Reassignment of 'self' in a method: Python 3.8

Question

In a method of a class:

def weight(self, grid):

    ...

    if self.is_vertical:
        self = self.T

    ...

I'd like to reassign self to its transposed value if the condition is true. Depending on the if-statement, I'd like to use self in a method later in its original or transposed condition.

As I understand, in a method, self is just a parameter name, but not the real reference or a pointer to an instance of a class, as in C++ or similar languages, so I can freely reassign it for use inside the scope of a method.

My question is why PyCharm's Inspection Info warns me that

...first parameter, such as 'self' or 'cls', is reassigned in a method. In most cases imaginable, there's no point in such reassignment, and it indicates an error.

while it works fine? Why does it indicates an error?

Answer 1

Why does it indicates an error?

Because there's no good reason to do it. Nothing uses the self variable automatically, so there's no need to reassign it. If you need a variable that sometimes refers to the object that the method was called on but could also hold some other value, use a different variable for this.

def mymethod(self):
    cur = self
    ...
    cur = cur.T
    ...

Note that self is just a local variable within this method. Reassigning self doesn't have any effect on the object itself, or the variable that the method was called on. It's practically useless to do this, so it almost always indicates that the programmer was confused. That's why Pycharm warns about it.

Since everyone expects self to refer to the object that the method was called on, reassigning it will also be confusing to other programmers. When working on code later in the method, they may not realize that self might not refer to that object. Imaging trying to have a conversation with someone who says "From now on, whenever I say 'me' or 'I', I actually mean that guy over there."

This is just the flip side of why we have the self and cls naming convention in the first place. As far as Python is concerned, you can use any name for the first parameter of a method. But we recommend everyone use these names so that when we read each others' code, we won't have to remember what variable refers to the current object in each method.

Python itself doesn't care, it won't cause an error message there.