Adding to a module's __dict__?

Steve Holden steve at
Tue Mar 2 14:33:11 CET 2010

Roy Smith wrote:
> In article <mailman.96.1267508316.23598.python-list at>,
>  Chris Rebert <clp2 at> wrote:
>> On Mon, Mar 1, 2010 at 8:27 PM, Roy Smith <roy at> wrote:
>>> >From inside a module, I want to add a key-value pair to the module's
>>> __dict__. Â I know I can just do:
>>> FOO = 'bar'
>>> at the module top-level, but I've got 'FOO' as a string and what I
>>> really need to do is
>>> __dict__['Foo'] = 'bar'
>>> When I do that, I get "NameError: name '__dict__' is not defined". Â Is
>>> it possible to do what I'm trying to do?
>> Yes; just modify the dict returned by the globals() built-in function
>> instead.
> Ah, cool.  Thanks.
>> It's usually not wise to do this and is better to use a
>> separate dict instead, but I'll assume you know what you're doing and
>> have good reasons to disregard the standard advice due to your
>> use-case.
> Why is it unwise?
> The use case is I'm importing a bunch of #define constants from a C header 
> file.  I've got triples that I want to associate; the constant name, the 
> value, and a string describing it.  The idea is I want to put in the 
> beginning of the module:
> declare('XYZ_FOO', 0, "The foo property")
> declare('XYZ_BAR', 1, "The bar property")
> declare('XYZ_BAZ', 2, "reserved for future use")
> and so on.  I'm going to have hundreds of these, so ease of use, ease of 
> maintenance, and niceness of presentation are important.
> My declare() function will not just set XYZ_FOO = 1 at module global scope, 
> but also insert entries in a variety of dicts so I can look up the 
> description string, map from a value back to the constant name, etc.
> I *could* do this in a separate dict, but the notational convenience of 
> being able to have the original constant names globally available is pretty 
> important.
And how important is it to make sure that whatever data your program
processes doesn't overwrite the actual variable names you want to use to
program the processing?

If you use this technique you are effectively making your program a
hostage to fortune, as you no longer control the namespace you are using
for the programming.

Steve Holden           +1 571 484 6266   +1 800 494 3119
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