python_cheatsheet.md

Python Cheatsheet 🐍

1. Basic Syntax

I/O & Variables

name = input("Enter name: ")    # Read string
age = int(input("Enter age: ")) # Read & convert to int
print(f"Hello {name}")          # F-string formatting

Conditionals

if score >= 90:
    print("A")
elif score >= 80:
    print("B")
else:
    print("C")

Loops

# For loop (0 to 4)
for i in range(5):
    print(i)

# For loop with step (10 down to 2)
for i in range(10, 0, -2):
    print(i)

# While loop
while count > 0:
    count -= 1

Functions

def my_function(param1, param2=10):
    result = param1 + param2
    return result

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2. String (str)

Immutable sequence of characters.

Initialization:

s = "Hello World"
s = 'Single quotes work too'

Function	Description	Example
s[i]	Character at index	'abc'[0] # 'a'
s[start:end]	Substring (Slicing)	'hello'[0:2] # 'he'
s.split(d)	String to List	'a,b'.split(',') # ['a','b']
"".join(list)	List to String	"-".join(['a','b']) # 'a-b'
s.strip()	Remove whitespace	' hi '.strip() # 'hi'
s.lower()	Case conversion	'A'.lower() # 'a'
len(s)	Length of string	len("abc") # 3

---

3. List (list)

Dynamic array (Mutable).

Initialization:

arr = []                        # Empty list
arr = [1, 2, 3]                 # With elements
arr = [0] * 5                   # 5 elements, all 0
arr = [x for x in range(5)]     # List comprehension

Function	Description	Complexity	Example
append(v)	Add to end	O(1)	arr.append(10)
pop()	Remove from end	O(1)	arr.pop()
pop(i)	Remove at index	O(N)	arr.pop(0)
insert(i, v)	Insert at index	O(N)	arr.insert(1, 5)
len(arr)	Number of elements	O(1)	len(arr)
arr[i]	Access at index	O(1)	x = arr[0]
arr[s:e]	Slicing (sub-list)	O(K)	arr[1:4]
sort()	In-place sort	O(N log N)	arr.sort()

---

4. Tuple (tuple)

Immutable sequence (cannot be changed after creation).

Initialization:

t = (1, 2, 3)
t = 1, 2, 3                     # Parentheses are optional
single_t = (5,)                 # Note the trailing comma for single element

Function	Description	Complexity	Example
t[i]	Access at index	O(1)	x = t[0]
len(t)	Number of elements	O(1)	len(t)
count(v)	Count occurrences	O(N)	t.count(1)
index(v)	Find first index	O(N)	t.index(2)

---

5. Dictionary (dict)

Key-Value pairs (Hash Table).

Initialization:

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

Function	Description	Complexity	Example
d[k] = v	Insert or update	O(1)	d["id"] = 1
d.get(k, def)	Access with default	O(1)	d.get("id", 0)
k in d	Check existence	O(1)	if "id" in d:
del d[k]	Remove key	O(1)	del d["id"]
len(d)	Number of pairs	O(1)	len(d)
d.keys()	Get all keys	O(1)	d.keys()
d.items()	Get all [k, v] pairs	O(1)	d.items()

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6. Set (set)

Unique values (Mutable).

Initialization:

s = set()                       # Empty set (must use set(), {} is a dict)
s = {1, 2, 3}

Function	Description	Complexity	Example
add(v)	Add unique value	O(1)	s.add(5)
remove(v)	Remove value (Error if missing)	O(1)	s.remove(5)
discard(v)	Remove (No error if missing)	O(1)	s.discard(5)
v in s	Check existence	O(1)	if 5 in s:
len(s)	Number of elements	O(1)	len(s)
s1 | s2	Union (OR)	O(N+M)	s1 | s2
s1 & s2	Intersection (AND)	O(min(N, M))	s1 & s2

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7. Math & Type Conversions

Function	Description	Example
int(s)	String/Float to Int	int("123")
str(n)	Number to String	str(123)
list(s)	String/Tuple to List	list("abc") # ['a','b','c']
abs(x)	Absolute value	abs(-5)
max(a, b)	Get maximum	max(10, 20)
min(a, b)	Get minimum	min(10, 20)
sum(list)	Sum of all elements	sum([1,2,3]) # 6