Rospy Services with method class

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Hi,

How can i use a class method for rospy.Service()?

The .srv file is

int64 a
int64 b
---
bool response

The code in the server.py is

s = rospy.Service('setVelocity', SetVelocity, self.setVelocity)

The signature of the setVelocity method

def setVelocity (self, r, l)

In the client.py i've

s = rospy.ServiceProxy('setVelocity', SetVelocity)
s(3,5)

and i get this error:

error processing request: setVelocity() takes exactly 3 arguments (2 given)

Sorry for my bad english