Semantic and documentation of Interpreter.prepare_main()

Interpreter.prepare_main() is documented as:

   .. method:: prepare_main(ns=None, **kwargs)
      
      Bind objects in the interpreter's :mod:`!__main__` module.
   
      Some objects are actually shared and some are copied efficiently,
      but most are copied via :mod:`pickle`.  See :ref:`interp-object-sharing`.

But it is not clear what ns is, and how kwargs is related. A module and a class are nemspaces, so can you pass them? Or the SimpleNamespace instance? No. Looking at the code, ns can only be None, a mapping or an iterable of pairs. Is it what was intended? Than it should be documented.

But accepting an iterable of pairs may be not intended. This can hardly be called a namespace. Then the code should be changed from

ns = dict(ns, **kwargs) if ns is not None else kwargs

to

ns = {**ns, **kwargs} if ns is not None else kwargs

And was it necessary to accept None? This is error-prone, as you can accidentally get None instead of a valid value (missing return or missing initialization). {} could be used as a default value, it is fine if it is not mutated. If exclude None, it can be simply written as:

ns = {**ns, **kwargs}

And I suggest to to change the name of the parameter, as it is not a general namespace. "dict" or "mapping" would better reflect its type. "updates" or "changes" could also be used, but then we should also rename "kwargs" to "updates2" or "changes2". Since this is positional-only parameter, changing the name does not break user code.

cc @ericsnowcurrently

从 Interpreter.prepare_main() 的文档及其实现入手，尤其要弄清 ns 和 kwargs 是如何组合的。确定预期接受的输入类型，以及是否应更改参数名称或行为，然后更新文档或相关代码，并添加或调整相关测试，使 API 语义明确。