[Python-Dev] cpython: locals dictionary in PyRun_String

[Matthew Keeter]

One more data point:
On the Python side, exec has documentation (https://docs.python.org/3/library/functions.html#exec)
that nicely reflects what’s going on in the frame code (where globals must be a dict but locals can be
any mapping object).

I’ll file a bug to see what people think about loosening the restrictions in the C API (which will make it
match exec’s documented restrictions).

Thanks,
Matt

On Jul 14, 2015, at 2:47 PM, Brett Cannon <brett at python.org> wrote:

> 
> 
> On Tue, Jul 14, 2015 at 11:39 AM Matthew Keeter <matt.j.keeter at gmail.com> wrote:
> The docs for PyRun_String say that both globals and locals should be dictionaries [1].
> 
> However, digging into the source [2] shows me that locals doesn’t need to be a dictionary;
> it just needs to implement the mapping protocol.  Is it a bad idea to rely on this fact?
> 
> (Context: I’m plugging a custom object into locals that uses __getitem__ to track lookups.)
> 
> As you pointed out in the code, that's in the frame creation code and not directly the PyRun_StringFlags code, so technically there is a chance for us to insert a PyDict_CheckExact() call before hitting the code you linked to.
> 
> As to whether we could loosen the documented restrictions so they are guaranteed, it would be best to file an issue at bugs.python.org requesting the restriction be officially loosened and if people are amenable then a test to make sure no one accidentally breaks the API promise. 

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