Dunder Methods and Composition

Scenario: Objects need readable logs and equality, while a deployment should contain services rather than become a service.

New words, in plain English

• Dunder method - a special double-underscore hook used by Python syntax
• Composition - building an object from other objects - a has-a relationship
• Equality - whether values should count as equivalent

__str__ controls friendly text, __repr__ debugging text, __eq__ equality, and __len__ length. Return NotImplemented for unsupported comparison types. Prefer composition for has-a relationships.

Analogy: A Python program is like a clear set of instructions for a careful helper: names label things, indentation groups steps, and errors explain where the helper became confused.

A small, real example

class Service:
    def __init__(self, name): self.name = name
    def __str__(self): return self.name
    def __eq__(self, other):
        if not isinstance(other, Service): return NotImplemented
        return self.name == other.name

Tip: Implement special methods only when their normal Python meaning fits naturally.

Goal: Practice this idea in the py-oop-inheritance challenge at /labs/python. Fix the broken program, run it yourself, then use check to prove the real end state.