Is it possible to apply my own decorators to builtin methods in Python?

Question

I've just come across Python decorators. Just out of interest, can you apply your own decorator to a built-in object method somehow? Say I wanted to apply this:

def remove_empty(fn):
    def filtered():
        return filter(lambda x: x != '', fn())
    return filtered

To this:

some_string.split('\n')

in order to remove empty strings. Is it possible? Or even a good idea?

Answer 1

It's possible in a sense; it depends on what exactly you mean. Decorator syntax like this...

@dec
def foo():
    pass

is really just sugar for this:

def foo():
    pass
foo = dec(foo)

So there's nothing to stop you from using a decorator on a predefined function in the global namespace.

func = dec(func)

But the methods of built-in classes live in the namespace of that class, and that namespace can't be modified directly, as chepner has already pointed out. That's a good thing, because it ensures that objects of type str will behave as expected! However, you could subclass str and decorate the method that way. (The below works in Python 2; in Python 3, pass the output of filter to a list. super also may work a little differently; I'll post a Python 3 update in the future.)

>>> def remove_empty(fn):
...     def filtered(*args, **kwargs):
...         return filter(lambda x: x != '', fn(*args, **kwargs))
...     return filtered
... 
>>> class WeirdString(str):
...     @remove_empty
...     def split(self, *args, **kwargs):
...         return super(WeirdString, self).split(*args, **kwargs)
... 
>>> 'This decorator is unnecessary\n\n\n'.split('\n')
['This decorator is unnecessary', '', '', '']
>>> WeirdString('This decorator is unnecessary\n\n\n').split('\n')
['This decorator is unnecessary']

Or more directly (and so more in the spirit of decorator use):

>>> class WeirdString2(str):
...     split = remove_empty(str.split)
... 
>>> WeirdString2('This decorator is unnecessary\n\n\n').split('\n')
['This decorator is unnecessary']

In the case of this particular example, I'd prefer an explicit filter. But I can imagine, for example, a subclass of a built-in class that does some memoization or something like that.

Answer 2

I'm afraid the answer is no. Decorators are applied when the function is defined, and str.split is pre-defined. You might think you could do something explicit like

str.split = remove_empty(str.split)

but that is not permitted:

Traceback (most recent call last):
  File "tmp.py", line 8, in <module>
    str.split = remove_empty(str.split)
TypeError: can't set attributes of built-in/extension type 'str'

Answer 3

Of course it is. Just write

remove_empty(lambda: some_string.split('\n'))()