When do we need Python Import Statements?

Question

A piece of code works that I don't see why. It shouldn't work from my understanding. The problem is illustrated easily below:

"Main.py"

from x import * #class x is defined
from y import * #class y is defined


xTypeObj = x()
yTypeObj = y()
yTypeObj.func(xTypeObj)

"x.py"

class x(object):

    def __init__...
        ...
    def functionThatReturnsAString(self):
        return "blah"

"y.py"

#NO IMPORT STATEMENT NEEDED?? WHY

class y(object):
    def __init__...
        ...
    def func(self, objOfTypeX):
        print(objOfTypeX.functionThatReturnsAString())

My question is why do I NOT need to have an import statement in "y.py" of the type

from x import functionThatReturnAString()

How does it figure out how to call this method?

Answer 1

Python is an object-oriented programming language. In such a language, values are objects, and objects can have methods.

The functionThatReturnsAString function is a method on a class, and objOfTypeX is an instance of that class. Instances of a class carry with them all the methods of it's class.

This is why, for example, list objects in python have an .append() method:

>>> alist = []
>>> alist.append(1)
>>> alist
[1]

The list class has a .append() method, and you do not need to import that method to be able to call it. All you need is a reference to a list instance.

_{^{Technically speaking, a python list is a type, but that distinction does not matter here. On the whole, types are the same things as classes, for the purpose of this discussion.}}

Please do go and read the Python Tutorial, it explains classes in a later chapter (but you may want to skim through the first set of chapters first).