nested Dictionaries

You can nest dictionaries inside dictionaries:

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my_family = {
    "child1": {"name": "John", "age": 10},
    "child2": {"name": "Jane", "age": 8}
}

Python If...Else

Python uses if, elif, and else for conditional statements:

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if x > 10:
    print("Greater")
else:
    print("Smaller")

Python While Loops

while loops execute a block of code as long as a condition is true:

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i = 1
while i < 5:
    print(i)
    i += 1

Python For Loops

for loops iterate over a sequence (like a list or a range):

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for i in range(5):
    print(i)

Python Functions

Functions are defined using the def keyword:

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def my_function():
    print("Hello from a function!")

Python Lambda

A lambda function is a small anonymous function:

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x = lambda a: a + 10
print(x(5))

Python Arrays

Arrays in Python can be created using the array module or with lists for simpler cases:

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import array as arr
my_array = arr.array('i', [1, 2, 3])

Python Classes/Objects

Classes are used to create objects. A class is like a blueprint for objects:

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class MyClass:
    def __init__(self, name):
        self.name = name

Python Inheritance

Inheritance allows one class to inherit methods and properties from another:

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class Parent:
    def __init__(self, name):
        self.name = name

class Child(Parent):
    pass

Python Iterators

An iterator is an object that contains a countable number of values. Use iter() to create an iterator:

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my_iter = iter([1, 2, 3])
print(next(my_iter))

Python Polymorphism

Polymorphism allows functions to operate on different object types, like classes sharing the same method name:

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class Cat:
    def speak(self):
        return "Meow"

class Dog:
    def speak(self):
        return "Bark"

Python Scope

Variables have a scope that defines where they can be accessed. Python has local, global, and nonlocal scopes.

Python Modules

Modules allow you to break down large programs into smaller, manageable files. You can import them using import:

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import math

Python Dates

Python's datetime module allows you to work with dates and times:

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import datetime
print(datetime.datetime.now())

Python Math

The math module offers various mathematical functions:

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import math
print(math.sqrt(16))

Python JSON

Python provides methods for working with JSON data using the json module:

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import json
x = json.dumps({"name": "John", "age": 30})

Python RegEx

The re module in Python allows you to work with regular expressions:

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import re
pattern = r"\bword\b"
text = "This is a word in a sentence."
match = re.search(pattern, text)

Python PIP

PIP is a package manager for installing and managing Python libraries. You can install packages using pip install <package-name>.

Python Try...Except

try...except is used for error handling in Python:

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try:
    x = 1 / 0
except ZeroDivisionError:
    print("Cannot divide by zero!")

Python User Input

You can get user input using the input() function:

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name = input("Enter your name: ")
print("Hello, " + name)

Python String Formatting

Python allows string formatting to include variables in strings:

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name = "John"
age = 30
txt = "My name is {} and I am {} years old"
print(txt.format(name, age))

Python File Handling

Python allows you to work with files. You can open, read, write, and delete files.

Python File Handling

You can open files using the open() function:

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file = open("filename.txt", "r")

Python Read Files

Read files using read() or readline():

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file = open("filename.txt", "r")
print(file.read())

Python Write/Create Files

Use write() or append() to write to a file:

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file = open("filename.txt", "w")
file.write("Hello, World!")

Python Delete Files

You can delete a file using the os.remove() method from the os module:

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import os
os.remove("filename.txt")