Mutable default arguments don't generally do what you want. Instead, try this:

class Node:
     def __init__(self, wordList=None, adjacencyList=None):
        if wordList is None:
            self.wordList = []
        else:
             self.wordList = wordList 
        if adjacencyList is None:
            self.adjacencyList = []
        else:
             self.adjacencyList = adjacencyList 

You didn't construct anything. You only created another reference to the animal class.

That class has name, height and weight attributes already, so the print() statements can access those attributes and print the values. You can also give those class attributes a different value, so myCat.name = 'Backstreet' works too. This change would also be visible through animal.name however.

myCat.printvars is a reference to the method you defined, but it is unbound; there is no instance here, only a class object, so there is nothing to set self to. You can pass in a value for self explicitly in this case, which is why myCat.printvars(myCat) works; you explicitly set self to myCat, and again that class object has the required attributes for this to work..

You can still create actual instances from the myCat reference:

an_actual_cat = myCat('Felix', 10, 15)

Just remember, all names in Python are references; you can always make more references to objects by assigning:

foo = an.animal
bar = an.animal

Now both foo and bar also point to the animal class.

Don't use class attributes for that, that's not what they are there for. Use default arguments:

def __init__(self, name='Undefined', number=-1):
    self.name = name
    self.number = number

If any of the arguments are mutable, then you need to use the usual idiom:

def __init__(self, foo=None):
    self.foo = [] if foo is None else foo