问题
I want to create decorator that shows which parameters were passed to function and methods. I have already written the code for functions, but methods are giving me a headaches.
This is function decorator that works as intended:
from functools import update_wrapper
class _PrintingArguments:
def __init__(self, function, default_comment, comment_variable):
self.function = function
self.comment_variable = comment_variable
self.default_comment = default_comment
update_wrapper(wrapped=function, wrapper=self)
def __call__(self, *args, **kwargs):
comment = kwargs.pop(self.comment_variable, self.default_comment)
params_str = [repr(arg) for arg in args] + ["{}={}".format(k, repr(v)) for k, v in kwargs.items()]
function_call_log = "{}({})".format(self.function.__name__, ", ".join(params_str))
print("Function execution - '{}'\n\t{}".format(comment, function_call_log))
function_return = self.function(*args, **kwargs)
print("\tFunction executed\n")
return function_return
def function_log(_function=None, default_comment="No comment.", comment_variable="comment"):
if _function is None:
def decorator(func):
return _PrintingArguments(function=func, default_comment=default_comment, comment_variable=comment_variable)
return decorator
else:
return _PrintingArguments(function=_function, default_comment=default_comment, comment_variable=comment_variable)
# example use:
@function_log
def a(*args, **kwargs):
pass
@function_log(default_comment="Hello World!", comment_variable="comment2")
def b(*args, **kwargs):
pass
a(0, x=1, y=2)
a(0, x=1, y=2, comment="Custom comment!")
b("a", "b", "c", asd="something")
b("a", "b", "c", asd="something", comment2="Custom comment for b!")
Output of the code execution:
Function execution - 'No comment.'
a(0, y=2, x=1)
Function executed
Function execution - 'Custom comment!'
a(0, y=2, x=1)
Function executed
Function execution - 'Hello World!'
b('a', 'b', 'c', asd='something')
Function executed
Function execution - 'Custom comment for b!'
b('a', 'b', 'c', asd='something')
Function executed
I have tried the exactly same decorator for methods:
class A:
def __init__(self):
pass
@function_log
def method1(self, *args, **kwargs):
print("\tself = {}".format(self))
@function_log(default_comment="Something", comment_variable="comment2")
def method2(self, *args, **kwargs):
print("\tself = {}".format(self))
a_obj = A()
a_obj.method1(0, 1, p1="abc", p2="xyz")
a_obj.method1(0, 1, p1="abc", p2="xyz", comment="My comment")
a_obj.method2("a", "b", p1="abc", p2="xyz")
a_obj.method2("a", "b", p1="abc", p2="xyz", comment="My comment 2")
The output is:
Function execution - 'No comment.'
method1(0, 1, p2='xyz', p1='abc')
self = 0
Function executed
Function execution - 'My comment'
method1(0, 1, p2='xyz', p1='abc')
self = 0
Function executed
Function execution - 'Something'
method2('a', 'b', p2='xyz', p1='abc')
self = a
Function executed
Function execution - 'Something'
method2('a', 'b', comment='My comment 2', p2='xyz', p1='abc')
self = a
Function executed
Parameter 'self' is not passed by my decorator to the method.
I want to write second decorator 'method_log' that would work pretty similar as 'function_log'.
For code:
class A:
def __init__(self):
pass
@method_log
def method1(self, *args, **kwargs):
print("\tself = {}".format(self))
@fmethod_log(default_comment="Something", comment_variable="comment2")
def method2(self, *args, **kwargs):
print("\tself = {}".format(self))
a_obj = A()
a_obj.method1(0, 1, p1="abc", p2="xyz")
a_obj.method1(0, 1, p1="abc", p2="xyz", comment="My comment")
a_obj.method2("a", "b", p1="abc", p2="xyz")
a_obj.method2("a", "b", p1="abc", p2="xyz", comment="My comment 2")
I want the output:
Method execution - 'No comment.'
method1(<__main__.A instance at ...>, 0, 1, p2='xyz', p1='abc')
self = <__main__.A instance at ...> #
Function executed
Method execution - 'My comment'
method1(<__main__.A instance at ...>, 0, 1, p2='xyz', p1='abc')
self = <__main__.A instance at ...>
Function executed
Method execution - 'Something'
method2(<__main__.A instance at ...>, 'a', 'b', p2='xyz', p1='abc')
self = <__main__.A instance at ...>
Function executed
Method execution - 'Something'
method2(<__main__.A instance at ...>, 'a', 'b', comment='My comment 2', p2='xyz', p1='abc')
self = <__main__.A instance at ...>
Function executed
回答1:
It's not working with you current design because of how classes work in Python.
