KeyError

KeyError is a built-in exception in Python that signals a missing key in a mapping. It is raised when attempting to retrieve an item from a mapping using a key that is not present, typically through the __getitem__ method, such as dict[key].

Example: d = {'a': 1}; value = d['b'] raises KeyError: 'b'. The error message includes the missing key in its representation.

KeyError is a subclass of LookupError in the Python exception hierarchy. In common mappings, such as regular dictionaries, a missing key triggers KeyError. In contrast, some mapping types like collections.defaultdict do not raise KeyError for missing keys because they create a default value instead.

Handling a KeyError can be done in several ways. You can use a try/except Block to catch the error and provide a fallback or alternate flow. Alternatively, dict.get(key, default) returns a specified default when the key is not present, avoiding an exception. You can also check for key presence with if key in d before accessing the value.

Notes: KeyError often indicates a logic error where code assumes a key exists without guaranteeing it. Debugging may involve tracing how dictionary keys are produced and populated, and ensuring proper key management and data flow. See also LookupError, dictionaries, and the __getitem__ protocol.