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The summary should be in the form of "Return <...>", and should have a trailing period. How about:
"Return whether the draft can be published."
We then also add some other indicators for our newly generated codebase docs. This docstring should have the form of:

"""Return [...]

[...]

Returns:
    bool:
    Whether the draft can be published.
"""


Always make sure all sentences are explanatory to those not familiar with the code and are coming into this as a new user, and always use correct grammar and punctuation.

Blank line between code blocks (if, for, etc.) and other code.
Same with the if statements below.

This function has always returned a boolean, and must continue to do so. Otherwise, we break other parts of the codebase, as well as third-party extensions.
We may also have various ways that a consumer would want to represent any error states here. A single string isn't going to really work.
Forms in Django have a concept of validation, where you call full_clean() on a form of model and it either returns or it raises a ValidationError for what it finds. I'd suggest a similar thing.
Let's define a validate_can_publish() method that does this work instead. Instead of building a list of strings, it can raise a django.core.exceptions.ValidationError when encountering things.
can_publish() can then call into this, and return a boolean depending on whether it raised an error.
That way, can_publish() can be a simple call, but anything needing more complex validation with error results can call validate_can_publish() instead.

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 undefined name 'validate_can_publish'

Hey Nathan, this is a solid start. However, you'll be wanting to make the changes to the Web API, specifically the ReviewRequestDraftResource (in webapi/resources/review_request_draft.py).
In that file, you'll want to look at update() which is responsible for setting values on a review request. The value taht ends up triggering this is public=1.
W

This should be self.validate_can_publish().

This needs translation.
Also, favour .append over += with lists.

This should really use .count instead of len((...).all())
e.g.
if draft.target_people.count() or draft.target_groups.count()

Same here.

It is more pythonic to do if draft.summary. However, if the summary is all whitespace, we probably don't want to allow it. Hence we should check draft.summary.strip().

Likewise, this should be if draft.description.trim().

You can just do if errors.

We already have a mechanism for reporting errors to the user. This checking should go in the API.
Have a look at lines 471--489 of reviewboard/webapi/resources/review_request.py for an example of data validation for review request drafts. Your logic will go there.

Feel free to ask my any questions about this review.

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