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Learn more about bidirectional Unicode charactersOriginal file line number Diff line number Diff line change @@ -0,0 +1,183 @@ # MIT License # # Copyright (c) 2024 Neil Webber # # Permission is hereby granted, free of charge, to any person obtaining a copy # of this software and associated documentation files (the "Software"), to deal # in the Software without restriction, including without limitation the rights # to use, copy, modify, merge, publish, distribute, sublicense, and/or sell # copies of the Software, and to permit persons to whom the Software is # furnished to do so, subject to the following conditions: # # The above copyright notice and this permission notice shall be included # in all copies or substantial portions of the Software. # # THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR # IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, # FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE # AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER # LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE # SOFTWARE. import functools import types # A DecoClass abstracts away a few details that are important to get correct # when implementing decorators as a class, with callable wrapper objects. # # To implement a decorator this way: # # class MyDecorator(DecoClass): # def __init__(self, *args, kw1="foo", kw2="bar", **kwargs): # super().__init__(*args, **kwargs) # self.kw1 = kw1 # args as needed per application # self.kw2 = kw2 # ... # # def __call__(self, *args, **kwargs): # # -- decorator-specific stuff goes here -- # rv = super().__call__(*args, **kwargs) # # -- decorator-specific stuff goes here -- # return rv # class DecoClass: """Base class for implementing a decorator as a class object.""" # NOTE ON PARAMETERIZED decorators and NAKED decorators # # If there are no arguments for the DECORATOR ITSELF, it will # usually be invoked like this: # # @DecoClass # def foo(): pass # # which ultimately becomes a call to the class constructor with # one argument, the decoration target: # # DecoClass(foo) # # If there are KEYWORD arguments for the DECORATOR ITSELF, that # looks like this: # # @DecoClass(clown='bozo') # def foo(): pass # # And the class constructor is called with the keyword arguments # given, and is expected to return a callable object 'D' that THEN is # invoked with the function as an argument: # # @DecoClass(clown='bozo') --returns--> D # D(foo) # # In the nested-function implementation of decorators this is handled # by yet a third level of function nesting (see any number of python # writeups about this topic for examples of that). # # In this implementation, the parameterized form returns a different # object (via __new__ magic) that in turn will be called to return # the decorated function object. def __new__(cls, f_or_nothing=None, *args, **kwargs): if f_or_nothing is None: return cls.__Deferred(cls, *args, **kwargs) # PARAMETERIZED case else: return super().__new__(cls) # NAKED case def __init__(self, f, /, **kwargs): """subclasses should provide their own and super() this.""" self.func = f functools.wraps(f)(self) # the callable object used for PARAMETERIZED decorators class __Deferred: def __init__(self, cls, *args, **kwargs): self.cls = cls self.args = args self.kwargs = kwargs def __call__(self, f): return self.cls(f, *self.args, **self.kwargs) def __call__(self, *args, **kwargs): """Call the wrapped function.""" return self.func(*args, **kwargs) # A decorator implemented as a callable object method will not work # for decorating bound methods unless the decorator object is a # non-data descriptor (i.e., defines a __get__() method). This does that. def __get__(self, instance, owner): if instance is None: return self return types.MethodType(self, instance) if __name__ == "__main__": import unittest # This is also an example of how to write a DecoClass decorator. # This decorator counts the nesting level of the decorated function. class RecursionCounter(DecoClass): def __init__(self, *args, depthlimit=None, **kwargs): super().__init__(*args, **kwargs) self.depthlimit = depthlimit self._count = 0 self.deepest = 0 def __call__(self, *args, **kwargs): if self.depthlimit is not None and self._count >= self.depthlimit: raise RecursionError(f"recursion limit exceeded") self._count += 1 self.deepest = max(self._count, self.deepest) try: rv = super().__call__(*args, **kwargs) finally: self._count -= 1 return rv @RecursionCounter def nakedfoo(n): if n > 1: nakedfoo(n-1) return nakedfoo.deepest @RecursionCounter() def nakedparmfoo(n): if n > 1: nakedparmfoo(n-1) return nakedparmfoo.deepest foolimit = 5 @RecursionCounter(depthlimit=foolimit) def limitedfoo(n): if n > 1: limitedfoo(n-1) return limitedfoo.deepest class TestMethods(unittest.TestCase): def test_naked_deco(self): self.assertEqual(nakedfoo(17), 17) def test_nakedparm_deco(self): self.assertEqual(nakedparmfoo(17), 17) def test_limited_deco(self): self.assertEqual(limitedfoo(foolimit), foolimit) with self.assertRaises(RecursionError): limitedfoo(foolimit+1) # this one is a method class Foo: @RecursionCounter def foo(self, a, b=1, /, *, clown='bozo'): return (a, b, clown) # commenting out __get__ in DecoClass will demonstrate what this tests def test_methodfoo(self): f = self.Foo() self.assertEqual(f.foo(17), (17, 1, 'bozo')) self.assertEqual(f.foo(3, 17, clown='krusty'), (3, 17, 'krusty')) unittest.main()