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Catalog / Python Cheatsheet

Python Cheatsheet

A comprehensive cheat sheet for Python, covering essential syntax, data structures, and common libraries. Useful for beginners and experienced developers alike.

Basics and Syntax

Variables and Data Types

Variables

Variables are dynamically typed; no explicit declaration needed.

x = 5
name = "Alice"

Data Types

Common data types: int, float, str, bool, list, tuple, dict, set

age = 30        # int
height = 5.9     # float
message = "Hello" # str
is_active = True  # bool

Type Conversion

Convert between data types:

str(10)    # Convert int to str
int("20")   # Convert str to int
float(5)   # Convert int to float

Operators

Arithmetic

+ (addition), - (subtraction), * (multiplication), / (division), // (floor division), % (modulus), ** (exponentiation)

Comparison

== (equal), != (not equal), > (greater than), < (less than), >= (greater than or equal), <= (less than or equal)

Logical

and, or, not

Assignment

=, +=, -=, *=, /=, %=, //=

Control Flow

If/Elif/Else

if condition:
    # Execute if condition is true
elif another_condition:
    # Execute if another_condition is true
else:
    # Execute if no condition is true

For Loop

for item in iterable:
    # Execute for each item

While Loop

while condition:
    # Execute while condition is true

Break and Continue

break   # Exit loop
continue  # Skip current iteration

Data Structures

Lists

Creating Lists

 
my_list = [1, 2, 3, 'a', 'b']

Accessing Elements

 
my_list[0]  # First element (1)
my_list[-1] # Last element ('b')
my_list[1:3] # Slice [2, 3]

List Methods

 
my_list.append(4)   # Add element
my_list.insert(1, 'z') # Insert at index
my_list.remove(1)    # Remove element
my_list.pop(1)       # Remove and return element at index
len(my_list)        # List length

Tuples

Creating Tuples

 
my_tuple = (1, 2, 3, 'a', 'b')

Tuples are immutable.

Accessing Elements

 
my_tuple[0]  # First element (1)
my_tuple[1:3] # Slice (2, 3)

Dictionaries

Creating Dictionaries

 
my_dict = {'name': 'Alice', 'age': 30}

Accessing Elements

 
my_dict['name']  # 'Alice'
my_dict.get('age', 0) # 30 (or 0 if not found)

Dictionary Methods

 
my_dict['city'] = 'New York'  # Add key-value pair
del my_dict['age']           # Delete key
my_dict.keys()             # Get keys
my_dict.values()           # Get values
my_dict.items()            # Get key-value pairs

Sets

Creating Sets

 
my_set = {1, 2, 3, 4, 5}

Sets contain unique elements.

Set Methods

 
my_set.add(6)       # Add element
my_set.remove(1)    # Remove element
my_set.union({7, 8})  # Union of sets
my_set.intersection({3, 4}) # Intersection of sets

Functions and Modules

Functions

Defining Functions

def greet(name):
    return f"Hello, {name}!"

Calling Functions

message = greet("Bob")
print(message) # Output: Hello, Bob!

Lambda Functions

add = lambda x, y: x + y
result = add(5, 3) # result is 8

Default Arguments

def power(base, exponent=2):
    return base ** exponent

print(power(3))    # Output: 9
print(power(3, 3)) # Output: 27

Modules

Importing Modules

 
import math
print(math.sqrt(25)) # Output: 5.0

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

import os as operating_system
print(operating_system.getcwd())

Common Modules

math, datetime, os, random, json, re

File I/O and Error Handling

File I/O

Reading Files

with open('my_file.txt', 'r') as f:
    content = f.read()
    print(content)

Writing Files

with open('my_file.txt', 'w') as f:
    f.write('Hello, file!')

Appending to Files

with open('my_file.txt', 'a') as f:
    f.write('\nNew line of text.')

Error Handling

Try/Except Blocks

try:
    result = 10 / 0
except ZeroDivisionError as e:
    print(f"Error: {e}")
finally:
    print("This will always execute.")

Raising Exceptions

raise ValueError("Invalid input")
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