How to warn about class (name) deprecation

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)

Why don’t you just sub-class? This way no user code should be broken.

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

Use inspect module to add placeholder for OldClass, then OldClsName is NewClsName check will pass, and a linter like pylint will inform this as error.

deprecate.py

import inspect
import warnings
from functools import wraps

def renamed(old_name):
    """Return decorator for renamed callable.

    Args:
        old_name (str): This name will still accessible,
            but call it will result a warn.

    Returns:
        decorator: this will do the setting about `old_name`
            in the caller's module namespace.
    """

    def _wrap(obj):
        assert callable(obj)

        def _warn():
            warnings.warn('Renamed: {} -> {}'
                        .format(old_name, obj.__name__),
                        DeprecationWarning, stacklevel=3)

        def _wrap_with_warn(func, is_inspect):
            @wraps(func)
            def _func(*args, **kwargs):
                if is_inspect:
                    # XXX: If use another name to call,
                    # you will not get the warning.
                    frame = inspect.currentframe().f_back
                    code = inspect.getframeinfo(frame).code_context
                    if [line for line in code
                            if old_name in line]:
                        _warn()
                else:
                    _warn()
                return func(*args, **kwargs)
            return _func

        # Make old name available.
        frame = inspect.currentframe().f_back
        assert old_name not in frame.f_globals, (
            'Name already in use.', old_name)

        if inspect.isclass(obj):
            obj.__init__ = _wrap_with_warn(obj.__init__, True)
            placeholder = obj
        else:
            placeholder = _wrap_with_warn(obj, False)

        frame.f_globals[old_name] = placeholder

        return obj

    return _wrap

test.py

from __future__ import print_function

from deprecate import renamed


@renamed('test1_old')
def test1():
    return 'test1'


@renamed('Test2_old')
class Test2(object):
    pass

    def __init__(self):
        self.data = 'test2_data'

    def method(self):
        return self.data

# pylint: disable=undefined-variable
# If not use this inline pylint option, 
# there will be E0602 for each old name.
assert(test1() == test1_old())
assert(Test2_old is Test2)
print('# Call new name')
print(Test2())
print('# Call old name')
print(Test2_old())

then run python -W all test.py:

test.py:22: DeprecationWarning: Renamed: test1_old -> test1
# Call new name
<__main__.Test2 object at 0x0000000007A147B8>
# Call old name
test.py:27: DeprecationWarning: Renamed: Test2_old -> Test2
<__main__.Test2 object at 0x0000000007A147B8>

Here is the list of requirements a solution should satisfy:

• Instantiation of a deprecated class should raise a warning
• Subclassing of a deprecated class should raise a warning
• Support isinstance and issubclass checks

---

Solution

This can be achieved with a custom metaclass:

class DeprecatedClassMeta(type):
    def __new__(cls, name, bases, classdict, *args, **kwargs):
        alias = classdict.get('_DeprecatedClassMeta__alias')

        if alias is not None:
            def new(cls, *args, **kwargs):
                alias = getattr(cls, '_DeprecatedClassMeta__alias')

                if alias is not None:
                    warn("{} has been renamed to {}, the alias will be "
                         "removed in the future".format(cls.__name__,
                             alias.__name__), DeprecationWarning, stacklevel=2)

                return alias(*args, **kwargs)

            classdict['__new__'] = new
            classdict['_DeprecatedClassMeta__alias'] = alias

        fixed_bases = []

        for b in bases:
            alias = getattr(b, '_DeprecatedClassMeta__alias', None)

            if alias is not None:
                warn("{} has been renamed to {}, the alias will be "
                     "removed in the future".format(b.__name__,
                         alias.__name__), DeprecationWarning, stacklevel=2)

            # Avoid duplicate base classes.
            b = alias or b
            if b not in fixed_bases:
                fixed_bases.append(b)

        fixed_bases = tuple(fixed_bases)

        return super().__new__(cls, name, fixed_bases, classdict,
                               *args, **kwargs)

    def __instancecheck__(cls, instance):
        return any(cls.__subclasscheck__(c)
            for c in {type(instance), instance.__class__})

    def __subclasscheck__(cls, subclass):
        if subclass is cls:
            return True
        else:
            return issubclass(subclass, getattr(cls,
                              '_DeprecatedClassMeta__alias'))

---

Explanation

DeprecatedClassMeta.__new__ method is called not only for a class it is a metaclass of but also for every subclass of this class. That gives an opportunity to ensure that no instance of DeprecatedClass will ever be instantiated or subclassed.

