← VCE Software Development U3O1

Docstrings for functions and classes

A triple-quoted string on the first line of a function or class body. It documents what the code does, its parameters and its return value. help() displays it.

Part of: VCE Software Development - U3O1 - Programming — Module 55, VCE Software Development - U4O1 - development and evaluation — Module 109, WA Computer Science ATAR Year 11 — Modular Programming — Module 43

Understanding Docstrings for functions and classes in VCE Software Development U3O1

What it does: The docstring lists the parameter (price) with its type and meaning, and states what the function returns. The triple-quoted string is the first line indented inside the class, so it documents the Student class. Python only treats a string as the docstring if it is the first statement in the body. help() will not find it elsewhere. help() displays the docstring of a function or class. Functions do not have a .comment attribute. A docstring needs three double quotes (or three single quotes) at each end. Two quotes make an empty string followed by errors.

Common mistakes: The docstring is closed with only two quotes. A triple-quoted docstring must end with three double quotes: """. The function header is missing its colon. It should be def greet(name): so the docstring can sit in the function body.

Class: _____________________  ·  Name: _____________________  ·  Date: _________

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This module's key terms are also in a printable crossword (Modules 48–71) and word search (Modules 10–71).

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Learning
Duo
Which line correctly starts a docstring for a function?
"""Return the area of a rectangle."""# Return the area of a rectangle.
Matches
def total(a, b): x = a + b """Add two numbers.""" return x
↓
The docstring is not on the first line of the function body, so Python does not treat it as the docstring and help() will not show it. Move it above x = a + b.
Applying
Meanings
Term
"""docstring"""
Meaning
A triple-quoted string on the first line of a function or class body. It documents what the code does, its parameters and its return value. help() displays it.
Practice
def average(a, b):
Testing
Gaps
help()
Check Answer
Spots
2 """Find rectangle area.
3 ""

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Quick Questions

1. Which line correctly starts a docstring for a function?
A. # Return the area of a rectangle.
B. """Return the area of a rectangle.""" ✓
A docstring is a triple-quoted string. A line starting with # is an ordinary comment, not a docstring.
2. Where must a function's docstring be placed?
A. On the first line inside the function body, straight after the def line ✓
B. At the very end of the function, after the return statement
Python only treats a string as the docstring if it is the first statement in the body. help() will not find it elsewhere.
3. Which function shows a docstring to the user in the Python shell?
A. print(calculate_total.comment)
B. help(calculate_total) ✓
help() displays the docstring of a function or class. Functions do not have a .comment attribute.
4. Which docstring best describes a parameter and the return value?
A. """Add tax to price. price: float. Returns the total as a float.""" ✓
B. """This function is very useful."""
A good docstring says what the function does, what each parameter is, and what it returns. Vague praise tells the reader nothing.
5. Which class definition has a docstring in the correct place?
A. class Student: """A student with a name and mark.""" ✓
B. class Student: name = "" """A student with a name and mark."""
For a class, the docstring must be the first statement inside the class body. Here the name assignment comes first, so the string is not a docstring.

Match the Pairs

"""Return the area of a rectangle.""" → A docstring: a triple-quoted string on the first line of a function body that says what the function does
help(calculate_area) → Displays the docstring of the calculate_area function
A docstring describes each parameter and the return value so other programmers know what to pass in and what comes back → """Add GST to a price. price: float, the cost before GST Returns: float, the cost with GST """
A class docstring goes on the first line inside the class body and describes what the class represents → class Student: """A student with a name and a mark."""
def total(a, b): x = a + b """Add two numbers.""" return x → The docstring is not on the first line of the function body, so Python does not treat it as the docstring and help() will not show it. Move it above x = a + b.
def total(a, b): "Add two numbers. Returns the sum." return a + b → Single quotes were used for a multi-line docstring, which causes a syntax error. Use triple quotes at the start and end.

Key Term

"""docstring"""
A triple-quoted string on the first line of a function or class body. It documents what the code does, its parameters and its return value. help() displays it.

Fill in the Blanks

1. class Student: """A student record.""" help(Student)
Hint: Type the name of the class whose docstring you want help() to display
2. def area(w, h): """Return the area.""" help(area)
Hint: Type the built-in function that displays a function's docstring
3. def area(w, h): """Return area. :param w: the width""" return w * h
Hint: Type the docstring tag used to describe a parameter
4. class Student: """A student record.""" count = 0
Hint: Type the keyword that starts a class definition, whose body begins with the docstring
5. def area(w, h): """Find area. :return: the area""" return w * h
Hint: Type the docstring tag used to describe the value a function gives back

Spot the Error

Question 1
A def area(w, h):
B """Find rectangle area.
C ""
D return w * h
Line C — The docstring is closed with only two quotes. A triple-quoted docstring must end with three double quotes: """.
Question 2
A def greet(name)
B """Print a greeting."""
C print("Hi " + name)
Line A — The function header is missing its colon. It should be def greet(name): so the docstring can sit in the function body.
Question 3
A define add(a, b):
B """Return a plus b."""
C return a + b
Line A — Python functions start with the keyword def, not define. The header should be def add(a, b):.
Question 4
A def square(n):
B # Return n squared.
C return n * n
D print(square(4))
Line B — A # comment is not a docstring, so help() cannot show it. The first line of the body should be a triple-quoted string: """Return n squared."""
Question 5
A def hello(name):
B """Print a greeting."""
C print("Hi " + name)
D help(hello())
Line D — help() needs the function object, not the result of calling it. hello() would run without its argument and fail. It should be help(hello).

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