Course 2 · Python BasicsLesson 13 of 14Free to read

def, parameters and return · Python from Zero

How do you build your own reusable tools that take inputs and give back results?

Why you need it

With the robot you used def to name a group of commands. Real functions go further: they take parameters (inputs) and return a value (output), just like len("abc") takes a string and returns 3. Functions let you test pieces separately, reuse them, and keep programs readable.

The intuition

A function is a small machine: something goes in, something comes out.

def area(width, height):
    return width * height

print(area(3, 4))

width and height are parameters: names for the inputs.
In area(3, 4), 3 and 4 are arguments: the actual values.
return sends a value back to where the function was called and ends the function. area(3, 4) becomes 12.

print only shows a value on the screen; return hands it back so the program can keep using it. A function without return gives back None.

Definition

Parameter and argument

A parameter is a variable named in a function definition; an argument is the value passed to it when the function is called.

Return value

The return statement ends a function and sends a value back to the caller; without it the function returns None.

Examples

A function that returns

Try it yourself

Click through the trace to see the jumps.

def area(width, height):
    return width * height

total = area(3, 4) + area(2, 5)
print(total)

Step by step

L1Define area. Nothing runs yet.
L4Call area(3, 4): width = 3, height = 4.width = 3, height = 4
L2Return 12. area(3, 4) is replaced by 12.→ 12
L4Call area(2, 5) → 10. 12 + 10 = 22.total = 22
L5Print 22.22

print is not return

Try it yourself

shout prints but returns nothing, so result is None.

def shout(word):
    print(word.upper())

def louder(word):
    return word.upper() + "!"

result = shout("hi")
print(result)
print(louder("hi"))

Step by step

L7shout prints HI, but has no return...HI
L8...so result is None.None
L9louder returns "HI!" and print shows it.HI!

Common mistakes

Printing instead of returning

If other code needs the result, return it. A function that only prints can't be used in a calculation.

Code after return

return ends the function immediately; lines after it in the same block never run.

Quick quiz

1. What does this print?

def f(x):
    return x * 2

print(f(3) + f(1))

A
```
8
```
B
```
62
```
C
```
f(3) + f(1)
```

Show the answer

Answer: A

f(3) is 6 and f(1) is 2; 6 + 2 = 8.

2. What does this print?

def g():
    print("hi")

result = g()
print(result)

A
```
hi
None
```
B
```
hi
hi
```
C
```
None
```

Show the answer

Answer: A

g prints hi, but returns nothing, so result is None.

Practice

3 exercises in three steps: predict what a program prints, change a program to meet a new requirement, then write one from scratch. Your code runs in the browser and is checked the moment you submit.

PredictCopies of values
Type exactly what this program prints.
ModifyReturn, don't print
square prints its result, so square(4) + 1 fails. Change it to return n × n.
WriteEven helpers
Write two functions. is_even(n) returns True if n is even, otherwise False. count_even(nums) returns how many numbers in the list are even, using is_even.

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Terms in this lesson

Term	Chinese	Definition
Parameter	参数	A name in a function definition that receives a value when the function is called.
Argument	实参	The actual value passed to a function in a call.
return	return 返回	The statement that ends a function and sends a value back to the caller.
None	None 空值	The value a function gives back when it has no return statement.