PYTHON
CHEAT SHEET.

Data Types

Type Conversion

int(3.14)      # → 3
float(42)     # → 42.0
str(123)      # → '123'
bool(0)       # → False
list((1,2,3)) # → [1, 2, 3]
tuple([1,2,3])# → (1, 2, 3)
dict([('a',1),('b',2)]) # → {'a': 1, 'b': 2}
set([1,2,2])  # → {1, 2}

Control Flow

if / elif / else

if x > 0:
    print("positive")
elif x == 0:
    print("zero")
else:
    print("negative")

Loops

# for loop over iterable
for i in range(5):
    print(i)       # 0 1 2 3 4

# enumerate with index
for idx, val in enumerate(['a','b','c']):
    print(idx, val)

# while loop
n = 0
while n < 3:
    print(n)
    n += 1

# break / continue / else
for x in range(10):
    if x == 3:
        continue      # skip 3
    if x == 7:
        break         # exit loop
    print(x)
else:
    print("loop completed")  # only if no break

Comprehensions

# List
squares = [x**2 for x in range(10)]       # [0,1,4,9,...]
evens   = [x for x in range(20) if x % 2 == 0]

# Dict
square_dict = {x: x**2 for x in range(5)}

# Set
unique_squares = {x**2 for x in [-2, -1, 1, 2]}

# Generator (lazy)
gen = (x**2 for x in range(10))

📐 Functions

Definition & Arguments

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

# Positional + keyword
greet("Alice")                    # → "Hello, Alice!"
greet("Bob", greeting="Hi")       # → "Hi, Bob!"

# *args (tuple) / **kwargs (dict)
def log(*args, **kwargs):
    print(args, kwargs)
log(1,2,3, a=4, b=5)   # (1,2,3) {'a':4, 'b':5}

# Lambda
square = lambda x: x**2
list(map(square, [1,2,3]))  # → [1,4,9]

Decorators (syntax sugar)

def timer(func):
    import time
    def wrapper(*args, **kwargs):
        start = time.time()
        result = func(*args, **kwargs)
        print(f"took {time.time()-start:.3f}s")
        return result
    return wrapper

@timer
def slow_function():
    time.sleep(1)

Common Operations

Built‑in Functions (Highlights)

len(obj)        # length
type(obj)        # type of object
id(obj)         # memory address
isinstance(obj, type)  # check type
range(start, stop, step)
map(func, iterable)
filter(func, iterable)
zip(iter1, iter2) # pair elements
sorted(iterable, key=..., reverse=...)
sum(iterable), min(iterable), max(iterable)
any(iterable), all(iterable)  # boolean checks
enumerate(iterable) # (index, value)

File I/O

# Read
with open('file.txt', 'r') as f:
    content = f.read()          # entire file
    line = f.readline()         # one line
    lines = f.readlines()       # list of lines

# Write (overwrite)
with open('out.txt', 'w') as f:
    f.write("Hello\n")

# Append
with open('out.txt', 'a') as f:
    f.write("more\n")

Exception Handling

try:
    result = 10 / 0
except ZeroDivisionError as e:
    print("Error:", e)
except Exception as e:
    print("Generic:", e)
else:
    print("No exception")  # only if no exception
finally:
    print("Always runs")

Standard Library Gems

Pythonic Tips

• Unpacking: a, b = (1, 2) and *rest, last = [1,2,3,4]
• Swap variables: a, b = b, a
• Chained comparisons: 0 < x < 10
• Truthiness: Empty containers, 0, None are False
• Context managers: with – for files, locks, connections
• Type hints (optional): def greet(name: str) -> str:
• Virtual environments: python -m venv .venv then source .venv/bin/activate