How is this valid Python syntax?

Question

The following is part of SqlAlchemy, I'm trying to figure out how this is valid Python syntax. I know the code works, I'm just trying to understand what is is doing here and how that is valid. These are NOT nested classes, they are stand alone classes. I would have expected a pass after each class declaration though and I'm pretty sure it not being there is significant.

class InstrumentedList(list):
    """An instrumented version of the built-in list."""


class InstrumentedSet(set):
    """An instrumented version of the built-in set."""


class InstrumentedDict(dict):
    """An instrumented version of the built-in dict."""


__canned_instrumentation = {
    list: InstrumentedList,
    set: InstrumentedSet,
    dict: InstrumentedDict,
}

__interfaces = {
    list: (
        {'appender': 'append', 'remover': 'remove',
         'iterator': '__iter__'}, _list_decorators()
    ),

    set: ({'appender': 'add',
           'remover': 'remove',
           'iterator': '__iter__'}, _set_decorators()
          ),

    # decorators are required for dicts and object collections.
    dict: ({'iterator': 'values'}, _dict_decorators()) if util.py3k
    else ({'iterator': 'itervalues'}, _dict_decorators()),
}

The class body has to contain something, but a docstring counts as that something. It's a statement, like pass. — jonrsharpe
– jonrsharpe, Commented Mar 15, 2018 at 18:03

adamkgray · Accepted Answer · 2018-03-15 18:17:43Z

1

It is valid python syntax as long as there is something inside of the class definition.

For example:

class FooBar:
    """ foobar """

This is okay because there is something (in this case a docstring, which can be accessed with the __doc__ attribute) in the class definition.

However, a class definition without anything in it is invalid syntax:

class FooBar:

When this is run, it produces the output:

SyntaxError: unexpected EOF while parsing

edited Mar 15, 2018 at 18:17

answered Mar 15, 2018 at 18:05

adamkgray

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1 Comment

boatcoder Over a year ago

I guess I'm still thinking about comments like C/C++ as things that aren't code. I have a new appreciation for docstrings now.

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