Python PEP: blank line after function definition?

Read Docstring Conventions.

It says that even if the function is really obvious you have to write a one-line docstring. And it says that:

There's no blank line either before or after the docstring.

So I would code something like

def hello_function():
    """Return 'hello' string."""
    return 'hello'

Projects use different docstring conventions.

For example, the pandas docstring guide explicitly requires you to put triple quotes into a line of their own.

Docstrings must be defined with three double-quotes. No blank lines should be left before or after the docstring. The text starts in the next line after the opening quotes. The closing quotes have their own line (meaning that they are not at the end of the last sentence).

As pointed out by @moliware, the Docstring Conventions state, under One-line Docstrings:

There's no blank line either before or after the docstring.

HOWEVER, it also says (under Multi-line Docstrings):

Insert a blank line after all docstrings (one-line or multi-line) that document a class -- generally speaking, the class's methods are separated from each other by a single blank line, and the docstring needs to be offset from the first method by a blank line.

My interpretation of all this: blank lines should never precede any docstring, and should only follow a docstring when it is for a class.