When a client calls a function that expects a parameter, the client must pass a parameter to the function.
The process behind parameter passing in Python is simple: the function call binds to the formal parameter
the object referenced by the actual parameter. The kinds of objects we have considered so far—integers,
floating-point numbers, and strings—are classified as immutable objects. This means a programmer cannot
change the value of the object. For example, the assignment
x = 4
binds the variable named x to the integer 4. We may change x by reassigning it, but we cannot change the
integer 4. Four is always four. Similarly, we may assign a string literal to a variable, as in
word = 'great'
but we cannot change the string object to which word refers. If the client’s actual parameter references
an immutable object, the function’s activity cannot affect the value of the actual parameter.