What does the error "AttributeError: 'Solution' object has no attribute 'alphanum'" mean?

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The error "AttributeError: 'Solution' object has no attribute 'alphanum'" means that in the code, there is an attempt to access the attribute 'alphanum' of an object of the class 'Solution', but this attribute is not defined or has not been initialized in the class or object. The name 'alphanum' is not recognized as a valid attribute of the object.