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Python exceptions

Every built-in exception, from KeyError to ExceptionGroup: what raises it, the exact message Python prints, and how to handle it. LIVE entries have an in-browser Trigger / Raise / Handle demo.

Something you asked for is not there: a key, an index, an attribute or a variable.

AttributeError(*args, name=None, obj=None)

Raised when an attribute reference or assignment fails — obj.x where obj has no x.

IndexError(*args)

Raised when a sequence index (list, tuple, str, range, bytes) is outside the valid range.

KeyError(*args)

Raised when a dict (or other mapping) lookup uses a key that is not there.

NameError(*args, name=None)

Raised when a bare name is used that is not defined in any enclosing scope.

UnboundLocalError

UnboundLocalError(*args)

Raised when a function reads a local variable before any value has been assigned to it.