Module 3 · Reusable code and honest errors Lesson 7 of 15
Functions
A function is a named block you can call again. It takes inputs (parameters), does one job, and returns a result.
The concept
A function is a named block you can call again with different inputs. def average(numbers): starts the definition, the indented lines below are the body, and return hands a value back to whoever called it. Parameters are the inputs in brackets; arguments are the values you pass when you call it.
The reason to use them is not tidiness, it is change. If the average is calculated in four places and the rule changes, you edit four places and miss one. With average(marks) you edit one line. Name a function for the job — clean_name, total_by_category — and keep it to one job; a function that also prints, saves and emails is three functions wearing one coat.
def average(numbers):
if not numbers:
return 0
return sum(numbers) / len(numbers)
print(average([72, 88, 65])) # 75.0
print(average([])) # 0, no crashCommon mistakes
The two beginner mistakes are printing the result instead of returning it (so the caller gets None and cannot reuse it) and writing a function that depends on a variable defined outside itself. Both make the function work in the one place it was tested and fail everywhere else.
Worked example
def average(numbers): return sum(numbers) / len(numbers) — then average([72, 88, 65]) works anywhere, including in a program you write next month.
Your practice task
Turn three pieces of your own code into functions and call each one twice with different values.
Do this before moving on. Reading a solution teaches the shape of it; typing it teaches you.
Key takeaway
A function is a named block you can call again with different inputs.
Keep your place in this course
A free account saves which lessons you have finished and unlocks the certificate. The lesson itself stays free to read.
Create free account or sign inCourse complete
You finished every lesson.
Your certificate is ready, and the projects you built are the proof to attach to an application.
Almost there
15 lessons left of 15
Finish every lesson to unlock the certificate. Each one ends in a practice task — do them and the final project brief writes itself.