Python Cheat Sheet

A Cheat Sheet 📜 to revise Python syntax in less time. Particularly useful for solving Data Structure and Algorithmic problems or a quick overview before an interview.

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Table of Contents

• Basics
• Data Structures
  • Lists
  • Dictionary
  • Counter
  • Deque
  • Heapq
  • Sets
  • Tuples
  • Strings
• Built-in Functions
• Advanced Topics
• Best Practices
• Tips & Gotchas

Basics

Data Types

Operator Precedence

Data Structures

Lists

Time Complexities:

nums = [1,2,3]

# Common Operations
nums.index(1)      # Find index
nums.append(1)     # Add to end
nums.insert(0,10)  # Add 10 from left (at index 0 which is start)
nums.remove(3)     # Remove value
nums.pop()         # Remove & return last element
nums.sort()        # In-place sort (TimSort: O(n log n))
nums.reverse()     # In-place reverse
nums.copy()        # Return shallow copy

# List Slicing
nums[start:stop:step]  # Generic slice syntax
nums[-1]    # Last item
nums[::-1]  # Reverse list
nums[1:]    # Everything after index 1
nums[:3]    # First three elements

Dictionary

Time Complexities:

d = {'a':1, 'b':2}

# Essential Operations
d.get('key', default)     # Safe access with default
d.setdefault('key', 0)    # Set if missing
d.items()                 # Key-value pairs
d.keys()                  # Just keys
d.values()               # Just values
d.pop(key)              # Remove and return value
d.update({key: value})  # Batch update

# Advanced Usage
from collections import defaultdict
d = defaultdict(list)     # Auto-initialize missing keys
d = defaultdict(int)      # Useful for counting

Counter

from collections import Counter

# Initialize
c = Counter(['a','a','b'])    # From iterable
c = Counter("hello")          # From string

# Operations
c.most_common(2)      # Top 2 frequent elements
c['a'] += 1           # Increment count
c.update("more")      # Add counts from iterable
c.total()             # Sum of all counts
c.elements()          # Returns an iterator over elements repeating 
c.subtract("ole")     # Subtract counts from iterable (can go negative)

Deque

Time Complexities:

from collections import deque

# Perfect for BFS - O(1) operations on both ends
d = deque()
d.append(1)          # Add right
d.appendleft(2)      # Add left
d.pop()              # Remove right
d.popleft()          # Remove left
d.extend([1,2,3])    # Extend right
d.extendleft([1,2,3])# Extend left
d.rotate(n)          # Rotate n steps right (negative for left)

Heapq

import heapq

# MinHeap Operations - All O(log n) except heapify
nums = [3,1,4,1,5]
heapq.heapify(nums)          # Convert to heap in-place: O(n)
heapq.heappush(nums, 2)      # Add element: O(log n)
smallest = heapq.heappop(nums)  # Remove smallest: O(log n)

# MaxHeap Trick: Multiply by -1
nums = [-x for x in nums]    # Convert to maxheap: O(n)
heapq.heapify(nums)          # O(n)
largest = -heapq.heappop(nums)  # Get largest: O(log n)

# Advanced Operations
k_largest = heapq.nlargest(k, nums)    # O(n * log k)
k_smallest = heapq.nsmallest(k, nums)  # O(n * log k)

# Custom Priority Queue
heap = []
heapq.heappush(heap, (priority, item))  # Sort by priority

Sets

Time Complexities:

s = {1,2,3}

# Common Operations
s.add(4)             # Add element
s.remove(4)          # Remove (raises error if missing)
s.discard(4)         # Remove (no error if missing)
s.pop()              # Remove and return arbitrary element

# Set Operations
a.union(b)           # Elements in a OR b
a.intersection(b)    # Elements in a AND b
a.difference(b)      # Elements in a but NOT in b
a.symmetric_difference(b)  # Elements in a OR b but NOT both
a.issubset(b)        # True if all elements of a are in b
a.issuperset(b)      # True if all elements of b are in a

Tuples

# Tuples are immutable lists
t = (1, 2, 3, 1)

# Essential Operations
t.count(1)      # Count occurrences of value
t.index(2)      # Find first index of value

# Useful Patterns
x, y = (1, 2)   # Tuple unpacking
coords = [(1,2), (3,4)]  # Tuple in collections

Strings

s = "hello world"

