Can you change the string of an instanced string class

Question

I have this class:

class A(str):            
    def __str__(self):
        return "the string"
    
    def __repr__(self):
        return "the repr"

a = A("change me")

Running print a returns the string

Running repr(a) returns the repr

Running a returns change me

The problem

Running the instance a returns the string used to instantiate the class. How can I change this string in the instance after instantiation?

The returns of __str__ and __repr___ are not the problem, as in certain situations rather than calling these methods the instance is called directly. So where you may expect __repr__ to return, the instance returns the instantiation string instead.

I have been unable to find any attributes within the string class to change this. Most solutions I've tried require you to re-instance the a variable, which I don't want to do. I want to change the string from within the current instance of the object class variable.

I have tried rebuilding the class through methods utilising __new__ and type(). However, this seems to require re-instancing the stored class variable, as recreating the class creates a duplicate and does not update the current instance.

I am limited to using Python 2.7, due to pipeline constraints.

Do you basically want a mutable string subclass? If so, do either of the following answer your question? [1](https://stackoverflow.com/questions/4520298/modify-subclassed-string-in-place) [2](https://stackoverflow.com/questions/10572624/mutable-strings-in-python) — bgfvdu3w, Jul 13 '22 at 05:44
The 1st reference didn't work for me; using `StringIO` returns an `instance` when running `a`, `<__main__.DelimitedStringIO instance at 0x00000245933C8788>`. However the 2nd link you provided might be working for me... — Adam Sirrelle, Jul 13 '22 at 06:10
Ah, no. :/ Subclassing this as an `Object` returns the object class instance MutableString rather than a string, as a subclass of `string` would. Thanks for the suggestion! — Adam Sirrelle, Jul 13 '22 at 06:16
Since you're using Python 2, `UserString.MutableString` is available. Have you tried that? — bgfvdu3w, Jul 13 '22 at 17:56

Adam Sirrelle · Accepted Answer · 2022-07-13T07:33:55.967

I have a potential answer, although it feels VERY bad and hacky.

class A(str):            
    def __str__(self):
        return "the string"

    def __repr__(self):
        return "the repr"
    
    def _instance_name(self):
        return [k for k, v in globals().items() if v is self]
    
    def change(self, new_string):
        name = self._instance_name()

        if name:
            globals()[name[0]] = type('A', (A, ), self.__dict__)(new_string)

a = A("change me")

Running a returns change me

a.change("I have changed")

Running a returns I have changed

Changing globals this way feels like welcoming the end times, but it's the only solution I've found so far. Any other potential methods would be appreciated!

Can you change the string of an instanced string class

The problem

1 Answers1