[Python-ideas] making a module callable

[Haoyi Li]

> Are you serious that replacing __main__ is a bad idea? If so, can you
explain why?

It also doesn't work, last I tried =( Strange things start breaking when
you replace __main__


On Fri, Nov 22, 2013 at 9:48 PM, Eric Snow <ericsnowcurrently at gmail.com>wrote:

> On Fri, Nov 22, 2013 at 9:07 PM, Steven D'Aprano <steve at pearwood.info>
> wrote:
> > On Fri, Nov 22, 2013 at 05:26:33PM -0700, Eric Snow wrote:
> >
> >> It's relevant because no one should be replacing __main__ in
> sys.modules. :)
> >
> > I see your smiley, which confuses me. Are you serious that replacing
> > __main__ is a bad idea? If so, can you explain why?
>
> I do think it's a bad idea.  It would be like replacing a builtin
> module in sys.modules, which is inadvisable (particularly key ones
> like sys).  Like builtins, __main__ is a special module, one created
> during interpreter startup.  It plays a special part in the REPL.
> Various parts of the stdlib have special-casing for __main__, which
> could be affected by replacement.  Replacing __main__ in sys.modules
> is, to me, just as inadvisable as replacing sys.
>
> The catch is that a script is exec'ed into the __main__ module's
> namespace, so during execution (nearly) all the import-related
> attributes relate to __main__.  In contrast, the equivalent module
> from the same file would be loaded into its own namespace, with its
> own import-related attributes, and cached independently at
> sys.modules[module_name].  This duality causes all sorts of grief (PEP
> 395 is a response to some of the pain points).  A key hangup is that
> __name__ is different depending on run-as-script or
> imported-as-module.
>
> That brings us back to the idea of a more formal
> replace-module-in-sys-modules API.  Any solution to that which uses
> __name__ to determine the module's name has to take into account that
> it may have been run as a script (where __name__ will be "__main__").
> If we simply used __name__ staight up we might end up replacing
> __main__ in sys.modules, which I suggest is a bad idea.  Hence the
> point of special-casing __main__.
>
> Sorry I wasn't clear.  Hopefully this was more so.
>
> -eric
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