Extract of _PySys_EndInit():

SET_SYS_FROM_STRING_BORROW("path", config->module_search_path);

sys.path is initialized from _PyMainInterpreterConfig.module_search_path object: the list of strings is not copied. As a consequence, when sys.path is modified, _PyMainInterpreterConfig is modified as well. For example, "import site" modifies sys.path.

I dislike this behavior. I prefer to see _PyMainInterpreterConfig as "constant": a snapshot of the configuration used to startup Python.

A side effect is that Py_NewInterpreter() copies the modified configuration, whereas starting from the "original" configuration. For the specific case of Py_NewInterpreter(), I'm not sure of what is the expected behavior... Py_NewInterpreter() imports the "site" module again, so it should reapply the same sys.path change.

Note: I found this bug while working on better tests for global, core and main configurations: https://github.com/python/cpython/pull/10524

Attached PR 10532 modifies _PySys_EndInit() to copy lists and dictionaries from the config.

Python has 3 kind of configurations:

* global configuration variables like Py_VerboseFlag
* core configuration: _PyCoreConfig
* main interpreter configuration: _PyMainInterpreterConfig

I tried to keep them consistency. Yesterday, I rewrote test_embed.InitConfigTests to really test that these 3 configurations are consistent... And I found multiple bugs :-) (I fixed them as well)

sys.flags is immutable, but some configurations are only used to "initialize" Python which can then be modified. sys.path is a good example.

I don't think that we can ensure that sys.path is always consistent with the main/core configuration (module_search_paths). It's possible to write "sys.path = ['/new/path']". There is no machinery at the module level to call a function when a sys module is *replaced*.

I propose to try to ensure that the configuration is not modified during Python lifecycle, and so that sys.path is a *copy* of the configuration. Same rationale for sys.warnoptions (list) and sys._xoptions (dict).

Attached PR 10532 implements this solution: copy lists and dicts.

New changeset 37cd982df02795905886ab36a2378ed557cb6f60 by Victor Stinner in branch 'master':
bpo-35239: _PySys_EndInit() copies module_search_path (GH-10532)
https://github.com/python/cpython/commit/37cd982df02795905886ab36a2378ed557cb6f60

New changeset d2be9a5c13221fb84c2221bbfd93efac6111e697 by Miss Islington (bot) in branch '3.7':
bpo-35239: _PySys_EndInit() copies module_search_path (GH-10532)
https://github.com/python/cpython/commit/d2be9a5c13221fb84c2221bbfd93efac6111e697

New changeset a5194115733f6ca8fc1ddbee43eabbde536900e6 by Victor Stinner in branch '3.7':
Revert "bpo-35239: _PySys_EndInit() copies module_search_path (GH-10532)" (GH-10660)
https://github.com/python/cpython/commit/a5194115733f6ca8fc1ddbee43eabbde536900e6

I has been decided to not backport the change to Python 3.7: see PR 10532 discusssion.