Example:
class Bird:
def __init__(self):
self.sound = "chirp!"
def reproduce_sound(self):
if self:
print(self.sound)
bird = Bird()
bird.reproduce_sound()
What does if self: mean? What is the case where the reproduce_sound function call prints nothing?
It checks the truth value of the instance and only prints if it is True. In your example the check doesn't do anything useful and will always print something. You can override the __bool__ method to change its default behaviour.
For example:
class Bird:
...
def __bool__(self):
return bool(self.sound)
Then:
b = Bird()
b.reproduce_sound() # Prints "chirp!"
b.sound = 0 # or any falsy value, such as None or ""
b.reproduce_sound() # Won't print anything because b == False
I have simple task: we have class TestingClass and we wanna simply test his methods. And we have class TestHelper for testing purpose.
class TestingClass():
def testing_method_1(self):
return False
def testing_method_2(self):
return True
class TestHelper():
def __init__(self, class_name):
pass
def add(self, func_name, func_result):
pass
def run(self):
pass
helper = TestHelper(TestingClass)
helper.add(“testing_method_1”, False)
helper.add(“testing_method_2”, True)
result = helper.run()
my target implement methods for class TestHelper . I solved it for myself but I think its bad realization. I initialize empty dictionary and test class. Next I add every method and test method result to dictionary. In run method I compare results.
class TestHelper():
def __init__(self, class_name):
self.test_class = class_name()
self.func_list = {}
def add(self, func_name, func_result):
temp = 'self.test_class.' + func_name
self.func_list[temp] = func_result
def run(self):
for f in self.func_list.keys():
if eval(f)() == self.func_list[f]:
print('yes')
else:
print('no')
Can I resolve it in best way? Thanks!
Using eval for this purpose is overkill.
You could instead use the getattr() function to retrieve the function and call it.
Read more: Python Docs
class TestHelper():
def __init__(self, class_name):
self.test_class = class_name()
self.func_list = {}
def add(self, func_name, func_result):
# { <function>:<desired result> }
self.func_list[func_name] = func_result
def run(self):
for func, desired_result in self.func_list.items():
if getattr(self.test_class, func)() is desired_result:
print('yes')
else:
print('no')
This code produces results:
...
>>> helper = TestHelper(TestingClass)
>>> helper.add("testing_method_1", False)
>>> helper.add("testing_method_2", True)
>>> result = helper.run()
yes
yes
Of course you should also test if the class even has an attribute with the given function name. You can use hasattr() for this.
target code:
test_box = TestBox(Checkers.begin_with("op") and Checkers.end_with("or"))
test_box.run("operator") # True
test_box.run("option") # False
What I think(may be in the wrong way): TestBox may be a class or a method, and begin_with and end_with are classmethod of class Checkers, but how to make Checkers.begin_with("op") and Checkers.end_with("or") evaluated until test_box.run() was called?
I have found a solution which use lambda, post here:
class TestBox:
def __init__(self, func):
self._func = func
def run(self, string):
Checkers.string = string
return self._func()
class Checkers:
string = None
#classmethod
def begin_with(cls, val):
return cls.string.startswith(val)
#classmethod
def end_with(cls, val):
return cls.string.endswith(val)
if __name__ == '__main__':
test_box = TestBox(lambda: Checkers.begin_with("op") or Checkers.end_with("or"))
print(test_box.run("operator")) # True
print(test_box.run("xxtion")) # False
print(test_box.run("xxtionor")) # True
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
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()
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)
You can decorate the decorator:
import decorator
class Test(object):
#decorator.decorator
def _decorator(foo, self):
foo(self)
#_decorator
def bar(self):
pass
I'm at an intermediate level with Python, and I've recently been playing around with Python context managers. I wanted to invert the order in which the enter and exit are run. So I wrote this context manager:
class ReversibleContextManager(object):
def __enter__(self, *args):
print('d')
return self
def __exit__(self, *args):
print('g')
return self
def __invert__(self):
self.__enter__, self.__exit__ = self.__exit__, self.__enter__
return self
It works fine forwards:
with ContextManager():
print('o')
d
o
g
But in reverse, we still get:
with ~ContextManager():
print('a')
d
o
g
If we call the enter and exit functions explicitly, as expected, we get:
with ReversibleContextManager() as c:
c.__enter__()
print('o')
c.__exit__()
d
d
o
g
g
BUT the order IS reversed for the instance's method!
with ~ReversibleContextManager() as c:
c.__enter__()
print('o')
c.__exit__()
d
g
o
d
g
So it looks like the with statement calls using the method bound to the Class rather than the instance (is this the right terminology?). Is this expected?
i.e.
what is called is:
c = ReversibleContextManager()
c.__invert__()
ReversibleContextManager.__enter__(c)
...in context...
ReversibleContextManager.__exit__(c)
Rather than what I expect:
c = ReversibleContextManager()
c.__invert__()
c.__enter__()
...in context...
c.__exit__()
So it looks like the with statement calls using the method bound to the Class rather than the instance (is this the right terminology?). Is this expected?
Absolutely. That's how Python generally looks up special methods. This is mostly so in cases like if you have a class Foo that implements __str__, print Foo calls type(Foo).__str__ instead of Foo.__str__.
As a workaround, you could create your reversed class inside the __invert__ function and return an instance of the new class.
class ReversibleContextManager(object):
def __enter__(self, *args):
print('enter')
return self
def __exit__(self, *args):
print('exit')
return self
def __invert__(self):
new = type("ReversibleContextManager",
(object,),
{'__enter__': self.__exit__,
'__exit__': self.__enter__})
return new()
>>> with ReversibleContextManager() as f:
... print("normal")
enter
normal
exit
>>> with ~ReversibleContextManager() as f:
... print("reversed")
exit
reversed
enter
A more verbose but pretty explicit way is to use two helper functions and just switch which one comes at enter and exit by a flag reverse:
class ReversibleContextManager(object):
def __init__(self, reverse=False):
self.reverse = reverse
def _enter(self, *args):
print('d')
return self
def _exit(self, *args):
print('g')
return self
def __enter__(self, *args):
if self.reverse:
return self._exit(*args)
return self._enter(*args)
def __exit__(self, *args):
if self.reverse:
return self._enter(*args)
return self._exit(*args)
def __invert__(self):
self.reverse = True
return self
>>> with ReversibleContextManager() as r:
print('o')
d
o
g
>>> with ~ReversibleContextManager() as r:
print('o')
g
o
d
>>> with ReversibleContextManager(reverse=True) as r:
print('o')
g
o
d
Another way:
class ReversibleContextManager(object):
inverted = False
def __init__(self):
if self.__class__.inverted:
self.__invert__()
self.__class__.inverted = False
def __enter__(self, *args):
print('d')
return self
def __exit__(self, *args):
print('g')
return self
def __invert__(self):
self.__class__.inverted = True
self.__class__.__enter__, self.__class__.__exit__ = self.__exit__, self.__enter__
return self