How to document a module constant in Python?

Unfortunately, variables (and constants) do not have docstrings. After all, the variable is just a name for an integer, and you wouldn't want to attach a docstring to the number 1 the way you would to a function or class object.

If you look at almost any module in the stdlib, like pickle, you will see that the only documentation they use is comments. And yes, that means that help(pickle) only shows this:

DATA
    APPEND = b'a'
    APPENDS = b'e'
    …

… completely ignoring the comments. If you want your docs to show up in the built-in help, you have to add them to the module's docstring, which is not exactly ideal.

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But Sphinx can do more than the built-in help can. You can configure it to extract the comments on the constants, or use autodata to do it semi-automatically. For example:

#: Indicates some unknown error.
API_ERROR = 1

Multiple #: lines before any assignment statement, or a single #: comment to the right of the statement, work effectively the same as docstrings on objects picked up by autodoc. Which includes handling inline rST, and auto-generating an rST header for the variable name; there's nothing extra you have to do to make that work.

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As a side note, you may want to consider using an enum instead of separate constants like this. If you're not using Python 3.4 (which you probably aren't yet…), there's a backport.enum package for 3.2+, or flufl.enum (which is not identical, but it is similar, as it was the main inspiration for the stdlib module) for 2.6+.

Enum instances (not flufl.enum, but the stdlib/backport version) can even have docstrings:

class MyErrors(enum.Enum):
    """Indicates some unknown error."""
    API_ERROR = 1

    """Indicates that the request was bad in some way."""
    BAD_REQUEST = 2

    """Indicates that the request is missing required parameters."""
    MISSING_PARAMS = 3

Although they unfortunately don't show up in help(MyErrors.MISSING_PARAMS), they are docstrings that Sphinx autodoc can pick up.

If you put a string after the variable, then sphinx will pick it up as the variable's documentation. I know it works because I do it all over the place. Like this:

FOO = 1
"""
Constant signifying foo.

Blah blah blah...
"""  # pylint: disable=W0105

The pylint directive tells pylint to avoid flagging the documentation as being a statement with no effect.

This is an older question, but I noted that a relevant answer was missing.

Or you can just include a description of the constants in the docstring of the module via .. py:data::. That way the documentation is also made available via the interactive help. Sphinx will render this nicely.

"""
Docstring for my module.

.. data:: API_ERROR

    Indicates some unknown error.

.. data:: BAD_REQUEST

    Indicates that the request was bad in some way.

.. data:: MISSING_PARAMS

    Indicates that the request is missing required parameters.
"""