Function Arguments Examples

Short, runnable programs that show python function arguments in action. Read the explanation, press Run, then change the code and try your own ideas. New to the topic? Start with the Python Function Arguments lesson.

1One function, four ways to call it

Positional, keyword and mixed calls all reach the same parameters. Keyword arguments let you skip an optional parameter in the middle.

One function, four ways to call it

def book_room(guest, nights, breakfast=False, floor=1):
    meal = "with breakfast" if breakfast else "room only"
    return f"{guest}: {nights} nights, {meal}, floor {floor}"

print(book_room("Asha", 3))
print(book_room("Asha", 3, True))
print(book_room("Asha", 3, floor=4))
print(book_room(nights=2, guest="Ben", breakfast=True))

Expected outputAsha: 3 nights, room only, floor 1
Asha: 3 nights, with breakfast, floor 1
Asha: 3 nights, room only, floor 4
Ben: 2 nights, with breakfast, floor 1

2Keyword-only flags and the error they prevent

dry_run can only be passed by name, so nobody can write a mysterious bare True at the end of the call. The try block prints the message Python raises when someone tries anyway.

Keyword-only flags and the error they prevent

def delete_files(folder, *, dry_run=True):
    mode = "Would delete" if dry_run else "Deleting"
    return f"{mode} everything in {folder}"

print(delete_files("/tmp/cache"))
print(delete_files("/tmp/cache", dry_run=False))

try:
    delete_files("/tmp/cache", False)
except TypeError as error:
    print("Error:", error)

Expected outputWould delete everything in /tmp/cache
Deleting everything in /tmp/cache
Error: delete_files() takes 1 positional argument but 2 were given

3Positional-only parameters

clamp(value, /, low, high) keeps a value inside a range. The first argument is just "the value", so it is positional-only, and the range limits can be given by name.

Positional-only parameters

def clamp(value, /, low=0, high=100):
    return max(low, min(value, high))

print(clamp(140))
print(clamp(-8))
print(clamp(15, low=20, high=30))
print(clamp(25, high=10))

Expected output100
0
20
10

4The None default in an audit log

history defaults to None, so each call that does not pass a list gets its own. The caller can still pass one list to collect several messages together.

The None default in an audit log

def log(message, history=None):
    if history is None:
        history = []
    history.append(message.upper())
    return history

print(log("started"))
print(log("finished"))

trail = []
log("login", trail)
log("logout", trail)
print(trail)

Expected output['STARTED']
['FINISHED']
['LOGIN', 'LOGOUT']

5Change in place or return a new list

curve_in_place changes the caller's list, while curved builds a new one. Both give the same marks, but only one touches the original.

Change in place or return a new list

def curve_in_place(marks, bonus):
    for i in range(len(marks)):
        marks[i] = min(marks[i] + bonus, 100)

def curved(marks, bonus):
    return [min(m + bonus, 100) for m in marks]

exam = [58, 95, 70]
print(curved(exam, 10), exam)
curve_in_place(exam, 10)
print(exam)

Expected output[68, 100, 80] [58, 95, 70]
[68, 100, 80]

6Returning several values from one function

stats returns a tuple of three numbers. The caller can unpack all three, or use _ for a value it does not need.

Returning several values from one function

def stats(scores):
    low = min(scores)
    high = max(scores)
    average = sum(scores) / len(scores)
    return low, high, average

low, high, average = stats([72, 90, 85, 61])
print(low, high, average)

_, best, _ = stats([40, 55, 48])
print("Best:", best)

Expected output61 90 77.0
Best: 55

Practise next

Python Function Arguments exercises → Back to the lesson

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