Arguments

Python gives you a lot of flexibility in how arguments are passed to functions. Understanding the options makes library documentation much easier to read.

Positional arguments

Matched to parameters by position.

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

describe("Ravi", 30)

The number of arguments must match, or Python raises TypeError.

Keyword arguments

Matched by name, so order does not matter and the call documents itself.

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

describe(age=30, name="Ravi")

Positional arguments must come before keyword arguments in a call.

Default values

A parameter with a default is optional.

def power(base, exponent=2):
    return base ** exponent

print(power(5))
print(power(5, 3))
print(power(exponent=4, base=2))

Parameters with defaults must come after those without.

def add(item, items=None):
    if items is None:
        items = []
    items.append(item)
    return items

print(add(1))
print(add(2))

*args: any number of positional arguments

The extra arguments arrive as a tuple.

def total(*numbers):
    print(numbers)
    return sum(numbers)

print(total(1, 2, 3))
print(total())

**kwargs: any number of keyword arguments

They arrive as a dictionary.

def show(**details):
    for key, value in details.items():
        print(f"{key} = {value}")

show(name="Ira", city="Goa", age=25)

Combining them

The order in a definition is always: positional, *args, keyword only, **kwargs.

def report(title, *values, unit="", **meta):
    print(title, values, unit, meta)

report("Temps", 31, 33, 29, unit="C", city="Jaipur")

Unpacking arguments in a call

* spreads a list into positional arguments; ** spreads a dictionary into keyword arguments.

def area(w, h):
    return w * h

size = [4, 5]
print(area(*size))

opts = {"w": 2, "h": 3}
print(area(**opts))

Keyword only and positional only

A bare * in the definition forces everything after it to be passed by keyword. A / forces everything before it to be positional.

def connect(host, port, *, timeout=10):
    print(host, port, timeout)

connect("db.local", 5432, timeout=5)
# connect("db.local", 5432, 5) would be an error

This is why you see calls like sorted(items, key=..., reverse=True) and never sorted(items, f, True).

Practice

1. Write average(*nums) that returns the mean of any number of values and returns 0 for no values.
2. Write make_tag(name, **attrs) that returns an HTML tag string, so make_tag("a", href="/x", id="l") gives <a href="/x" id="l">.
3. Rewrite power so that the exponent must be given by keyword.