This module defines an object type which can compactly represent an array of basic values: characters, integers, floating point numbers. Arrays are sequence types and behave very much like lists, except that the type of objects stored in them is constrained. The type is specified at object creation time by using a type code, which is a single character.
You can’t put strings in an array that is restricted to only containing unsigned char values (which is what 'B' indicates).