Can one write something like:
class Test(object):
def _decorator(self, foo):
foo()
@self._decorator
def bar(self):
pass
This fails: self in @self is unknown
I also tried:
@Test._decorator(self)
which also fails: Test unknown
I would like to temporarily change some instance variables in the decorator and then run the decorated method, before changing them back.
Would something like this do what you need?
class Test(object):
def _decorator(foo):
def magic( self ) :
print "start magic"
foo( self )
print "end magic"
return magic
@_decorator
def bar( self ) :
print "normal call"
test = Test()
test.bar()
This avoids the call to self to access the decorator and leaves it hidden in the class namespace as a regular method.
>>> import stackoverflow
>>> test = stackoverflow.Test()
>>> test.bar()
start magic
normal call
end magic
>>>
edited to answer question in comments:
How to use the hidden decorator in another class
class Test(object):
def _decorator(foo):
def magic( self ) :
print "start magic"
foo( self )
print "end magic"
return magic
@_decorator
def bar( self ) :
print "normal call"
_decorator = staticmethod( _decorator )
class TestB( Test ):
@Test._decorator
def bar( self ):
print "override bar in"
super( TestB, self ).bar()
print "override bar out"
print "Normal:"
test = Test()
test.bar()
print
print "Inherited:"
b = TestB()
b.bar()
print
Output:
Normal:
start magic
normal call
end magic
Inherited:
start magic
override bar in
start magic
normal call
end magic
override bar out
end magic
E0213: Method should have "self" as first argument (no-self-argument). –
Steviestevy ...; vv = foo( self ); print( "override bar out" ); return vv; ... –
Broaddus _decorator should be such a "class function". –
Steviestevy staticmethod function does. the _decorator = staticmethod( _decorator ) line at the end of the class definition overwrites the regular binding that would become a method, and binds the same function but as a static method for use outside the class. for normal "class functions", which python calls static methods ( receiving neither self instance nor class instance ), you can just put @staticmethod before the def for the function you want to be static. We need the decorator above to be a regular function while decorating, hence binding at the end –
Broaddus staticmethod makes it unusable as a decorator itself. I'm wondering if it's a design choice, so that we don't define decorators this way. Or if this is something that may change in a future version of Python, to allow for this use case. –
Steviestevy class Wat: print( "unexpected" ) works just fine, if unexpectedly :). using the staticmethod decorator turns the function into an object that tells the class not to pass it self like it would a normal function. I don't know what check the class does to determine how to call the functions, but if it just special cases staticmethod, one could add a __call__ method to it that calls its contained __func__ to make it callable in the class block –
Broaddus @staticmethod decorator on the _decorator method and the statement _decorator = staticmethod(_decorator), it works perfectly. However, this code does not work in Python 2.7 if either the @staticmethod decorator is used or the _decorator = staticmethod(_decorator) statement is removed. Does this mean that the new version of Python has changed the underlying mechanism? –
Chiromancy <unbound method ...> objects with python 3.0. The concept of “unbound methods” has been removed from the language. When referencing a method as a class attribute, you now get a plain function object. So you don't have to staticmethod it to use it out of class, like in the 2.* series. The only difference being in 3.0 sans staticmethod, if you (grossly/horribly) used _decorator from an instance of the class to decorate in another class, it would get self injected into its args and break. so, ah, don't do that. –
Broaddus What you're wanting to do isn't possible. Take, for instance, whether or not the code below looks valid:
class Test(object):
def _decorator(self, foo):
foo()
def bar(self):
pass
bar = self._decorator(bar)
It, of course, isn't valid since self isn't defined at that point. The same goes for Test as it won't be defined until the class itself is defined (which its in the process of). I'm showing you this code snippet because this is what your decorator snippet transforms into.
So, as you can see, accessing the instance in a decorator like that isn't really possible since decorators are applied during the definition of whatever function/method they are attached to and not during instantiation.
