How can I decorate all inherited methods in a subclass

Question

class Reader:
    def __init__(self):
        pass

    def fetch_page(self):
        with open('/dev/blockingdevice/mypage.txt') as f:
            return f.read()

    def fetch_another_page(self):
        with open('/dev/blockingdevice/another_mypage.txt') as f:
            return f.read()   

class Wrapper(Reader):
    def __init__(self):
        super().__init__()

    def sanity_check(func):
        def wrapper():
            txt = func()
            if 'banned_word' in txt:
                raise Exception('Device has banned word on it!')
        return wrapper

    @sanity_check
    <how to automatically put this decorator on each function of base class? >

w = Wrapper()
w.fetch_page()
w.fetch_another_page()

How can I make sure that sanity_check's wrapper was run automatically when calling fetch_page and fetch_another_page on an instance of the Wrapper class?

Answer 1

If using python3.6 or above, you can accomplish this using __init_subclass__

Simple implementation: (for the real thing you probably want a registry and functools.wraps, etc):

class Reader:
    def __init_subclass__(cls):
        cls.fetch_page = cls.sanity_check(cls.fetch_page)
        cls.fetch_another_page = cls.sanity_check(cls.fetch_another_page)

    def fetch_page(self):
        return 'banned_word'

    def fetch_another_page(self):
        return 'not a banned word'

class Wrapper(Reader):
    def sanity_check(func):
        def wrapper(*args, **kw):
            txt = func(*args, **kw)
            if 'banned_word' in txt:
                raise Exception('Device has banned word on it!')
            return txt
        return wrapper

Demo:

In [55]: w = Wrapper()

In [56]: w.fetch_another_page()
Out[56]: 'not a banned word'

In [57]: w.fetch_page()
---------------------------------------------------------------------------
Exception                                 Traceback (most recent call last)
<ipython-input-57-4bb80bcb068e> in <module>()
----> 1 w.fetch_page()
...

Exception: Device has banned word on it!

Edit:In case you can't change the baseclass, you can subclass and create an Adapter class:

class Reader:

    def fetch_page(self):
        return 'banned_word'

    def fetch_another_page(self):
        return 'not a banned word'

class ReadAdapter(Reader):
    def __init_subclass__(cls):
        cls.fetch_page = cls.sanity_check(cls.fetch_page)
        cls.fetch_another_page = cls.sanity_check(cls.fetch_another_page)

class Wrapper(ReadAdapter):
    def sanity_check(func):
        def wrapper(*args, **kw):
            txt = func(*args, **kw)
            if 'banned_word' in txt:
                raise Exception('Device has banned word on it!')
            return txt
        return wrapper

Should provide the same result.