Python none attribute access

Skill viditkbhatnagar/immunize/src/immunize/patterns/python-none-attribute-access

A curated pattern library that stops AI coding assistants from repeating common runtime errors. No API key. No LLM calls at runtime.

Install

npx -y skills add viditkbhatnagar/immunize --skill python-none-attribute-access

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What its author says it does

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Use when writing Python that calls a function returning an Optional value (e.g., dict.get, re.search) before accessing attributes on the result.

SKILL.md

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python-none-attribute-access

Before dereferencing an Optional, check for None. dict.get(key) returns None when the key is missing; re.search(...) returns None on no match; many ORMs return None for "not found." Accessing any attribute on that None raises:

AttributeError: 'NoneType' object has no attribute '...'

Type checkers flag this, but runtime code often slips through — the AttributeError only surfaces on the unlucky input.

Example

Wrong — crashes when user_id is not in the registry:

def lookup_display_name(registry: dict[str, User], user_id: str) -> str:
    user = registry.get(user_id)
    return user.name

Right — handle the None case explicitly:

def lookup_display_name(registry: dict[str, User], user_id: str) -> str:
    user = registry.get(user_id)
    if user is None:
        return "unknown"
    return user.name

Don't catch AttributeError

Wrapping the access in try: ... except AttributeError: hides real bugs — a typo in an attribute name becomes silent. The correct fix is an explicit None check, a sentinel default, or failing fast with a clearer error.

Sentinel alternatives

dict.get(key, default) — pass a fallback directly.
if key not in registry: — fail fast with a specific error message.
registry.setdefault(key, default_user) — ensure the slot exists.

Pick the option whose semantics match what the caller expects to see when the value is missing.