If you need class-level access, try this:
class Test(object):
@classmethod
def _decorator(cls, foo):
foo()
def bar(self):
pass
Test.bar = Test._decorator(Test.bar)
import functools
class Example:
def wrapper(func):
@functools.wraps(func)
def wrap(self, *args, **kwargs):
print("inside wrap")
return func(self, *args, **kwargs)
return wrap
@wrapper
def method(self):
print("METHOD")
wrapper = staticmethod(wrapper)
e = Example()
e.method()
@foo, not @foo() –
Manslaughter wrapper be self? –
Couperin wrapper = staticmethod(wrapper) is below @wrapper. Had wrapper = staticmethod(wrapper) occurred first (or had the more usual @staticmethod decorator been used), it would indeed give a TypeError. I'm not actually sure what making it a static method accomplishes in this case. –
Narcho self as the first argument of the method. Let's say we do want to make the wrapper method static, we need to do so after the definition on any other method that uses this wrapper method, to avoid the automatic transformation to instance method. I would suggest playing with the debugger and placing breakpoints on the method definitions to understand this better. Also check the official docs for staticmethod –
Ferrochromium This is one way to access(and have used) self from inside a decorator defined inside the same class:
class Thing(object):
def __init__(self, name):
self.name = name
def debug_name(function):
def debug_wrapper(*args):
self = args[0]
print 'self.name = ' + self.name
print 'running function {}()'.format(function.__name__)
function(*args)
print 'self.name = ' + self.name
return debug_wrapper
@debug_name
def set_name(self, new_name):
self.name = new_name
Output (tested on Python 2.7.10):
>>> a = Thing('A')
>>> a.name
'A'
>>> a.set_name('B')
self.name = A
running function set_name()
self.name = B
>>> a.name
'B'
The example above is silly, but it works.
Here's an expansion on Michael Speer's answer to take it a few steps further:
An instance method decorator which takes arguments and acts on a function with arguments and a return value.
class Test(object):
"Prints if x == y. Throws an error otherwise."
def __init__(self, x):
self.x = x
def _outer_decorator(y):
def _decorator(foo):
def magic(self, *args, **kwargs) :
print("start magic")
if self.x == y:
return foo(self, *args, **kwargs)
else:
raise ValueError("x ({}) != y ({})".format(self.x, y))
print("end magic")
return magic
return _decorator
@_outer_decorator(y=3)
def bar(self, *args, **kwargs) :
print("normal call")
print("args: {}".format(args))
print("kwargs: {}".format(kwargs))
return 27
And then
In [2]:
test = Test(3)
test.bar(
13,
'Test',
q=9,
lollipop=[1,2,3]
)
start magic
normal call
args: (13, 'Test')
kwargs: {'q': 9, 'lollipop': [1, 2, 3]}
Out[2]:
27
In [3]:
test = Test(4)
test.bar(
13,
'Test',
q=9,
lollipop=[1,2,3]
)
start magic
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
<ipython-input-3-576146b3d37e> in <module>()
4 'Test',
5 q=9,
----> 6 lollipop=[1,2,3]
7 )
<ipython-input-1-428f22ac6c9b> in magic(self, *args, **kwargs)
11 return foo(self, *args, **kwargs)
12 else:
---> 13 raise ValueError("x ({}) != y ({})".format(self.x, y))
14 print("end magic")
15 return magic
ValueError: x (4) != y (3)
I found this question while researching a very similar problem. My solution is to split the problem into two parts. First, you need to capture the data that you want to associate with the class methods. In this case, handler_for will associate a Unix command with handler for that command's output.
class OutputAnalysis(object):
"analyze the output of diagnostic commands"
def handler_for(name):
"decorator to associate a function with a command"
def wrapper(func):
func.handler_for = name
return func
return wrapper
# associate mount_p with 'mount_-p.txt'
@handler_for('mount -p')
def mount_p(self, slurped):
pass
Now that we've associated some data with each class method, we need to gather that data and store it in a class attribute.
OutputAnalysis.cmd_handler = {}
for value in OutputAnalysis.__dict__.itervalues():
try:
OutputAnalysis.cmd_handler[value.handler_for] = value
except AttributeError:
pass
I use this type of decorator in some debugging situations, it allows overriding class properties by decorating, without having to find the calling function.
class myclass(object):
def __init__(self):
self.property = "HELLO"
@adecorator(property="GOODBYE")
def method(self):
print self.property
Here is the decorator code
class adecorator (object):
def __init__ (self, *args, **kwargs):
# store arguments passed to the decorator
self.args = args
self.kwargs = kwargs
def __call__(self, func):
def newf(*args, **kwargs):
#the 'self' for a method function is passed as args[0]
slf = args[0]
# replace and store the attributes
saved = {}
for k,v in self.kwargs.items():
if hasattr(slf, k):
saved[k] = getattr(slf,k)
setattr(slf, k, v)
# call the method
ret = func(*args, **kwargs)
#put things back
for k,v in saved.items():
setattr(slf, k, v)
return ret
newf.__doc__ = func.__doc__
return newf
Note: because I've used a class decorator you'll need to use @adecorator() with the brackets on to decorate functions, even if you don't pass any arguments to the decorator class constructor.
