CODESARRAY

Lesson 8, by Codesarray, , 4 min read

Python functions: def, return, default arguments and *args

Write your own Python functions with def, return a result, use default and keyword arguments, understand local variables, and collect any number of arguments with *args.

A Python function is a named block of code you can run again and again. By the end of this page you will write your own with def, return a result with return, give parameters default values, pass arguments by name, know why a variable made inside a function stays inside it, and collect any number of arguments with *args.

def, call and return

You write def, a name, brackets and a colon. The indented lines are the body. def area(width, height=1): starts a function called area. Calling it, area(3, 4), runs the body with width as 3 and height as 4. The return line hands the result back, so you can print it or store it. A function with no return gives back None.

Parameters and defaults

Parameters are the names in the brackets. A default, like height=1, is used when you leave that argument out, so area(5) uses a height of 1 and gives 5. You can also pass arguments by name: area(height=2, width=6). Names make a call easy to read, and the order stops mattering.

Local variables

A variable made inside a function is local. In greet, message exists only while the function runs. Using message after the call raises NameError: name 'message' is not defined. A function sends values out through return, not by leaking names.

*args and **kwargs

Sometimes you do not know how many arguments will come. A star before a name, *nums, collects them all into a tuple. total(1, 2, 3) puts (1, 2, 3) into nums, and sum(nums) adds them up. A double star, **kwargs, does the same for named arguments, collecting them into a dictionary.

def area(width, height=1):
    """Return width times height."""
    return width * height

print(area(3, 4))
print(area(5))
print(area(height=2, width=6))

def total(*nums):
    return sum(nums)

print(total(1, 2, 3))

def greet(name):
    message = "Hi " + name
    return message

print(greet("Ada"))
12
5
12
6
Hi Ada

Line 1 defines area with a default height of 1. Line 2 is the docstring, a sentence saying what the function does. Line 3 returns width * height. Line 5 calls it with 3 and 4 and prints 12. Line 6 leaves out the height, so it prints 5. Line 7 passes both by name and prints 12. Line 9 defines total with *nums, line 10 returns the sum, and line 12 prints 6. Lines 14 to 16 define greet, which builds a local message and returns it, so line 18 prints Hi Ada.

What you learned

  • def defines a function, and return sends a value back; with no return you get None.
  • A default value fills in an argument you leave out, and you can pass arguments by name.
  • Variables made inside a function are local to it.
  • *args collects any number of arguments into a tuple, and **kwargs collects named ones into a dictionary.
  • A docstring on the first line of the body says what the function does.

Common questions

What is the difference between a parameter and an argument? A parameter is the name in the definition, like width. An argument is the value you pass when you call it, like 3.

**Does print return a value?** No. print shows text and returns None. If you want a result you can reuse, return it instead of printing it.

Can a function return more than one value? Yes. return 1, 2 returns a tuple, and you can unpack it: a, b = f().

Can a default come before a parameter without one? No. def area(height=1, width) is a SyntaxError. Put parameters without defaults first.

**What do the names args and kwargs mean?** They are only a habit. The stars do the work, so *nums behaves the same as *args.

This lesson follows on from Python dictionaries. Next in the series: comprehensions.

This article goes with the video Functions. Watch it on YouTube.