init and Instance State

Use __init__ to define invariants, validate inputs early, and keep instance state consistent across all code paths.

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init Sets Invariants

Constructor validation prevents bad state from entering your system. Fail early so errors are close to the source.

Validate on Construction

class TimeoutConfig:
    def __init__(self, timeout_seconds: int) -> None:
        if timeout_seconds <= 0:
            raise ValueError("timeout_seconds must be > 0")
        self.timeout_seconds = timeout_seconds

Prefer Immutable-Like State

When possible, keep objects stable after construction and provide explicit methods for changes.

Operational Checklist

Validate required parameters in __init__.
Document expected units (seconds vs milliseconds).
Initialize all fields; avoid creating attributes later ad-hoc.

Failure Modes

Partially initialized objects: attributes appear later and cause runtime surprises.
Late validation: invalid state travels far before crashing.

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