How can I use a class instance variable as an argument for a method decorator in Python?
Question
How can I use a class instance variable as an argument for a method decorator in Python? The following is a minimal example shows what I'm trying to do. It obviously fails as the decorator function does not have access to the reference to the instance and I have no idea how to get access to the reference from the decorator.
def decorator1(arg1):
def wrapper(function):
print "decorator argument: %s" % arg1
return function
return wrapper
class Foo(object):
def __init__(self, arg1):
self.var1 = arg1
@decorator1(self.var1)
def method1(self):
print "method1"
foo = Foo("abc")
foo.method1()
Solution
It's not going to work; the decorator is called during class creation time, which is long before an instance is created (if that ever happens). So if your "decorator" needs the instance, you have to do the "decorating" at instantiation time:
def get_decorator(arg1):
def my_decorator(function):
print "get_decorator argument: %s" % arg1
return function
return my_decorator
class Foo(object):
def __init__(self, arg1):
self.var1 = arg1
self.method1 = get_decorator(self.var1)(self.method1)
def method1(self):
print "method1"
foo = Foo("abc")
foo.method1()
Note that I changed the function names according to their meanings; the actual "decorator", i.e. the function that (potentially) modifies the method, is wrapper
in your case, not decorator1
.
OTHER TIPS
Your “warper” function is actually a decorator, rather than a warper. Your “decorator1” function is a decorator constructor. If you want to have access to self.var1 in runtime you have to make a warper not decorator:
def decorator(function):
def wrapper(self,*args,**kwargs):
print "Doing something with self.var1==%s" % self.var1
return function(self,*args,**kwargs)
return wrapper
class Foo(object):
def __init__(self, arg1):
self.var1 = arg1
@decorator
def method1(self):
print "method1"
foo = Foo("abc")
foo.method1()
If you want to have more generic decorator, it's better idea to declare a callable class:
class decorator:
def __init__(self,varname):
self.varname = varname
def __call__(self,function):
varname=self.varname
def wrapper(self,*args,**kwargs):
print "Doing something with self.%s==%s" % (varname,getattr(self,varname))
return function(self,*args,**kwargs)
return wrapper
Using:
@decorator("var1")
The decorator is executed when the class is defined, so you can't pass an instance variable to it.
Here's how we used to do this in the olden days.
class Foo(object):
def __init__(self, arg1):
self.var1 = arg1
def method1(self):
self.lock()
try:
self.do_method1()
except Exception:
pass # Might want to log this
finally:
self.unlock()
def do_method1(self):
print "method1"
def lock(self):
print "locking: %s" % self.arg1
def unlock(self):
print "unlocking: %s" % self.arg1
Now, a subclass only needs to o override do_method1
to get the benefits of the "wrapping". Done the old way, without any with
statement.
Yes, it's long-winded. However, it doesn't involve any magic, either.