How to call a method of a class from another class?

Question

I want to call a method of a class from another class. I'd like to do something like this.

class A:
 def meth(self):
 B.meth1()
class B:
 def meth1(self):
 pass
a = A()
a.meth()

I get the following error :

TypeError: unbound method meth1() must be called with B instance as first argument (got nothing instead)

What am I doing wrong?

Update:

The example above is probably a bit vague. Here's what I exactly intend to do :

class A:
 def meth(self, data):
 if data[0] == '/':
 B.handleCOMMAND(data)
class B:
 def handleCOMMAND(self, data):
 """
 Some code to handle some command.
 """

Update 2:

class A:
 def meth(self, data):
 if data[0] == '/':
 B.handleCOMMAND(data)
class B:
 def __init__(self):
 self.state = 'CHAT'
 def handleCOMMAND(self, data):
 if data.strip().split()[1] == 'send':
 self.state == 'RECV-FILE'

The main problem I am facing is that :

'self.state' is an instance variable in class B. And depending on what 'data' meth() of class A gets, handleCOMMAND of class B needs to be invoked which is supposed to alter the value of 'self.state'

• BrenBarn: What do you want the effect of that to be? In general a B method needs a B instance to work, but if you're calling from A, you don't have a B instance. (If B.meth1 doesn't need an instance, make it a classmethod instead. If it doesn't even need the class, take it out of the class and make it a function.)
• Ryan Saxe: if you just want to be able to use one of class B methods for class A, you should use inheritance for this. Is this what you want?
• mgilson: Maybe you want B.meth1 to be a staticmethod? Without more details, it's really hard to know anything for sure.
• abarnert: @RyanSaxe: That might be the answer, but it's certainly not guaranteed, and probably not even the most likely possibility. (As mgilson says, without knowing his design, it's impossible to be sure.)
• Ryan Saxe: This is very true, which is why I asked if that's what he wanted...But the way he asks it it seems as though he wants to be able to call methods from class B in class A, which is inheritance. What do you mean by most likely possibility? possibility of what he is asking?

Answer 1

You can't call an instance method unless you have an instance to call it on.

Without knowing what your design goals are, it's impossible to tell you where the instance should come from, but there are two particularly common patterns.

First, the method might take a B instance as a parameter. For example:

class A:
 def meth(self, b):
 b.meth1()
class B:
 def meth1(self):
 pass
a = A()
b = B()
a.meth(b)

To make this more concrete, imagine that A is a Car, and B is a TollRoad, and my_car.drive(my_road) has to call my_road.pay_toll().

---

Alternatively, each A instance may own a B instance as a member. For example:

class A:
 def __init__(self, b):
 self.b = b
 def meth(self):
 self.b.meth1()
class B:
 def meth1(self):
 pass
b = B() 
a = A(b)
a.meth()

To make this more concrete, imagine that A is a Car again, and B is a ServiceCenter, and my_car.schedule_service() needs to call my_car.service_center.get_next_appointment().

---

As a variation to the latter, each B instance may be a sort of part-of-the-whole of an A instance, constructed automatically by the A instance. For example:

class A:
 def __init__(self):
 self.b = B()
 def meth(self):
 self.b.meth1()
class B:
 def meth1(self):
 pass
a = A()
a.meth()

For this one, imagine that A is a Car again, and B is an Engine. So, my_car.drive() starts off calling my_car.engine.start(). But, unlike a ServiceCenter, which is a thing that lives on its own and is used by many cars, an Engine is only used by the car it's part of, so there's no good reason to create the engine separately and pass it to the car (unless you're modeling a car factory).

Answer 2

B is being used as a static class.

class A:
 def meth(self):
 B.meth1()
class B:
 @staticmethod
 def meth1():
 pass
a = A()
a.meth()

• abarnert: I'm not sure why this got upvoted, since there are two earlier answers that say the exact same thing, with more explanation. But, as with those two, this is probably not what the OP wants—or, if it is, he probably shouldn't want it.

Answer 3

class A:
 def meth(self):
 "create instance of B class"
 b = B()
 b.meth1()

Also you can declare B.meth1() as static:

class B:
 @staticmethod
 def meth1():
 pass

Then you can call it like this:

class A:
 def meth(self):
 "create instance of B class"
 B.meth1()

• abarnert: This does get rid of the error, but it's probably not what the OP actually wants to do.
• hek2mgl: Check my update. I've mentioned the static approach as well now
• abarnert: And that's even less likely to be what he wants. There are not many good uses for @staticmethods. (Of course there are _some_—but it's uncommon enough that if a novice thinks he wants a staticmethod he probably wants to rethink his design.)

Answer 4

Here B.meth1 is an instance method, and you are attempting to use it as a class method. You can either re-define B.meth1 to be a classmethod:

class B:
 @classmethod
 def meth1(cls):
 pass

Or, you can call it as an instance method by instantiating the B class:

class A:
 def meth(self):
 B().meth1()

• abarnert: This is pretty much the same as hek2mgl's answer, and it has the same problems: It will get rid of the error, but it probably won't do what the OP is hoping to do (unless he's got an unusual use case, or a bad design).