Push Down Method

Push Down Method moves a method from a base class into the subclasses that actually need it. The base class stops promising a capability that does not belong to every subtype. 1

Original version: even the scanner inherits printing

public abstract class Device
{
    public string Name { get; } = "device";
    public string Print() { return "printed"; }
}
public sealed class Printer : Device { }
public sealed class Scanner : Device { }

class Device:
    name = "device"

    def print_page(self):
        return "printed"


class Printer(Device):
    pass


class Scanner(Device):
    pass

These return strings as teaching stand-ins rather than controlling hardware. The structural mistake is visible anyway: every device exposes printing, including a scanner that has no such capability in this model.

Revised version: printing belongs to printers

public abstract class Device
{
    public string Name { get; } = "device";
}
public sealed class Printer : Device
{
    public string Print() { return "printed"; }
}
public sealed class Scanner : Device { }

class Device:
    name = "device"


class Printer(Device):
    def print_page(self):
        return "printed"


class Scanner(Device):
    pass

Printer calls still return the same result. The scanner no longer advertises the irrelevant operation. Common device data remains in the base class.

Removing a false promise is still an API change

A caller that accepted any Device and called printing will no longer compile in C#, and a Python caller making the same assumption may fail at runtime. Change that caller to require the actual printing capability, perhaps through a focused interface, rather than reintroducing type checks everywhere.

If several subclasses share the operation, a more specific intermediate base class or composed helper may be useful. Duplicating the method into many unrelated subclasses can signal that inheritance is the wrong way to organize the capability.

The domain decides what makes sense. Some physical devices both print and scan; this particular model defines separate capabilities. Do not infer software contracts solely from a familiar noun or preserve an inappropriate base method just to reuse its implementation.

Sources

1. Martin Fowler — Push Down Method.