Python Cheat Sheet

Tip: Python is dynamically typed — no need to declare type before assignment. Use type() or isinstance() to check types at runtime.

Note: Tuples can be used as dictionary keys (if they contain only immutable items), but lists cannot.

len(), type(), id(), str(), int(), float(), bool(), list(), tuple(), set(), dict(), sorted(), min(), max(), sum(), abs(), round(), isinstance(), hash(), callable(), chr(), ord(), bin(), oct(), hex(), dir(), help(), eval(), exec(), input(), print(), open(), range(), enumerate(), zip(), filter(), map(), reversed(), slice(), iter(), next(), any(), all(), delattr(), getattr(), setattr(), hasattr(), globals(), locals(), vars(), __import__(), compile(), memoryview(), property(), staticmethod(), classmethod(), super(), repr(), ascii(), format(), frozenset(), bytearray(), bytes()

Most used: len(), type(), range(), print(), input(), enumerate(), zip(), sorted(), map(), filter()

.upper(), .lower(), .strip(), .split(), .join(), .rstrip(), .lstrip(), .capitalize(), .title(), .replace(), .find(), .index(), .count(), .startswith(), .endswith(), .isdigit(), .isalpha(), .isalnum(), .islower(), .isupper(), .zfill(), .format(), .center(), .ljust(), .rjust(), .partition(), .rpartition(), .splitlines(), .swapcase(), .casefold(), .encode(), .expandtabs(), .isdecimal(), .isidentifier(), .isnumeric(), .isprintable(), .isspace(), .maketrans(), .translate(), .rsplit(), .rfind(), .rindex(), .format_map()

Most used: .upper(), .lower(), .strip(), .split(), .join(), .replace()

# List comprehension (fast, Pythonic)
squares = [x*x for x in range(5)]        # [0,1,4,9,16]
evens   = [x for x in range(10) if x%2==0]

# Sort with custom key
words = ["banana", "apple", "cherry"]
words.sort(key=len)

# Reverse sort
nums = [3, 1, 4, 1, 5]
nums.sort(reverse=True)                  # [5, 4, 3, 1, 1]

Most used: .append(), .extend(), .pop(), .sort(), .reverse(), .remove()

# Set operators
a | b   # union
a & b   # intersection
a - b   # difference
a ^ b   # symmetric difference

# Set comparisons
s1.issubset(s2)      # s1 ⊆ s2
s1.issuperset(s2)    # s1 ⊇ s2
s1.isdisjoint(s2)    # no common elements

# Dict comprehension
squares = {x: x*x for x in range(5)}

# Merge dicts (Python 3.9+)
merged = d1 | d2

# Integer & Float attributes
(10).numerator        # 10
(10).denominator      # 1

# Short-circuit behavior
x = 0 or "default"    # "default"
y = 5 and 10          # 10

# Special patterns
for i, val in enumerate(["a","b"]): print(i, val)
for n, a in zip(names, ages): print(n, a)
for i in reversed([1,2,3]): print(i)
for i in sorted([3,1,2]): print(i)

Precedence: ** → ~+- → */%// → +- → <<>> → & → ^ → | → comparisons → ==!= → assignments → is/is not → in/not in → not → and → or

# Special parameters
def func(a, b, /): ...        # positional only
def func(*, a, b): ...        # keyword only
def func(a, /, b, *, c): ...  # combined

# Function attributes
hello.__name__        # 'hello'
hello.__doc__         # docstring
add.__annotations__   # {'a': int, 'b': int, 'return': int}

Hierarchy: BaseException → Exception → ArithmeticError, LookupError, etc.

# Inheritance types
class A: pass
class B(A): pass                        # Single
class C(A): pass
class D(B, C): pass                     # Multiple
class E(D): pass                        # Multilevel
# Hierarchical: multiple children from one parent
# Hybrid: combination of above

# Special methods
__init__, __str__, __repr__, __len__,
__getitem__, __call__, __eq__, __slots__

# Class & static methods
@classmethod def create(cls): return cls()
@staticmethod def add(a, b): return a + b

# Property decorator
@property def radius(self): return self.__radius
@radius.setter def radius(self, value): self.__radius = value

Polymorphism types: Compile-time (overloading), Runtime (overriding), Duck typing, Operator overloading.

# Reading methods
f.read(), f.read(n), f.readline(), f.readlines()
for line in f:  # memory efficient

# File methods
f.seek(pos), f.tell(), f.close(), f.flush()

# OS operations
os.path.exists(), os.remove(), os.rename()
os.listdir(), os.mkdir(), os.getcwd()

# CSV / JSON / Pickle
csv.reader(f), csv.writer(f)
json.load(f), json.dump(data, f)
pickle.dump(data, f), pickle.load(f)

Advanced: iterators, generators, threading, multiprocessing, async/await, memory management, metaclasses, descriptors, context managers, decorators.

# Swap without temp
a, b = b, a

# Multiple return
x, y = get_coords()

# Default dict value
value = dict.get(key, default)

# Safe key check
if 'key' in dict: ...

# Short-circuit default
name = user_input or "Guest"

Common errors: NameError (undefined), UnboundLocalError, AttributeError, TypeError.

Naming: snake_case (vars), UPPER_CASE (constants), CamelCase (classes), _private, __dunder__.