13.3. EdX Python Style Guide

This section describes the requirements and conventions used to contribute Python programming to the edX platform.

13.3.1. Principles

Generally, do not edit files just to change the style. But do aim for this style with new or modified code.

See also Code Quality.

13.3.1.1. Write a Good repr() for Each Class

Each class should have a __repr__() method defined, so that calling repr() on an instance of the class returns something meaningful that distinguishes objects from each other to a human being. This is useful for debugging purposes.

13.3.2. Syntax and Organization

Follow PEP 8.

  • 4-space indents (no tabs)
  • Names like this: modules_and_packages, functions_and_methods, local_variables, GLOBALS, CONSTANTS, MultiWordClasses
  • Acronyms should count as one word: RobustHtmlParser, not RobustHTMLParser
  • Trailing commas are good: they prevent having to edit the last line in a list when adding a new last line. You can use them in lists, dicts, function calls, etc.
  • EXCEPT: we aren’t (yet) limiting code lines to 79 characters. Use 120 as a limit for code. Please use 79 chars as a limit for docstring lines though, so that the text remains readable.

13.3.2.1. Breaking Long Lines

Follow these guidelines:

  • Try to refactor the code to not need such long lines. This is often the best option, and is often overlooked for some reason. More, shorter lines are good.
  • If you need to break a function call over more than one line, put a newline after the open paren, and move the arguments to their own line. DO NOT indent everything to where the open paren is. This makes the code too indented, and makes different function calls near each other indented different amounts.

# NO NO NO!:

results = my_object.some_method(arg1,    # this is very
                                arg2,    # very ugly and makes
                                arg3,    # the code squished over on the right.
                                )

# YES:

results = my_object.some_method(
    arg1,
    arg2,
    arg3,
)

# OR:

results = my_object.some_method(
    arg1, arg2, arg3
)

Important points:

  • Do not over-indent to make things line up with punctuation on the first line.
  • Closing paren should be on a line by itself, indented the same as the first line.
  • The first line ends with the open paren.

13.3.2.2. Imports Order

PEP8 recommends a most-general to most-specific import order, which means this order:

  • Standard library imports
  • Third Party Library imports
  • Local imports

Alphabetize each group of imports, and use a single blank line to separate groups.

Note

Most Open edX repositories use the isort library, which will automatically order imports to follow PEP8.

13.3.3. Pylint Guidelines and Practices

  • For unused args, you can prefix the arguments with an underscore (_) to mark them as unused (as convention), and pylint will accept that.

  • Adding a TODO in one place requires you to make a pylint fix in another (just to force us to clean up more code).

  • No bare except clauses. except: should be except Exception:, which will prevent it from catching system-exiting exceptions, which we probably should not be doing anyway. If we need to, we can catch BaseException (There’s no point in catching BaseException, that includes the exceptions we didn’t want to catch with except: in the first place.) (ref: http://docs.python.org/2/library/exceptions.html#bltin-exceptions). Catching Exception, however, will still generate a Pylint warning (“W0703: catching too general exception.”) If you still feel that catching Exception is justified, silence the pylint warning with a pragma: # pylint: disable=broad- except.

  • Although we try to be vigilant and resolve all quality violations, some Pylint violations are just too challenging to resolve, so we opt to ignore them via use of a pragma. A pragma tells Pylint to ignore the violation in the given line. An example is:

    self.assertEquals(msg, form._errors['course_id'][0])  # pylint: disable=protected-access
    

    The pragma starts with a # two spaces after the end of the line. We prefer that you use the full name of the error (pylint: disable=unused- argument as opposed to pylint: disable=W0613), so that it is more clear what you are disabling in the line.

13.3.3.1. Classes Versus Dictionaries

It’s better to use a class or a namedtuple to pass around data that has a fixed shape than to use a dict. It makes it easier to debug (because there is a fixed, named set of attributes), and it helps prevent accidental errors of either setting new attributes into the dictionary (which might, for instance, get serialized unexpectedly), or might be typos.

13.3.4. Docstrings

Follow PEP 257.

  • Write docstrings for all modules, classes, and functions.
  • Always format docstrings using the multi-line convention, even if there’s only one line of content (see below).
  • Use three double-quotes for all docstrings.
  • Start with a one-line summary. If you can’t fit a summary onto one line, think harder, or refactor the code.
  • Write in Sphinx-friendly prose style. Put double backquotes around code names (variables, parameters, methods, etc).

The preferred style is so-called “Google Style” with readable headers for different sections, and all arguments and return values defined.

Note

There is one exception to the preferred style. REST APIs created using Django REST Framework (DRF) must use a hybrid format that is suitable both for DRF and ReadTheDocs. For more information see the edX REST API Conventions.

For additional information see these references.

Here’s how you write documentation in a mostly “Google Style” manner:

def func(arg1, arg2):
    """
    Summary line.

    Extended description of function.

    Arguments:
        arg1 (int): Description of arg1
        arg2 (str): Description of arg2

    Returns:
        bool: Description of return value
    """

Note

There are some exceptions:

  • The summary line is on the second line, including single-line comments (see below)
  • Use the full word “Arguments”.

Most of our code is written using an older style:

def calculate_grade(course, student):
    """
    Sum up the grade for a student in a particular course.

    Navigates the entire course, adding up the student's grades. Note that
    blah blah blah, and also beware that blah blah blah.

    `course` is an `EdxCourseThingy`. The student must be registered in the
    course, or a `NotRegistered` exception will be raised.

    `student` is an `EdxStudentThingy`.

    Returns a dict with two keys: `total` is a float, the student's total
    score, and `outof` is the maximum possible score.
    """

If you only have a single line in your docstring, first consider that this is almost certainly not enough documentation, and write some more. But if you do have just one line, format it in a similar way to a multi-line docstring:

def foo(a, b):
    """
    Computes the foo of a and b.
    """

Not like this:

def foo(a, b):
    """Computes the foo of a and b.""" # NO NO NO

We intentionally stray from PEP 257 in this case. The formatting inconsistency between single and multi-line docstrings can result in merge conflicts when upstream and downstream branches change the same docstring. See this GitHub comment for more context.