New submission from Paul Ganssle <p.gans...@gmail.com>:
It seems that test.support.import_fresh_module gets tripped up with its module
blocking when you attempt to get a fresh copy of a submodule of a module where
you are also importing the module that you are trying to block (bit of a doozy
of a sentence there...). So, for example, with the following configuration in
mymodule/__init__.py:
from .other import other
try:
from ._a import attr
except ImportError:
from ._b import attr
(Assuming _a.attr = "A" and _b.attr = "B"), if you attempt to do:
m = test.support.import_fresh_module("mymodule",
fresh=("mymodule._other",), blocked=("mymodule._a"))
Then you'll find that m.attr is pulled from _a.attr. Here's a small script to
demonstrate:
from test.support import import_fresh_module
import sys
def import_ab(fresh_other):
fresh = ("mymodule._other", ) if fresh_other else ()
mods_out = []
for to_block in "_b", "_a":
blocked = (f"mymodule.{to_block}",)
mods_out.append(import_fresh_module("mymodule",
fresh=fresh, blocked=blocked))
return mods_out
for fresh_other in [True, False]:
mymodule_a, mymodule_b = import_ab(fresh_other)
qualifier = "With" if fresh_other else "Without"
print(f"{qualifier} a fresh import of mymodule._other")
print(f"a: {mymodule_a.attr}")
print(f"b: {mymodule_b.attr}")
print()
When you run it with a suitably configured module on Python 3.8:
$ python importer.py
With a fresh import of mymodule._other
a: A
b: A
Without a fresh import of mymodule._other
a: A
b: B
It also happens if you add `mymodule._a` or `mymodule._b` to the fresh list
when you are trying to block the other one.
I *think* the problem is that in the step where _save_and_remove_module is
called on fresh_name (see here:
https://github.com/python/cpython/blob/76db37b1d37a9daadd9e5b320f2d5a53cd1352ec/Lib/test/support/__init__.py#L328-L329),
it's necessarily populating `sys.modules` with a fresh import of the top-level
module we're trying to import (mymodule) *before* the blocking goes into
effect, then the final call to importlib.import_module just hits that cache.
I think either of the following options will fix this issue:
1. Switching the order of how "fresh" and "blocked" are resolved or
2. Deleting `sys.modules[name]` if it exists immediately before calling
`importlib.import_module(name)
That said, I'm still having some weird statefulness problems if I block a C
module's import and *then* block a Python module's import, so there may be some
other underlying pathology to the current approach.
----------
components: Tests
files: test_support_repro.zip
messages: 365702
nosy: brett.cannon, eric.snow, ncoghlan, p-ganssle
priority: normal
severity: normal
status: open
title: test.support.import_fresh_module fails to correctly block submodules
when fresh is specified
type: behavior
versions: Python 3.7, Python 3.8, Python 3.9
Added file: https://bugs.python.org/file49031/test_support_repro.zip
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue40173>
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