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I have renamed a python class being a part of a library. I am willing to leave a possibility to use its previous name for some time but would like to warn user that it's deprecated and will be removed somewhere in the future.

I think that to provide backward compatibility it will be enough to use an alias like that:

class NewClsName:

OldClsName = NewClsName

I have no idea how to mark the OldClsName as deprecated in an elegant way. Maybe I could make OldClsName a function which emits a warning (to logs) and constructs the NewClsName object from its parameters (using *args and **kvargs) but it doesn't seem elegant enough (or maybe it is?).

Unfortunately, I don't know how Python standard library deprecation warnings work but I imagine that there may be some nice magic to deal with deprecation, e.g. allowing treating it as errors or silencing depending on some interpreter's command line option.

The question is: How to warn users about using an obsolete class alias (or obsolete class in general).

EDIT: The function approach doesn't work for me (I already gave it a try) because the class has some class methods (factory methods) which can't be called when the OldClsName is defined as a function. Following code won't work:

class NewClsName(object):
    def CreateVariant1( cls, ... ):

    def CreateVariant2( cls, ... ):

def OldClsName(*args, **kwargs):
    warnings.warn("The 'OldClsName' class was renamed [...]",
                  DeprecationWarning )
    return NewClsName(*args, **kwargs)

OldClsName.CreateVariant1( ... )

Because of:

AttributeError: 'function' object has no attribute 'CreateVariant1'

Is inheritance my only option? To be honest, it doesn't look very clean to me - it affects class hierarchy through introduction of unnecessary derivation. Additionally, OldClsName is not NewClsName what is not an issue in most cases but may be a problem in case of poorly written code using the library.

I could also create a dummy, unrelated OldClsName class and implement a constructor as well as wrappers for all class methods in it, but it is even worse solution, in my opinion.

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2 Answers

up vote 4 down vote accepted


Yup, I think that's pretty standard practice:

def OldClsName(*args, **kwargs):
    from warnings import warn
    warn("get with the program!")
    return NewClsName(*args, **kwargs)

The only tricky thing is if you have things that subclass from OldClsName - then we have to get clever. If you just need to keep access to class methods, this should do it:

class DeprecationHelper(object):
    def __init__(self, new_target):
        self.new_target = new_target

    def _warn(self):
        from warnings import warn
        warn("Get with the program!")

        return self.new_target(*args, **kwargs)

    def __getattr__(self, attr):
        return getattr(self.new_target, attr)

OldClsName = DeprecationHelper(NewClsName)

I haven't tested it, but that should give you the idea - __call__ will handle the normal-instantation route, __getattr__ will capture accesses to the class methods & still generate the warning, without messing with your class heirarchy.

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Supporting inheritance should be easy as well - just write a class OldClsName(NewClsName): # and overload __new__. –  delnan Jan 25 '12 at 19:05
+1 for confirming that this solution is used and for mentioning the warnings module. Unfortunately the function solution doesn't work for me (see edited question). Maybe you have some other clean solutions? :D –  Dariusz Walczak Jan 25 '12 at 21:51
Updated with an example of using a wrapper object that can proxy both calls & attribute accesses to the new class. –  AdamKG Jan 25 '12 at 22:02
Ahh, nice, nice. Works perfectly, thank you very much :) –  Dariusz Walczak Jan 26 '12 at 9:49
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Please have a look at warnings.warn.

As you'll see, the example in the documentation is a deprecation warning:

def deprecation(message):
    warnings.warn(message, DeprecationWarning, stacklevel=2)
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