Use another class' methods without decorators and inheritance

Question

I have a class, which have several methods of its own, not shown here for simplicity:

class Foo:
  def __init__(self, arg: str):
    self.bar = arg

Let's say, aside from its own methods, I want Foo's instances to use str's methods in its bar property's (assured to be a string) stead. This is possible with the __getattr__ dunder method:

class Foo:
  def __getattr__(self, item):
    return getattr(self.foo, item)

The call's result should be .bar's new value. However, since Python strings are immutable, a string resulted from a method call (say, str.strip()) will need to be re-assigned, which doesn't look very nice. Also, an if is needed in case that call doesn't return a string:

result = instance_of_Foo.strip()
if isinstance(result, str):
  instance_of_Foo.bar = result
else:
  ...

I solved this problem with a decorator:

def decorator(function, *, self):
  def wrapper(*args, **kwargs):
    result = function(*args, **kwargs)
        
    if isinstance(result, str):
      self.bar = result
    else:
      return result

  return wrapper

class Foo:
  def __init__(self, arg: str):
    self.bar = arg

  def __getattr__(self, item):
    method = decorator(getattr(self.bar, item), self = self)
    return method

foo = Foo(' foo ')
print(foo.bar) # ' foo '

foo.strip()
print(foo.bar) # 'foo'

...but there surely is a more "Pythonic" way, preferably using dunder methods instead of a decorator, to intercept the call, isn't there? Note that my class cannot substitute a string (Liskov principle violation), so inheritance is out of the question.

Answer 1

To answer my own question:

You (or I) can use a wrapper and cache the __getattr__ dunder method. However, chepner's answer should be preferred as it can handle an arbitrary given function and is better designed.

from functools import cache

class Foo:
  def __init__(self, arg: str):
    self.bar = arg

  @cache
  def __getattr__(self, item):
    method = getattr(self.bar, item)
        
    def wrapper(*args, **kwargs):
      result = method(*args, **kwargs)
            
      if isinstance(result, str):
        self.bar = result
      else:
        return result
    
    print(f'{id(wrapper)}')
    
    return wrapper

Try it:

foo = Foo(' foo ')
print(foo.bar)  # ' foo '

foo.strip()     # id(wrapper) = 2345672443040
print(foo.bar)  # 'foo'

foo.center(7)   # id(wrapper) = 2345681396384
print(foo.bar)  # '  foo  '

foo.center(9)   # Nothing, cached.
print(foo.bar)  # '   foo   '

foo.strip(' ')  # With an argument, also nothing.
print(foo.bar)  # 'foo'