The simple way to do it. All you need is to put the decorator method outside the class. You can still use it inside.
def my_decorator(func):
#this is the key line. There's the aditional self parameter
def wrap(self, *args, **kwargs):
# you can use self here as if you were inside the class
return func(self, *args, **kwargs)
return wrap
class Test(object):
@my_decorator
def bar(self):
pass
Declare in inner class. This solution is pretty solid and recommended.
class Test(object):
class Decorators(object):
@staticmethod
def decorator(foo):
def magic(self, *args, **kwargs) :
print("start magic")
foo(self, *args, **kwargs)
print("end magic")
return magic
@Decorators.decorator
def bar( self ) :
print("normal call")
test = Test()
test.bar()
The result:
>>> test = Test()
>>> test.bar()
start magic
normal call
end magic
>>>
Decorators seem better suited to modify the functionality of an entire object (including function objects) versus the functionality of an object method which in general will depend on instance attributes. For example:
def mod_bar(cls):
# returns modified class
def decorate(fcn):
# returns decorated function
def new_fcn(self):
print self.start_str
print fcn(self)
print self.end_str
return new_fcn
cls.bar = decorate(cls.bar)
return cls
@mod_bar
class Test(object):
def __init__(self):
self.start_str = "starting dec"
self.end_str = "ending dec"
def bar(self):
return "bar"
The output is:
>>> import Test
>>> a = Test()
>>> a.bar()
starting dec
bar
ending dec
I have a Implementation of Decorators that Might Help
import functools
import datetime
class Decorator(object):
def __init__(self):
pass
def execution_time(func):
@functools.wraps(func)
def wrap(self, *args, **kwargs):
""" Wrapper Function """
start = datetime.datetime.now()
Tem = func(self, *args, **kwargs)
end = datetime.datetime.now()
print("Exection Time:{}".format(end-start))
return Tem
return wrap
class Test(Decorator):
def __init__(self):
self._MethodName = Test.funca.__name__
@Decorator.execution_time
def funca(self):
print("Running Function : {}".format(self._MethodName))
return True
if __name__ == "__main__":
obj = Test()
data = obj.funca()
print(data)
Use a static method and include an additional parameter (self) in the inner function (wrapper) of the decorator.
class Test:
@staticmethod
def _decorator(f):
@functools.wraps(f)
def _wrapper(self, *args, **kwargs):
# do some serious decorating (incl. calls to self!)
print(self)
return f(self, *args, **kwargs)
return _wrapper
@_decorator
def bar(self):
return 42
You can decorate the decorator:
import decorator
class Test(object):
@decorator.decorator
def _decorator(foo, self):
foo(self)
@_decorator
def bar(self):
pass
For Python 3 and for the linters sake
def methoddecorator(deco: Callable[[Any, Callable], Callable]):
"""
Decorator to implement method decorators in the same class
Example of usage:
class A:
@methoddecorator
def my_methods_deco(self, method):
@wraps(method)
def wrapper(this: 'A', *args, **kwargs):
# do smth
# N.B. for instance access use this, not self!
return method(this, *args, **kwargs)
return wrapper
@my_methods_deco
def my_method(self, a, b):
...
"""
@functools.wraps(deco)
def wrapped_deco(method):
return deco(NotImplemented, method)
return wrapped_deco
Use this uber-decorator to patch the classes.
BTW, this code does not support decorator parameters like @deco(param=...), but more complicated one does.
def methoddecorator(deco):
"""
Decorator to implement method decorators in the same class
Supports optionally parametrized decorators
Example of usage:
class A:
@methoddecorator
def my_methods_deco(self, _method=None, param1=None, param2=None):
@wraps(method)
def wrapper(this: 'A', *args, **kwargs):
# do smth
# deco params are also available here
return method(this, *args, **kwargs)
return wrapper
@my_methods_deco
def my_method1(self, a, b):
...
@my_methods_deco(param1=11, param2=12)
def my_method2(self, a, b):
...
"""
@wraps(deco)
def wrapped_deco(_method=None, **kwargs):
return (
deco(NotImplemented, _method)
if _method is not None
else partial(deco, NotImplemented, **kwargs)
)
return wrapped_deco
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