When is a str not a str?

Question

I have an issue in Python3 where a str is not a str. I'm working through a 2to3 project, and I'm getting a failure due to a type check in a third party class. For what it's worth this didn't happen under Python 2. The following example does not error but is similar to my issue:

class TheirClass():
    # I don't love that they're using `id` here, but they are
    def __init__(self, name, id=None):
        if isinstance(name, str):
            self.name = name
        else:
            raise TypeError('name is not a str')

my_object = TheirClass('a name')
print('Yep that worked')

I haven't yet been able to reduce this to a MCVE, but I do know the following things:

• I am passing a str into the constructor for TheirClass.name
• Outside TheirClass.__init__ isinstance(name, str) == True
• Inside TheirClass.__init__ isinstance(name, str) == False
• Inside TheirClass.__init__ type(name) returns <class str>
• Inside TheirClass.__init__ dir(name) == dir(str)
• inspect.getmro reports that my pseudo-str doesn't have an __mro__
• After renaming id in their code I was able to see that id(str) != id(type(name))
• The id of the name object does not change between the two method calls

As far as I can tell nobody is redefining isinstance or str. But the id of str does change from outside constructor of TheirClass to inside of it.

Answer 1

The final clue was that the id of str changed. While there was no assignment of str or def str there was an import. Specifically:

from past.builtins import str