Syntax Basics

Python's syntax is famously compact, but a few of its rules surprise newcomers from brace-based languages. This chapter walks through variables, indentation, functions and their flexible argument handling, control flow, and f-strings, the everyday way to build text.

Variables and names

There is no declaration keyword: a variable comes into existence the first time you assign to it. Think of a variable as a label attached to a value. The value has a type; the label does not, so the same name can later point at something of a different type.

count = 3
name = "Python"
pi = 3.14159
is_ready = True

# assign several names at once, then swap them
x, y = 1, 2
x, y = y, x

# convention: snake_case for variables and functions, UPPER_CASE for constants
MAX_RETRIES = 5

Most projects follow the PEP 8 style guide: snake_case for functions and variables, PascalCase for classes, and UPPER_CASE for module-level constants.

Indentation is the block structure

Instead of braces, Python uses indentation. Statements that open a block, such as if, for, def and class, end with a colon, and every line indented beneath them at the same depth belongs to that block. Four spaces per level is the standard. Mixing tabs and spaces causes errors, so configure your editor to insert spaces. Comments start with #, and a triple-quoted string placed directly under a function or class becomes its docstring, which help() and editors display as documentation.

Functions

Define functions with def and hand back a result with return; a function that never returns explicitly gives back None. Parameters can have defaults, and callers may pass arguments by name, which makes call sites self-documenting.

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


greet("Mina")                      # 'Hello, Mina!'
greet("Mina", greeting="Hi")       # 'Hi, Mina!'
greet(greeting="Hey", name="Joon") # 'Hey, Joon!'

When a function should accept a variable number of arguments, use *args to collect extra positional arguments into a tuple and **kwargs to collect extra keyword arguments into a dict. A bare * in the parameter list makes everything after it keyword-only.

def log(message, *args, level="INFO", **kwargs):
    print(level, message, args, kwargs)


log("start", 1, 2, user="kim")
# INFO start (1, 2) {'user': 'kim'}


def connect(host, *, timeout=10):
    ...


connect("db.local", timeout=5)  # timeout can only be passed by name