How a module is being marked as imported?

[Jean-Charles Lefebvre]

Hi all,

The short version: How CPython marks a module as being fully imported, if it does, so that the same import statement ran from another C thread at the same time does not collide? Or, reversely, does not think the module is not already fully imported?

The full version: I'm running CPython 3.5.1, embedded into a C++ application on Windows. The application is heavily multi-threaded so several C threads call some Python code at the same time (different Python modules), sharing interpreter's resources by acquiring/releasing the GIL frequently DURING the calls, at language boundaries.

Sometimes (but always only once per application instance), a call to os.path.expandvars raises the AttributeError exception with message: module 'string' has no attribute 'ascii_letters'. It is raised by the ntpath.expandvars function (line 372). When I noticed the late import statement of the 'string' module at the line above, I thought that MAYBE, it could be because the interpreter is ran in an heavily multi-threaded environment and that the GIL acquiring/releasing occurred at a bad timing? Making me wonder how the import mechanism interacts with the GIL, if it does?