Hello,

Recently as part of the effort of untangling the tests of ElementTree and general code improvements (e.g. http://bugs.python.org/issue15651), I ran into something strange about PEP 3121-compliant modules. I'll demonstrate with csv, just as an example.

PEP 3121 mandates this function to look up the module-specific state in the current sub-interpreter:

  PyObject* PyState_FindModule(struct PyModuleDef*);

This appears to make the following assumption: a given sub-interpreter only imports any C extension *once*. If it happens more than once, the assumption breaks in troubling ways. In normal code, it should never happen more than once because of the caching in sys.modules; However, many of our tests monkey-patch sys.modules (mainly by calling test.support.import_fresh_module) and hell breaks use. Here's a simple example:

----
import sys

csv = __import__('csv')
csv.register_dialect('unixpwd', delimiter=':', quoting=csv.QUOTE_NONE)

print(csv.list_dialects())
# ==> ['unixpwd', 'excel-tab', 'excel', 'unix']

del sys.modules['csv']              # FUN
del sys.modules['_csv']
some_other_csv = __import__('csv')

print(csv.list_dialects())
# ==> ['excel-tab', 'excel', 'unix']
----

Note how doing some sys.modules acrobatics and re-importing suddenly changes the internal state of a previously imported module. This happens because:

1. The first import of 'csv' (which then imports `_csv) creates module-specific state on the heap and associates it with the current sub-interpreter. The list of dialects, amongst other things, is in that state.
2. The 'del's wipe 'csv' and '_csv' from the cache.
3. The second import of 'csv' also creates/initializes a new '_csv' module because it's not in sys.modules. This *replaces* the per-sub-interpreter cached version of the module's state with the clean state of a new module

So essentially, while PEP 3121 moves state from C-file globals to per-module state, the state is still global, and this fact can be exposed from pure Python code.

The above is a toy example. Here's a more serious case I ran into with ET, but once again is demonstrated with 'csv' for simplicity:

----

import io
from test.support import import_fresh_module
 
import csv
 
csv_other = import_fresh_module('csv', fresh=['_csv', 'csv'])
 
f = io.StringIO('foo\x00,bar\nbaz,42')
reader = csv.reader(f)
 
try:
    for row in reader:
        print(row)
except csv.Error as e:
    print('Caught csv.error', e)
except Exception as e:
    print('Caught Exception', e)
----