Understanding Different Types of Arguments in Python Function Definitions

Python functions are defined using the def keyword followed by the function name and a parenthesized list of parameters. The function body starts on the next indented line.

Default (optional) arguments provide a value in the function definition using the = assignment operator. If a caller supplies a value, it overrides the default; otherwise the default is used. Multiple default arguments are allowed, but they must appear after any non‑default parameters.

Example:

def add(a, b=5, c=10):
    return a + b + c

This function can be called in three ways:

print(add(3))        # Output: 18
print(add(3, 4))     # Output: 14
print(add(2, 3, 4))  # Output: 9

Keyword arguments allow callers to specify parameters by name, using the syntax kwarg=value . The order of arguments is irrelevant, but each keyword must correspond to a parameter defined in the function.

print(add(b=10, c=15, a=20))  # Output: 45
print(add(a=10))                # Output: 25

Positional arguments are passed based on their order in the call and must match the order of the function’s parameters. Positional arguments must appear before any keyword arguments.

print(add(10, 20, 30))          # Output: 60
print(add(10, c=30, b=20))          # Output: 60

Variable‑length arguments enable functions to accept an arbitrary number of arguments. A single asterisk * collects extra positional arguments into a tuple, while a double asterisk ** collects extra keyword arguments into a dictionary.

def add(*b):
    result = 0
    for i in b:
        result += i
    return result

print(add(1, 2, 3, 4, 5))  # Output: 15
print(add(10, 20))          # Output: 30

def fn(**a):
    for i in a.items():
        print(i)

fn(numbers=5, colors="blue", fruits="apple")
# Output:
# ('numbers', 5)
# ('colors', 'blue')
# ('fruits', 'apple')

Positional‑only and keyword‑only parameters are indicated with / and * in the parameter list. Parameters before / are positional‑only; parameters after * are keyword‑only.

def add(a, b, /, c, d):
    return a + b + c + d

print(add(3, 4, 5, 6))          # Output: 18
print(add(3, 4, c=1, d=2))      # Output: 10

Attempting to pass a positional‑only argument as a keyword, or a keyword‑only argument positionally, raises a TypeError .

Key takeaways include using default arguments wisely (avoiding mutable defaults), ordering arguments correctly, and choosing the appropriate parameter style to improve readability and prevent errors.