Can a simple difference in Python3 variable names alter the way code runs?

Question

This code…

class Person:
    num_of_people = 0

    def __init__(self, name):
        self.name = name
        Person.num_of_people += 1

    def __del__(self):
        Person.num_of_people -= 1

    def __str__(self):
        return 'Hello, my name is ' + self.name

cb = Person('Corey')
kb = Person('Katie')
v = Person('Val')

Produces the following error…

Exception AttributeError: "'NoneType' object has no attribute 'num_of_people'" in <bound method Person.__del__ of <__main__.Person object at 0x7f5593632590>> ignored

But this code does not.

class Person:
    num_of_people = 0

    def __init__(self, name):
        self.name = name
        Person.num_of_people += 1

    def __del__(self):
        Person.num_of_people -= 1

    def __str__(self):
        return 'Hello, my name is ' + self.name

cb = Person('Corey')
kb = Person('Katie')
vb = Person('Val')

The only difference I see is the last variable name is “vb” vs. “v”.

I am leaning Python and am working on the OOP stuff now.

Asked By: Corey

||

Source

Answer 1

Yes, although it is not the so much the variable name that causes this, not directly.

When Python exits, all modules are deleted too. The way modules are cleaned up is by setting all globals in a module to None (so those references no longer refer to the original objects). Those globals are keys in a dictionary object, and as dictionaries are ordered arbitrarily, renaming one variable can change the order in which variables are cleared.

When you renamed v to vb, you altered the order in which variables are cleared, and now Person is cleared last.

One work-around is to use type(self).num_of_people -= 1 in the __del__ method instead:

def __del__(self):
    type(self).num_of_people -= 1

because the instance will always have a reference to the class still, or test if Person is not set to None:

def __del__(self):
    if Person is not None:
        Person.num_of_people -= 1

Two notes:

CPython 3.4 no longer sets globals to None (in most cases), as per Safe Object Finalization; see PEP 442.
CPython 3.3 automatically applies a randomized hash salt to the str keys used in a globals dictionary; this makes the behaviour you observed even more random, merely re-running your code several times may or may not trigger the error message.

Answered By: Martijn Pieters

Can a simple difference in Python3 variable names alter the way code runs?

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