Interface Segregation Principle

Don’t force clients to depend on methods they don’t use.

Intent

Prefer many small interfaces over one fat interface. A print-only device should only know about printing.

Problem

Machine demands print, fax, and scan. An old printer can only print. It still must stub fax and raise on scan — a lie forced by the base type.

Solution

Split into Printer, Scanner, and Fax. Implement only what you need. A multi-function device composes the small roles.

Printer+Scanner+Fax→MultiFunctionDevice

Structure

Piece	Role
Machine / OldFashionedPrinter	Fat interface (violation)
Printer / Scanner / Fax	Segregated interfaces
*Impl classes	Focused implementations
MultiFunctionDeviceImpl	Delegates to _printer, _scanner, _fax

Compare code

Violation — fat Machine

class OldFashionedPrinter(Machine):
    def print(self, document): ...
    def fax(self, document): pass  # forced
    def scan(self, document):
        raise NotImplementedError  # forced

Compliant — small interfaces + compose

class PrinterImpl(Printer):
    def print(self, document):
        print(document)

class MultiFunctionDeviceImpl(MultiFunctionDevice):
    def __init__(self, printer, scanner, fax):
        self._printer = printer
        self._scanner = scanner
        self._fax = fax

    def fax(self, document):
        self._fax.fax(document)

Walkthrough

OldFashionedPrinter inherits Machine and must deal with fax/scan even though it cannot do them.

Printer, Scanner, and Fax each declare one concern. Implementations only take what they need.

MultiFunctionDeviceImpl delegates to injected collaborators. Underscored fields avoid shadowing the methods.

Quick check

Which design follows ISP?

Run locally

uv run isp

Expected:

Hello, world!
Scanning Hello, world!
Faxing Hello, world!

Source: main.py · tutorial README

Takeaway

• Fat interfaces force stub methods and lies
• Split by client need
• Compose multi-capability types from small roles