Stamp Coupling

The name stamp coupling is historical and refers to data structures that are “stamped” and passed around as a whole.

Think of records, structs, objects or data packets. Every time when you pass around class instances, technically you use stamp coupling.

But it is when only a part of the data in the stamp is used, when we call it out, because that is when problems might arise.

class Shape:
····def move(self) -> None:
········print("Move")

····def draw(self) -> None:
········print("Draw")

def move(shape: Shape) -> None:
····shape.move()
····shape.draw()

def redraw(shape: Shape) -> None:  # Too much information.
····shape.draw()                   # Only .draw() is used.

Objects of type Shape are accepted in functions move and redraw. Notice how redraw takes a shape but only uses draw from it.

That is the moment where we call-out there is stamp coupling going on. A solution could be to introduce a protocol class that functions as a interface.

from typing import Protocol

class Movable(Protocol):
····def move(self) -> None: ...
····def draw(self) -> None: ...

class Drawable(Protocol):
····def draw(self) -> None: ...

class Shape:
····def move(self) -> None:
········print("Move")

····def draw(self) -> None:
········print("Draw")

def move(shape: Movable) -> None:  # Exactly needed
····shape.move()
····shape.draw()

def redraw(shape: Drawable) -> None:  # Exactly needed
····shape.draw()

Yes, it introduces extra code but the coupling has become smaller. There is no risk of calling wrong methods in the functions anymore.