When indexing a dictionary with a non-existent key, what exception is raised?

When indexing a dictionary using a key that does not exist, a KeyError is raised. This exception specifically indicates that the requested key cannot be found in the dictionary.

Dictionaries in Python are designed to hold key-value pairs, and each key must be unique within its own scope. When you attempt to access a key that is not present, Python does not find a corresponding value to return, which leads to the exception being raised.

For instance, if you have a dictionary defined as my_dict = {'a': 1, 'b': 2}, and you try to access my_dict['c'], since 'c' is not defined in the dictionary, Python raises a KeyError. This behavior is crucial for debugging, as it alerts the programmer that there has been an attempt to access an invalid key in the data structure.