Function objects are created by function definitions. The only
operation on a function object is to call it:
There are really two flavors of function objects: built-in functions and user-defined functions. Both support the same operation (to call the function), but the implementation is different, hence the different object types.
The implementation adds two special read-only attributes:
f.func_code is a function's code
the dictionary used as the function's global name space (this is the
m.__dict__ where m is the module in which
the function f was defined).