One exit is enough
A function returns at the first return it reaches. You rarely need more than one, and a single clear exit is easier to follow than several.
Running a function with a result
A function can hand back a value with return. That value is what the call becomes, so it can be stored or printed.
What you will see
3
None
About 1 min of reading
Quick check
Before this new lesson, one small question about the lesson you just finished. Recall it, do not look it up.
Try It
Return a value and use it.
1
Define a function that returns a doubled number.
def double(n):
return n * 2
Before you run it, say out loud what it will print. Then open the answer.
What it prints
no output yet
2
Call it inside a print.
print(double(6))
Before you run it, say out loud what it will print. Then open the answer.
What it prints
12
3
Store the result in a name.
x = double(7)
print(x)
Before you run it, say out loud what it will print. Then open the answer.
What it prints
14
4
Call a function that only prints and store it.
def show(n):
print(n)
y = show(3)
print(y)
Before you run it, say out loud what it will print. Then open the answer.
What it prints
3
None
show prints 3 but returns nothing, so y is None. Printing is not returning.
Put it in order
Before you write anything, put these lines back in the right order. Click a line to move it up one step. This support step is easier than writing code from scratch, and it is here to make the last example stick.
Data Flow
return gives a value back to the call, and that value can be stored or used in a larger expression.
1
double defined
↓
2
call double(5)
↓
3
return hands back 10
↓
4
print shows 10
Why this works
When you have a choice
Print versus return
print shows a value to the reader. return gives a value back to the program. A function that only prints cannot be used in a calculation, which is the mistake beginners make most.
Examples
Return sends a value back
def double(n):
return n * 2
print(double(5))return hands the value back to the call. double(5) becomes 10, and print shows it.
Store the result
def double(n):
return n * 2
answer = double(4)
print(answer)The returned value is stored in answer like any other value. A function is a way to make a value you do not write down.
No return means None
def shout(text):
print(text.upper())
result = shout("hi")
print(result)A function with no return prints but gives back None. Storing its result stores None, which is the difference between showing and returning.
Return stops the body
def first(n):
return n
print("never")
print(first(1))Code after a return never runs. return is the exit, so the print below it is dead.
Words this lesson uses
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.
← What a function is
Previous Lesson
What a function is