Function Arguments

Pass values to functions safely with positional, keyword, default, variable, and named arguments.

12 min read 10 quiz questions

Ways to pass arguments

Arguments can be matched by their position or by a parameter name. A positional argument must appear in the same order as the parameters.

A keyword argument uses name=value, so its order is clear.

def introduce(name, age):
    print(name, age)

introduce("Mina", 12)
introduce(age=12, name="Mina")

Example 1: Positional and keyword arguments

def introduce(name, age):
    print(f"{name} is {age}.")

introduce("Mina", 12)
introduce(age=12, name="Mina")

The first call is positional: the first value goes to name and the second goes to age. The second call names both parameters, so Python can match them in either order.

Default parameter values

Example 2: Default values

def welcome(name, greeting="Hello"):
    print(f"{greeting}, {name}!")

welcome("Kai")
welcome("Kai", "Welcome back")

greeting has a default value. Python uses it when you do not provide that argument, and uses your value when you do.

Avoid mutable default values

Example 3: A shared default list

def add_item(item, items=[]):
    items.append(item)
    return items

print(add_item("tea"))
print(add_item("coffee"))

Example 4: Collect extra arguments

def show_order(item, *extras, **details):
    print(item)
    print(extras)
    print(details)

show_order("Book", "Bookmark", "Pen", priority="high", gift=True)

*extras collects extra positional arguments in a tuple. **details collects extra keyword arguments in a dictionary.

Key points

Positional arguments match parameter order.
Keyword arguments name the parameter they fill.
Defaults make an argument optional.
Do not use a list or dict as a default value.
*args and **kwargs collect extra arguments.

Worked example

A cinema booking: seat number is required, but the snack bundle is optional and can be passed by keyword for clarity.

Example

def book(name, show, snack=None):
    line = f"{name}: {show}"
    if snack:
        line += f" + {snack}"
    return line

print(book("Rafi", "7pm", snack="popcorn"))

How it works, step by step

Positional arguments fill parameters in order: name then show.
snack=... was passed by KEYWORD, so its label makes the call self-documenting.
The default None lets callers skip it.

References & Further Reading

Related entries from PyLabs's own reference library.

zip() (Pair items from two or more iterables.)
map() (Apply a function to each item in one or more iterables.)

Your notes

Practice exercise

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Write a function named repeat with parameters word and times=2. Print word repeated times times. Call repeat("go", 3).

Expected output: gogogo

# Write your code below

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Function Arguments quiz

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