Immunize Python None Attribute Access

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.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: viditkbhatnagar
• Source: viditkbhatnagar/immunize
• License: Apache-2.0
• Homepage: https://pypi.org/project/immunize/

Install and usage instructions live in the source repository linked above.