Return from __call__ immediately if a value gets changed in the class

In contrast to my other answer @Copperfield had the suggestion of using the Python @property decorators to do this, which could be handy for some applications. This saves you from having to changing the code where you assign self.a = 1, and it will just "automatically" notice whenever you happen to make such an assignment. It's a bit more "under the hood", i.e. less transparent to readers of the code, but could still be useful:

class Property:

    def __init__(self):
        ## initializing the attribute (Note change to _a, not a)
        self._a = 0
    def __call__(self, val):
        try:
            self.func1(val)
            self.func2(val)
            self.func3(val)
        except MyCustomException:
            return
    def func1(self,value):
        if value==20:
            self.a = 1
        print("all okay in func1")
    def func2(self,value):
        if value==40:
           self.a = 1
        print("all okay in func2")
    def func3(self,value):
        if value==60:
           self.a = 1
        print("all okay in func3")

    @property
    def a(self):
        return self._a

    @a.setter
    def a(self, new_value):
        self._a = new_value
        if self._a == 1:
            throw MyCustomException

What is happening here is that we have specified a as a property. On the surface it works just like a normal member variable in how you assign and access it, however with the special @a.setter decorator we can define a custom function that runs whenever you set its value. So here we can make it check what value is being assigned, and throw a MyCustomException if that value is 1, which we can then catch after skipping the functions that you didn't want to run.

How's this?

class Property:

    def __init__(self):
        ## initializing the attribute
        self.a = 0
    def set_a_to_1_and_raise(self):
        self.a = 1
        throw MyCustomException 
    def __call__(self, val):
        try:
            self.func1(val)
            self.func2(val)
            self.func3(val)
        except MyCustomException:
            return
    def func1(self,value):
        if value==20:
            self.set_a_to_1_and_raise()
        print("all okay in func1")
    def func2(self,value):
        if value==40:
           self.set_a_to_1_and_raise()
        print("all okay in func2")
    def func3(self,value):
        if value==60:
           self.set_a_to_1_and_raise()
        print("all okay in func3")

Edit: Added @tdelaney's nice suggestion regarding the name of the function self.set_a_to_1_and_raise() to make it obvious that it raises an exception when you call it.