Functions & Arguments
Defining reusable logic: parameters, defaults, *args/**kwargs, and return values.
Watch the lesson
4 videos on CodeWithMunnaX
A function is defined with def and returns None implicitly if there's no return.
python
def greet(name, greeting="Hello"):
return f"{greeting}, {name}!"
print(greet("CodeWithMunnaX")) # Hello, CodeWithMunnaX!
print(greet("CodeWithMunnaX", "Welcome")) # Welcome, CodeWithMunnaX!
print(greet(name="CodeWithMunnaX", greeting="Hey")) # keyword arguments — order stops matteringParameters without a default (name) are required; parameters with one (greeting="Hello") are optional. Once you give one parameter a default, every parameter after it must also have one.
Variable-length arguments
*args collects extra positional arguments into a tuple; **kwargs collects extra keyword arguments into a dict. This is how functions like print() accept any number of arguments.
python
def total_views(*counts, **labels):
print("counts:", counts) # a tuple
print("labels:", labels) # a dict
return sum(counts)
result = total_views(120, 340, 500, channel="CodeWithMunnaX")
print("total:", result)Why default mutable arguments can bite you
Never use a mutable default value like [] or {} — it's created once, when the function is defined, and shared across every call that doesn't pass its own value.
python
def add_video(title, playlist=[]): # BUG: shared list across calls
playlist.append(title)
return playlist
print(add_video("Intro")) # ['Intro']
print(add_video("Variables")) # ['Intro', 'Variables'] -- 'Intro' leaked in!Fix it with None as a sentinel and create the real default inside the function body:
python
def add_video(title, playlist=None):
if playlist is None:
playlist = []
playlist.append(title)
return playlistType hints (optional, but good practice)
Type hints don't change runtime behavior — Python doesn't enforce them — but they document intent and let editors/tools catch mistakes.
python
def greet(name: str, greeting: str = "Hello") -> str:
return f"{greeting}, {name}!"Key points to remember
- A function with no return statement returns None.
- *args collects extra positional arguments into a tuple; **kwargs collects extra keyword arguments into a dict.
- Never use a mutable object (list/dict) as a default argument — it's created once at definition time and shared across every call.
- Once one parameter has a default value, every parameter after it must have one too.
- Type hints (name: str, -> str) document intent but aren't enforced by Python at runtime.
Try it yourself
*args, **kwargs, and the mutable-default trap
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