Python’s empty function does not require a pass statement?

Question

I stumbled upon an interesting and unexpected feature of Python:

def fun():
    """Foo’s docstring"""

is a valid function? According to PEP 257, “A docstring is a string literal that occurs as the first statement in a module, function, class, or method definition” meaning that the docstring itself is considered a statement?

I would have expected that at least a pass statement is required here. The above example contradicts the Python Zen of “explicit is better than implicit” as pass states an explicit intent, and a docstring does not.

Can anybody shed some light on the intent?

What's unclear? As that quote tells you, the string is the first statement, only one statement is required by [the grammar](https://docs.python.org/3/reference/grammar.html) for the function to be syntactically valid. That's *not* an empty function, in other words. — jonrsharpe, Sep 10 '17 at 22:16
The Zen of Python doesn't tell you about the grammar. If you think a `pass` statement after the docstring makes the code clearer in intent then by all means add one, but it isn't *required* for the code to run. It would only complicate the parser to have the requirement that `suite` contains at least one statement, except in the case that it's in a `funcdef` and that first statement is a string literal, in which case it must contain at least two. *"Simple is better than complex."* — jonrsharpe, Sep 10 '17 at 22:20
Yes, the Python parser does allow you to do some things that violate the Zen of Python, but so what? What exactly is your question? — jwodder, Sep 10 '17 at 22:25
@jonrsharpe: I understand that. I just find it odd/surprising that a _string literal_ is also considered a _statement_ when placed in a certain location, and as a consequence allows an empty function like the one above that, to me, conflicts with other required uses of `pass`. — Jens, Sep 10 '17 at 22:26
What do you mean *"considered a statement when placed in a certain location"*? A string literal on its own is an expression, thus an expression statement. The fact that this is *also* special-cased as a docstring when such a statement is the first in a function/class/module/method doesn't change that. — jonrsharpe, Sep 10 '17 at 22:35

score 12 · Answer 1 · answered Sep 10 '17 at 22:32

A string literal is just like any other literal. It also works if you just put in an integer:

def func():
    1

However, it doesn't work if you only use a comment:

def func():
    # test

# IndentationError: expected an indented block

Even though it's also added as docstring (saved in the __doc__ attribute) it's also a function level constant:

def func():
    """I'm a function"""

>>> func.__code__.co_consts
("I'm a function", None)

So the presence of a string literal as only content of a function doesn't change how the function is actually "parsed" and "compiled" itself. Well, apart from the fact that it also got a not-None __doc__ attribute.

It's actually very handy for abstractmethods (see for example "Body of abstract method in Python"), where you don't need an actual function body.

Python’s empty function does not require a pass statement?

1 Answers1