Python basics cheatsheet — print, variables, lists, loops, functions

Print

Most common Python statement.

print("Hello, World!")

f-string

Inline expression in a string. The default since 3.6.

name = "Ada"
age = 36
print(f"{name} is {age}")

Type check

Find out what something is.

isinstance(x, (int, float))

Cast

Convert between types — always explicit.

n = int("42")
s = str(3.14)

Multiline string

Triple-quoted; preserves line breaks.

doc = """
Line 1
Line 2
"""

String split / join

Tokenise text and put it back.

"a,b,c".split(",")  # → ["a","b","c"]
",".join(["a","b","c"])

if / elif / else

Branch on a condition. No switch in Python.

if x > 0:
    ...
elif x == 0:
    ...
else:
    ...

Truthy / falsy

Empty, 0, None, "" are falsy. Everything else is truthy.

if data:
    process(data)

Ternary

One-line if/else for assignment.

sign = "+" if x >= 0 else "-"

List

Ordered, mutable, indexable.

xs = [1, 2, 3]
xs.append(4)

Tuple

Like list, but immutable. Good for fixed records.

point = (3, 4)

Set

Unique values, O(1) membership.

unique = set(xs)
3 in unique

Dict

Hash map from keys to values. Order-preserving since 3.7.

user = {"name": "Ada", "age": 36}

Slice

Substring/sublist with `[start:stop:step]`.

first3 = xs[:3]
reverse = xs[::-1]

List comp

Filtered / mapped list in one line.

evens = [x for x in xs if x % 2 == 0]

Dict comp

Build a dict in one expression.

squared = {x: x*x for x in range(5)}

for over list

Default loop.

for x in xs:
    process(x)

enumerate

Loop with an index counter.

for i, x in enumerate(xs, start=1):
    print(i, x)

zip

Walk two iterables in parallel.

for name, age in zip(names, ages):
    ...

while

Loop until a condition flips. Less common.

while not done:
    step()

break / continue

Exit / skip an iteration.

for x in xs:
    if x < 0:
        continue
    if x > 100:
        break

def

Define a function.

def greet(name: str) -> str:
    return f"Hello, {name}"

Defaults

Fallback values for parameters.

def repeat(x, n=2):
    return x * n

*args / **kwargs

Capture variable positional + keyword args.

def f(*args, **kwargs):
    ...

Lambda

Anonymous one-liner. Use for short callbacks.

add = lambda a, b: a + b

try / except

Handle exceptions; be specific.

try:
    n = int(s)
except ValueError:
    n = 0

raise

Throw a specific exception.

raise ValueError("must be positive")

with open

Read/write a file with auto-cleanup.

with open("data.txt") as f:
    text = f.read()