Why python docstring is interpreted differently from comment

Question

Let's say, I've got a function like this:

def myFunc():
    # useful function to calculate stuff

This will produce an indentation error, unless I add pass:

def myFunc():
    # useful function to calculate stuff
    pass

However, if I replace a comment with docstring, no pass is necessary:

def myFunc():
    """useful function to calculate stuff"""

This seems like an odd feature as neither of these are used in the program, as far as I know. So, why does it behave like this?

Answer 1

A docstring isn't just a comment. It actually has meaning to the interpreter. In the case with a docstring, you could do myFunc.__doc__ and actually get your docstring back (In the other case with a pass, the result myFunc.__doc__ would be None).

In other words, you are actually adding some code to the function body to modify it's behavior (in some circumstances), so no pass is necessary.

Answer 2

A comment is outright ignored by the interpreter, so omitting a block after an indent is a syntax error. However, a docstring is a real Python object--at its most basic, a literal str. A lone expression is a valid block of code:

'This is a string. It is a valid (though pretty useless) line of Python code.'

In the case of docstrings in particular, there's also some additional functionality going on, such as being used to set the __doc__ attribute.

>>> def myFunc():
...     '''MyDocString'''
...
>>> print(myFunc.__doc__)
MyDocString

Note that this also works for classes:

>>> class MyClass(object):
...     '''MyClassDocString'''
...
>>> print(MyClass.__doc__)
MyClassDocString