CS 111
Summer 1 2020

Python Coding Conventions and Standards

Python Coding Conventions and Standards

The following coding conventions and standards apply to all work in CS111. You should always follow these standards, and adherance will be rewarded in the grading.

Note: this is a summary of the rules found in PEP 8, the official style guide for Python code, written by the authors of the Python language.

Header Comments

You must begin each file with a brief header comment, which must include the following elements: The file name Your name, BU email, and the date you created the file * A brief description of the work contained in the file.

For example:

# File: hello_world.py
# Author: Aaron Stevens (azs@bu.edu), 3/24/2006
# Description: Our first Python program, in which we demonstrate 
# writing output to the console window.


Comments should help the reader of your code to understand it. They may help you to understand it if you need to rework your code at some later point. In the case of a program submitted for this course, they also serve to convince the grader that you grasp the what, how and why of your code (as opposed to having hacked it into working). In particular, please observe the following guidelines:

Documentation strings

Documentation strings (or docstrings) can be attached to any module (at the top of the .py file), function (after the def line), or class (after the class line). They are simple strings that are better than surrounding comments, since they can be reviewed using Python’s help() function, among other uses. (More information is available in PEP 257, Docstring Conventions.)

Use the triple-double quotation marks (""") to start and end a docstring. The docstring should begin on the line with the opening """, as follows:

def factorial(n):
    """Return the factorial of n."""

If the docstring can’t fit on one line, move the ending """ to its own line after the string and keep writing the docstring. Do not add an extra newline after the opening """. For example:

def replace(secret_word, display_word, letter):
    """Return a list of characters that results from taking the
    existing display word list and revealing any and all occurrences
    of the specified letter, as dictated by secret_word.

Another point to note: PEP 257 states that function docstrings should be a phrase ending with a period that describes the effect of the function as a command, rather than describing it (see the examples above).

Variable and Function Names

Choose your names carefully. As part of program documentation, names must clearly indicate both the kind of thing being named and its role in the program. Whenever possible, write short, obvious names for very local contexts (i.e., a for loop counter could be called i) and longer names for bigger contexts (examples to follow). Avoid cryptic abbreviations (e.g., use increment_event_count() rather than incevcnt()). Avoid choosing similar names for different things (e.g., index, indx, index2), or, on the other hand, similar names for very different things (example: if keys is a list of strings, then don’t use key for an integer).


Python uses indentation to manage the visiblity and scope of statements in your code. You must use the correct indentation to create code that is syntactically correct.

Spyder is a software program that makes it easier to write and run Python code. We will use Spyder extensively in this course when we write programs, since it provides text editor features like code highlighting and automatic indentation.

In an Spyder Editor window, pressing the tab key will cause Spyder to increase the indent level. By default, Spyder will insert the appropriate number of ASCII space characters to get the text cursor to the next level of indentation.

In other text editors, pressing the tab key might insert a tab character (a special character indicating an indent). This is not ideal — if you aren’t using Spyder, you should configure your editor to use spaces.


Using blank lines

While the horizontal position of code on a line (i.e., indentation) conveys important information to the Python interpreter, the interpreter ignores the vertical spacing of code in a file. For example, this function definition

def distance(yards):
    print("yards: " + str(yards))
    feet = yards * 3
    print("feet: " + str(feet))
    inches = feet * 12
    print("inches: " + str(inches))
    return inches

is equivalent to this one:

def distance(yards):
    print("yards: " + str(yards))

    feet = yards * 3
    print("feet: " + str(feet))

    inches = feet * 12
    print("inches: " + str(inches))

    return inches

However, the second definition* is more readable. Therefore putting blank lines between logical sections of code is encouraged, to increase readability.

Spaces in between elements on a line

The Python interpreter also ignores the spaces between binary operators like = or +. Therefore the expressions a=b and a = b are equivalent. You should get into the habit of using a single space between the operands in these expressions. However, a couple of notable exceptions to this whitespace rule can be found below:


Updated by Aaron Stevens, Fall 2019