Make UserString behave like str

I found a hack while looking at the source code of UserString. You could do something like this:

from collections import UserString
from functools import cachedproperty
def double(s):
    print('Computing...')
    return s + s
class LazyString(UserString):
    # Specify slots to make the class usable as a dict key
    slots = ('originalstr',)
    # No good very bad hack, but it works
    class = str
    def init(self, originalstr: str):
        self.originalstr = originalstr
    # cachedproperty so that double is called only once
    @cachedproperty
    def data(self) -> str:
        return double(self.originalstr)
s = LazyString("test")

Then,

>>> s # Lzay computation here, repr is called
Computing...
'testtest'
>>> print(s) # str is called
testtest
>>> isinstance(s, str) # Check passed
True
>>> {s: 1} # Can be used a  dict key
{'testtest': 1}
>>> len(s) # len ok
8
>>> s[5] # Indexation ok
'e'

This should work for the few use-cases you described, though there is probably an edge case that breaks everything, as per usual.

Disclaimer: any new string created from using a method (or indexing) LazyString will be an str, not a LazyString. This is better since it prevent issues with doubling down on the custom functions call.

Depending on what OP wants, this approach doesn’t really work. For example, len(s) returns 4, and s[4] causes an IndexError (for all practical purposes I’d expect the results 8 and 't', respectively).

That's fair, though I'd say OP posted their requirements. I'll change my answer real quick.