Search python class variables by value

I have a class with static variables to search for error / status code. Take the HTTP status code as an example

class Foo(object):
    OK = 200
    Not_Modified = 304
    Forbidden = 403
    Internal_Server_Error = 500

Now I need to get the verbal status ('OK', 'Not_Modified', etc.) based on the code (200, 403, etc.). I cannot change the structure of the class, as other programs use it. So I created a dictionary description_by_valcontaining {code : description}:

from collections import Hashable

class Foo(object):
    OK = 200
    Not_Modified = 304
    Forbidden = 403
    Internal_Server_Error = 500
    description_by_val = dict((value, key)
        for key, value in locals().iteritems()
            if not key.startswith("__") and value and isinstance(value, Hashable))


>>> Foo.description_by_val[200]
'OK'

Now I have questions in terms of code performance and practice.

• Every time I call Foo.description_by_val, it will lead to a dictionary regeneration? this is bad, even the data set is very small, because it will cause millions of times.
• , ? , , .

?

:

, - description_by_val, , .

>>> from collections import Hashable
>>> 
>>> def show(key):
...     print key
...     return True
... 
>>> 
>>> class Foo(object):
...     OK = 200
...     Not_Modified = 304
...     Forbidden = 403
...     Internal_Server_Error = 500
...     description_by_val = dict((value, key)
...         for key, value in locals().iteritems()
...             if not key.startswith("__") and key and isinstance(value, Hashable) and show(key))
... 
OK
Forbidden
Internal_Server_Error
Not_Modified
>>> 
>>> Foo.description_by_val
{200: 'OK', 304: 'Not_Modified', 403: 'Forbidden', 500: 'Internal_Server_Error'}
>>> Foo.description_by_val
{200: 'OK', 304: 'Not_Modified', 403: 'Forbidden', 500: 'Internal_Server_Error'}
>>> Foo.description_by_val[200]
'OK'

, . , :)