When a class is instantiated, the functions on it get bound to the instance -
they become bound methods, so that self is automatically passed.
You can see it happen:
class A:
def method1(self):
pass
>>> A.method1
<function A.method1 at 0x7f303298ef28>
>>> a_instance = A()
>>> a_instance.method1
<bound method A.method1 of <__main__.A object at 0x7f303a36c518>>
When A is instantiated, method1 is magically transformed from a
function into a bound method.
Your decorator replaces method1 - instead of a real function,
it is now an instance of _PrintingArguments. The magic
that turns functions into bound methods is not applied to random
objects, even if they define __call__ so that they behave like a function. (But that magic can be applied, if your class implements the Descriptor protocol, see ShadowRanger's answer!).
class Decorator:
def __init__(self, func):
self.func = func
def __call__(self, *args, **kwargs):
return self.func(*args, **kwargs)
class A:
@Decorator
def method1(self):
pass
>>> A.method1
<__main__.Decorator object at 0x7f303a36cbe0>
>>> a_instance = A()
>>> a_instance.method1
<__main__.Decorator object at 0x7f303a36cbe0>
There is no magic. method1 on the instance of A is not a bound method,
it's just a random object with a __call__ method, which will not have
self passed automatically.
If you want to decorate methods you have to replace the decorated function
with another real function, an arbitrary object with __call__ will not do.
You could adapt your current code to return a real function:
import functools
class _PrintingArguments:
def __init__(self, default_comment, comment_variable):
self.comment_variable = comment_variable
self.default_comment = default_comment
def __call__(self, function):
@functools.wraps(function)
def decorated(*args, **kwargs):
comment = kwargs.pop(self.comment_variable, self.default_comment)
params_str = [repr(arg) for arg in args] + ["{}={}".format(k, repr(v)) for k, v in kwargs.items()]
function_call_log = "{}({})".format(function.__name__, ", ".join(params_str))
print("Function execution - '{}'\n\t{}".format(comment, function_call_log))
function_return = function(*args, **kwargs)
print("\tFunction executed\n")
return function_return
return decorated
def function_log(_function=None, default_comment="No comment.", comment_variable="comment"):
decorator = _PrintingArguments(
default_comment=default_comment,
comment_variable=comment_variable,
)
if _function is None:
return decorator
else:
return decorator(_function)
回答2:
If you want _PrintingArguments to bind the same way as a plain function, this is actually possible, you just need to implement the descriptor protocol yourself to match the way built-in functions behave. Conveniently, Python provides types.MethodType, which can be used to create a bound method from any callable, given an instance to bind to, so we use that to implement our descriptor's __get__:
import types
class _PrintingArguments:
# __init__ and __call__ unchanged
def __get__(self, instance, owner):
if instance is None:
return self # Accessed from class, return unchanged
return types.MethodType(self, instance) # Accessed from instance, bind to instance
This works as you expect on Python 3 (Try it online!). On Python 2 it's even simpler (because unbound methods exist, so the call to types.MethodType can be made unconditionally):
import types
class _PrintingArguments(object): # Explicit inheritance from object needed for new-style class on Py2
# __init__ and __call__ unchanged
def __get__(self, instance, owner):
return types.MethodType(self, instance, owner) # Also pass owner
Try it online!
For slightly better performance (on Python 2 only), you could instead do:
class _PrintingArguments(object): # Explicit inheritance from object needed for new-style class on Py2
# __init__ and __call__ unchanged
# Defined outside class, immediately after dedent
_PrintingArguments.__get__ = types.MethodType(types.MethodType, None, _PrintingArguments)
which moves the implementation of __get__ to the C layer by making an unbound method out of types.MethodType itself, removing byte code interpreter overhead from each call.
来源：https://stackoverflow.com/questions/57807258/passing-self-parameter-during-methods-decorating-in-python