Instantiation is simple. The metaclass overrides the __new__ method of DeprecatedClass to always return an instance of NewClass.

Subclassing is not much harder. DeprecatedClassMeta.__new__ receives a list of base classes and needs to replace instances of DeprecatedClass with NewClass.

Finally, the isinstance and issubclass checks are implemented via __instancecheck__ and __subclasscheck__ defined in PEP 3119.

---

Test

class NewClass:
    foo = 1


class NewClassSubclass(NewClass):
    pass


class DeprecatedClass(metaclass=DeprecatedClassMeta):
    _DeprecatedClassMeta__alias = NewClass


class DeprecatedClassSubclass(DeprecatedClass):
    foo = 2


class DeprecatedClassSubSubclass(DeprecatedClassSubclass):
    foo = 3


assert issubclass(DeprecatedClass, DeprecatedClass)
assert issubclass(DeprecatedClassSubclass, DeprecatedClass)
assert issubclass(DeprecatedClassSubSubclass, DeprecatedClass)
assert issubclass(NewClass, DeprecatedClass)
assert issubclass(NewClassSubclass, DeprecatedClass)

assert issubclass(DeprecatedClassSubclass, NewClass)
assert issubclass(DeprecatedClassSubSubclass, NewClass)

assert isinstance(DeprecatedClass(), DeprecatedClass)
assert isinstance(DeprecatedClassSubclass(), DeprecatedClass)
assert isinstance(DeprecatedClassSubSubclass(), DeprecatedClass)
assert isinstance(NewClass(), DeprecatedClass)
assert isinstance(NewClassSubclass(), DeprecatedClass)

assert isinstance(DeprecatedClassSubclass(), NewClass)
assert isinstance(DeprecatedClassSubSubclass(), NewClass)

assert NewClass().foo == 1
assert DeprecatedClass().foo == 1
assert DeprecatedClassSubclass().foo == 2
assert DeprecatedClassSubSubclass().foo == 3

In python >= 3.6 you can easily handle warning on subclassing:

from warnings import warn

class OldClassName(NewClassName):
    def __init_subclass__(self):
        warn("Class has been renamed NewClassName", DeprecationWarning, 2)

Overloading __new__ should allow you to warn when the old class constructor is called directly, but I haven’t tested that since I don’t need it right now.

Since Python 3.7, you can provide a customization of module attribute access using __getattr__ (and __dir__). Everything is explained in PEP 562.
In the bellow example, I implemented __getattr__ and __dir__ in order to deprecate the “OldClsName” in favor of “NewClsNam”:

# your_lib.py

import warnings

__all__ = ["NewClsName"]

DEPRECATED_NAMES = [('OldClsName', 'NewClsName')]


class NewClsName:
    @classmethod
    def create_variant1(cls):
        return cls()


def __getattr__(name):
    for old_name, new_name in DEPRECATED_NAMES:
        if name == old_name:
            warnings.warn(f"The '{old_name}' class or function is renamed '{new_name}'",
                          DeprecationWarning,
                          stacklevel=2)
            return globals()[new_name]
    raise AttributeError(f"module {__name__} has no attribute {name}")


def __dir__():
    return sorted(__all__ + [names[0] for names in DEPRECATED_NAMES])

In the __getattr__ function, if a deprecated class or function name is found, a warning message is emitted, showing the source file and line number of the caller (with stacklevel=2).

In the user code, we could have:

# your_lib_usage.py
from your_lib import NewClsName
from your_lib import OldClsName


def use_new_class():
    obj = NewClsName.create_variant1()
    print(obj.__class__.__name__ + " is created in use_new_class")


def use_old_class():
    obj = OldClsName.create_variant1()
    print(obj.__class__.__name__ + " is created in use_old_class")


if __name__ == '__main__':
    use_new_class()
    use_old_class()

When the user run his script your_lib_usage.py, it will get something like this:

NewClsName is created in use_new_class
NewClsName is created in use_old_class
/path/to/your_lib_usage.py:3: DeprecationWarning: The 'OldClsName' class or function is renamed 'NewClsName'
  from your_lib import OldClsName

Note: the stack trace is usually written in STDERR.

To see the error warnings, you may need to add a “-W” flag in the Python command line, for instance:

python -W always your_lib_usage.py