# Essential Methods
s.split()            # Split on whitespace
s.split(',')         # Split on comma
s.strip()            # Remove leading/trailing whitespace
s.lower()            # Convert to lowercase
s.upper()            # Convert to uppercase
s.isalnum()          # Check if alphanumeric
s.isalpha()          # Check if alphabetic
s.isdigit()          # Check if all digits
s.find('sub')        # Index of substring (-1 if not found)
s.count('sub')       # Count occurrences
s.replace('old', 'new')  # Replace all occurrences

# ASCII Conversion
ord('a')             # Char to ASCII (97)
chr(97)              # ASCII to char ('a')

# Join Lists
''.join(['a','b'])   # Concatenate list elements

Built-in Functions

# Iteration Helpers
enumerate(lst)        # Index + value pairs
zip(lst1, lst2)      # Parallel iteration
map(fn, lst)         # Apply function to all elements
filter(fn, lst)      # Keep elements where fn returns True
any(lst)             # True if any element is True
all(lst)             # True if all elements are True

# Binary Search (import bisect)
bisect.bisect(lst, x)     # Find insertion point
bisect.bisect_left(lst, x)# Find leftmost insertion point
bisect.insort(lst, x)     # Insert maintaining sort

# Type Conversion
int('42')            # String to int
str(42)              # Int to string
list('abc')          # String to list
''.join(['a','b'])   # List to string
set([1,2,2])         # List to set

# Math
abs(-5)              # Absolute value
pow(2, 3)            # Power
round(3.14159, 2)    # Round to decimals

Advanced Topics

Custom Sorting with cmp_to_key

from functools import cmp_to_key

def compare(item1, item2):
    # Return -1: item1 comes first
    # Return 1:  item2 comes first
    # Return 0:  items are equal
    if item1 < item2:
        return -1
    elif item1 > item2:
        return 1
    return 0

# Sort using custom comparison
sorted_list = sorted(items, key=cmp_to_key(compare))

Taking Multiple Inputs

# Basic multiple input
x, y = input("Enter two values: ").split()

# Multiple integers
x, y = map(int, input("Enter two numbers: ").split())

# List of integers
nums = list(map(int, input("Enter numbers: ").split()))

# Multiple inputs with custom separator
values = input("Enter comma-separated values: ").split(',')

# List comprehension method
x, y = [int(x) for x in input("Enter two numbers: ").split()]

Math Module Essentials

import math

# Constants
math.pi       # 3.141592653589793
math.e        # 2.718281828459045

# Common Functions
math.ceil(2.3)        # 3 - Smallest integer greater than x
math.floor(2.3)       # 2 - Largest integer less than x
math.gcd(a, b)        # Greatest common divisor
math.log(x, base)     # Logarithm with specified base
math.sqrt(x)          # Square root
math.pow(x, y)        # x^y (prefer x ** y for integers)

# Trigonometry
math.degrees(rad)     # Convert radians to degrees
math.radians(deg)     # Convert degrees to radians

Important Python Integer Operations

# Binary representation
bin(10)              # '0b1010'
format(10, 'b')      # '1010' (without prefix)

# Division and Modulo
divmod(10, 3)        # (3, 1) - returns (quotient, remainder)

# Negative number handling
x = -3
y = 2
print(x // y)        # -2 (floor division)
print(int(x/y))      # -1 (preferred for negative numbers)
print(x % y)         # 1 (Python's modulo with negative numbers)

Best Practices

Documentation

def binary_search(arr, target):
    """
    Find target in sorted array using binary search.
    Args:
        arr: Sorted list of numbers
        target: Number to find
    Returns:
        Index of target or -1 if not found
    """
    pass

Testing

# Use assertions for edge cases
assert binary_search([], 1) == -1, "Empty array should return -1"
assert binary_search([1], 1) == 0, "Single element array should work"

Tips & Gotchas

1. Integer Division:

# Use int() for consistent negative number handling
print(-3//2)        # Returns -2
print(int(-3/2))    # Returns -1 (usually desired)

2. Default Dictionaries:

# Prefer defaultdict for frequency counting
from collections import defaultdict
freq = defaultdict(int)
for x in lst:
    freq[x] += 1    # No KeyError if x is new

3. Heap Priority:

# For custom priority in heapq, use tuples
heap = []
heapq.heappush(heap, (priority, item))

4. List Comprehension:

# Often clearer than map/filter
squares = [x*x for x in range(10) if x % 2 == 0]

5. String Building:

# Use join() in