Technically a function body needs at least one statement. A docstring is just an expression statement (a string), so a function definition with just a docstring is synctatically valid Python. I've seen people say multiline string literals are Python's version of multiline comments, but that's really just convention; it's a noop expression statement. Same as doing

    def foo():
        4

Which is also an expression statement as a function body, and also does nothing. Contrast to actually using a comment as a function body; comments aren't statements (nor expressions, so they can't be used as an expression statement):

    def foo():
        # this doesn't work

> IndentationError: expected an indented block after function definition on line 1

Of course, this doesn't really matter at all, and I get that it feels strange. I've just been thinking about grammars and syntax lately, and it's been interesting to now have the vocabulary and mental model to understand these unintuitive things :)