One of the difficulties when comparing programming languages is that they sometimes use the same words to describe similar things, but always with differences. Sometimes the differences are large enough that we want to use different words, but often they are not.

This came up the other day when someone said, “__init__() is not a constructor.” I understand this idea, I’ve heard it before. I just don’t think it’s a good way to help people understand Python.

The reason he said it is because in C++ (and maybe other languages), an object is not truly a usable object of its class until the constructor returns. The function is called the “constructor” because it is the code that constructs the object.

In Python, an object called self is passed into __init__, and it is already a usable object of its class before __init__ is called. So a C++ expert will say, “__init__() is not a constructor.”

I look at it differently: a constructor is the code a class author writes to fully get the object ready for use. That’s what __init__() does in Python. To deal with the difference between Python and C++, I’d say, “constructors in Python and C++ are different: in Python, the object is ready to use before the constructor is called, but in C++, it isn’t until the constructor finishes.”

If you haven’t used C++, you might not even see what the big deal is. There are aspects of C++ behavior where this dividing line between “isn’t of the class” and “is of the class” makes a difference (non-virtual methods, for example). Python doesn’t have those behaviors, so drawing a strict line isn’t as necessary.

So let’s use “constructor” for the user’s view of the code, rather than the internal implementation’s view. Part of Python’s construction of an object is handled by __new__, but you almost never need to deal with __new__, so let’s keep it simple.

Constructor is just one example of two languages using the same word for similar but slightly different things:

• A classic example is “variable.” Some people say, “Python has no variables,” when what they mean is, “Python variables work differently than C variables.” They work exactly the same as JavaScript variables. (For more on this, see my talk Python Names and Values.)
• Python lists (which are just like JavaScript arrays) are different than Lisp lists (which are linked lists).
• Java classes have many differences from Python classes: access modifiers on the Java side, multiple inheritance and special methods on the Python side.
• Python functions (which can have side-effects) are not like Haskell functions (which cannot).
• And let’s not even get started on strings!

Different programming languages are different, but with similarities. We have to re-use words to talk about them. Then we can explore where the constructs are the same, and where they are different.