Clarification on classes in python

Question

I am new to Python (coming from MATLAB) and was beginning to learn that classes were a good way to modularize a program. I understand that they are like a "blueprint" that can streamline processes, but I think my understanding of that is faulty. I was reading the documentation and came across this example:

class Complex:
 def __init__(self, realpart, imagpart):
 self.r = realpart
 self.i = imagpart
x = Complex(3.0, -4.5)
x.r, x.i

with an output of

(3.0, -4.5)

And it stumped me. There are no functions known as realpart or imagpart to be passed and yet they are renamed as r and i and instantly work. They don't appear to be built in functions either. How are realpart and imagpart doing anything?

NOTE: I apologize for the ambiguous title and tags, I'm not really sure what this would be called. I'd appreciate suggestions.

• Ason: Two great answers. for MATLAB users switching to python; Python's init function inside classes pass information dramatically different than MATLAB's functions! Thanks DSM for an extended answer.
• carmenism: The __init__ function "inside classes" is markedly different than MATLAB's functions because __init__ it creates an instance of a class, in this case Complex. In MATLAB, you define functions that you pass values to and return some kind of result usually. Python can do that too, but what you're doing here is instantiating an object, so you're storing realpart and imagpart in the object. Unlike python, MATLAB is not object-oriented, though it comes somewhat close with something called functional objects.
• Ason: @nullpointer Thank you for the elaboration!
• carmenism: You're welcome. Adjusting to Object Oriented Programming can be a bit jarring at first. I know it's hard but you'll have to try to not compare too many things to how they were in MATLAB. Good luck with your python endeavors!
• Ason: Thanks again, I will try to keep my Python and MATLAB practices separated.

Answer 1

How are realpart and imagpart doing anything?

They're not doing anything: they're just the names given to the arguments to __init__. We could call them apple and pear if we liked. When you type

x = Complex(3.0, -4.5)

__init__ is passed three arguments:

def __init__(self, realpart, imagpart):

the instance itself (traditionally called self), the first explicit argument passed (3.0), and the second explicit argument passed (-4.5). It then stores these as instance variables in self.r and self.i.

• Ason: So, by calling x=complex(3,-4.5), I am in essance giving realpart and imagpart a value? That can then be called by their shorter names r and i?
• BrenBarn: Are you reading the code? You can't just ignore what it says in __init__. The reason you can refer to them by r and i is because the code assigns those shorter names to them. They don't magically get extra names.
• Ason: wow, ok.. I was definitely thinking of this backwards. Thank you guys.
• DSM: realpart and imagpart are given values, yeah, but only for the lifetime of the __init__ method. It's like def f(a,b): return a+b; f(1,2) does bind a=1 and b=2, but only locally. As BrenBarn says, it's the self.r = realpart which pushes the local realpart into the instance.

Answer 2

realpart and imagpart are the arguments to the class initializer __init__. When you call Complex(3.0, -4.5), those arguments are passed to that function. The function happens to set some attributes on the object to save the values of those arguments. It does this with the lines

 self.r = realpart
 self.i = imagpart

These lines create attributes on the object. When you create an object called x, it will have those attributes accessible as x.r and x.i.

I'm not sure why you're asking about "functions" called realpart and imagpart. They are just ordinary variables.

You should read the Python tutorial to familiarize yourself with the basics of Python.