Why is imperative mood important for docstrings?

From the docstring of check_imperative_mood itself:

  """D401: First line should be in imperative mood: 'Do', not 'Does'.

   [Docstring] prescribes the function or method's effect as a command:
    ("Do this", "Return that"), not as a description; e.g. don't write
    "Returns the pathname ...".

(We'll ignore the irony that this docstring itself would fail the test.)

Why is it important? Because that's the explicit convention for Python docstrings, as detailed in PEP 257. There's nothing particularly special about it - it doesn't seem obvious to me that one of "Multiplies two integers and returns the product" and "Multiply two integers and return the product" is clearly better than the other. But it is explicitly specified in the documentation.

Consider the following example candidate for a docstring:

Make a row from a given bit string or with the given number of columns.

In English, this is a complete, grammatical sentence that begins with a capital letter and ends with a period. It's a sentence because it has a subject (implicitly, "you"), an object, "row," and a predicate (verb), "make."

Now consider an alternative:

Makes a row from a given bit string or with the given number of columns.

In English, this is ungrammatical. It is an adjectival phrase, therefore it should not begin with a capital letter and should not end with a period. Let's fix that problem:

makes a row from a given bit string or with the given number of columns

As an adjectival phrase, its antecedent --- its target --- is not explicit. Of course, we know it's the item being "docstringed," but, grammatically, it's dangling. That's a problem. Aesthetically, it's ugly, and that's another problem. So we fixed one problem and added two more.

For people who care about clear, anambiguous communication in grammatical English, the first proposal is clearly superior. I will guess that's the reason the Pythonistas chose the first proposal. In summary, "Docstrings shall be complete, grammatical sentences, specifically in the imperative mood."

It is not complete to just say it is about a convention or a consistency (otherwise, the follow-up question would be, "consistent with what?").

It is actually an explicit requirement from - albeit buried down deep in - the canonical PEP 257 Docstring Conventions. Quoted below:

def kos_root():
    """Return the pathname of the KOS root directory."""
    ...

Notes:

• The docstring is a phrase ending in a period. It prescribes the function or method's effect as a command ("Do this", "Return that"), not as a description; e.g. don't write "Returns the pathname ...".

That pydocstyle docstring was actually quoted from PEP 257 paragraph above.