Where can I find method definitions for classes in python?

Python has something called a "docstring", analagous to Javadoc. Most methods are written with docstrings (the standard library is pretty good about this, at least), and most IDEs will automatically show you the docstring for any given method you're about to reference, in the same way that they would show you the javadoc. Docstring looks like this:

def my_method(some_params):
    '''
    docstring goes here, within the triple-quotes
    '''

When you're on a python terminal, you can look directly at the docstring for a given method by doing

>>> help(my_method)

You can also import a package and look at a particular method:

>>> import collections
>>> help(collections.Counter.keys)

This then shows you the following:

keys(...)
    D.keys() -> a set-like object providing a view on D's keys

You can also use help() on a class, which will show the class's docstring and the docstrings for each method in the class:

>>> help(collections.Counter)

or on an instance, which would show more or less the same information:

>>> x = collections.Counter()
>>> help(x)

In your particular case, Counter is a subclass of the built-in dict, which might account for the lack of explicit documentation on the methods they share. dict.get(key), for example, returns the value for the given key, so I'd assume Counter.get(key) would return something similar.

If looking at the docstring doesn't really help you figure out your issue, don't be afraid to just open up a terminal and experiment with the inputs and outputs!