LookupError

LookupError is a built-in exception class in Python that represents failures when a key or index lookup on a container does not succeed. It serves as a general category for errors related to retrieving items from mappings or sequences.

In the Python exception hierarchy, LookupError inherits from Exception, and has two common direct subclasses: KeyError and IndexError. KeyError is raised when a mapping key is not found, while IndexError occurs when indexing or slicing a sequence goes out of range.

Examples ofLookupError situations include retrieving an absent dictionary key or accessing an invalid list index. For instance, d = {'x': 1}; d['y'] raises KeyError, and lst = [0, 1, 2]; lst[5] raises IndexError. These exceptions are typically thrown by the underlying container operations such as __getitem__.

Usage-wise, code can catch LookupError to handle all lookup-related failures in a uniform way, but this is broader than necessary. Most code handles KeyError and IndexError separately to respond to the specific context—missing keys versus out-of-range indices.

Notes: LookupError is part of the standard Python exception set and is primarily intended for built-in container lookups. Membership tests using the in operator do not raise LookupError when an element is not present; they return False instead. Custom data structures may implement their own lookup logic and may raise LookupError if appropriate.

See also: KeyError, IndexError.