How to add abilities to strings in Python

Question

This is more of a curiosity question than anything else. I'm new with Python and playing around with it. I've just looked at the base64 module. What if instead of doing:

import base64
string = 'Foo Bar'
encoded = base664.b64encode

I wanted to do something like:

>>> class b64string():
>>>   <something>
>>>
>>> string = b64string('Foo Bar')
>>> string
'Foo Bar'
>>> string.encode64()
'Rm9vIEJhcg=='
>>> string
'Rm9vIEJhcg=='
>>> string.assign('QmFyIEZvbw==')
>>> string
'QmFyIEZvbw=='
>>> string.b64decode()
'Bar Foo'
>>> string
'Bar Foo'

Is there a simple, pythonic way to create that class?

I've begun with this:

>>> class b64string(base64):
...   def __init__(self, v):
...     self.value=v

And already I get:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Error when calling the metaclass bases
    module.__init__() takes at most 2 arguments (3 given)

And don't get me started on (just to see what would happen):

>>> class b64string(str, base64): pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Error when calling the metaclass bases
    metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases

I know how to do it manually by listing all of the attributes of base64 in a new class and calling them with the stored value as argument. But is there a neat, pythonic way to do this? Is it a bad idea to do it? The idea would be, if needed, to do it with many such modules and have "super strings" that would have as modules all the things I would need to do with them. Is that bad? Is it un-pythonic? If it is pythonic, how is it done?

Answer 1

I don't think creating such complex string-like classes is a good idea, but if you really want to, here's a simple snippet that runs your examples.

First, we define a class that's a generic string-wrapper. Its core is a __getattr__ function that forwards every method call to a given self.module, adding self.string as the first parameter and remembering the result on self.string.

import base64

class ModuledString(object):
    def __init__(self, string):
        self.string = string

    def __getattr__(self, attrname):
        def func(*args, **kwargs):
            result = getattr(self.module, attrname)(self.string, *args, **kwargs)
            self.string = result
            return result
        return func

    def __str__(self):
        return str(self.string)

Creating a string-wrapper with base64 capabilities is then easy:

class B64String(ModuledString):
    module = base64

if __name__ == '__main__':
    string = B64String('Foo Bar')
    print string
    # 'Foo Bar'
    print string.b64encode()
    # 'Rm9vIEJhcg=='
    print string
    # 'Rm9vIEJhcg=='
    string.string = 'QmFyIEZvbw=='
    print string
    # 'QmFyIEZvbw=='
    print string.b64decode()
    # 'Bar Foo'

Note that the above examples work only because b64encode and b64decode take a string as the first argument and return a string as the result (there is no validation in my __getattr__ function). A random function from some random module would probably raise some kind of exception. So, after all, it would be better to restrict the usage to a predefined set of functions from a given module, but it should be easy now.

I repeat, I don't recommend using such code in any serious project, only for fun.