New submission from Eric Snow <ericsnowcurren...@gmail.com>:
The sys.modules dict is a special object. It is the only variable of the
CPython interpreter object that is exposed in the sys module[1]. Everything
else in sys lives in the module. However, the modules dict lives in the
interpreter object and is bound to the sys module separately. No other
variable of the interpreter object gets this treatment.
This situation sets up an unexpected behavior for sys.modules. There are many
places, mostly in Python/import.c, where the modules dict gets used and not by
pulling from sys.modules. These places use interp->modules directly[2]. So if
sys.modules is re-bound, the imp module is using/reporting an out of sync
modules dict.
One could argue that re-binding a module global is risky and should be avoided.
I agree. Here is the use case that prompted me to march ahead anyway:
class BaseTest(TestCase):
@classmethod
def setUpClass(cls):
cls.sysmodules = sys.modules
sys.modules = sys.modules.copy()
@classmethod
def tearDownClass(cls):
sys.modules = cls.sysmodules
I was writing some import related tests and wanted sys.modules to be returned
to its initial state after each test. I realise that Lib/test/support.py
provides CleanImport and others address this, but you have to provide the
module names to clean up. This is an unfortunate hassle sometimes when several
layers of imports happen during the import of the module you care about.
So the result was an exception when I tried importing an extension module, like
"_sqlite3". This is because in importdl.h the new module is added to the dict
returned by PyImport_GetModuleDict(), not to the one at sys.modules.
For now I am doing the following to get the same effect:
class BaseTest(TestCase):
@classmethod
def setUpClass(cls):
cls.sysmodules = sys.modules.copy()
@classmethod
def tearDownClass(cls):
for name in sys.modules:
del sys.modules[name]
for name in cls.sysmodules:
sys.modules[name] = cls.sysmodules[name]
However, this is less efficient, sort of. I expect that the current direct use
of interp->modules in the CPython code is [much?] more efficient than
PySys_GetObject("modules") calls.
Proposal
In light of all this I recommend that either use of interp->modules be replaced
by PySys_GetObject("modules") calls, or the sys module documentation[3] be
updated to make clear that sys.modules should not be re-bound (in a CPython
implementation detail note). I'm guessing that the first option is right out.
The documentation addition would be just right.
[1] variables of the interpreter object found by grepping "interp->" in the
CPython source:
modules
modules_by_index
next
codec_search_path
codec_search_cache
codec_error_registry
codecs_initialized
fscodec_initialized
modules_reloading
builtins
sysdict
tstate_head
tscdump
dlopenflags
[2] see PyImport_GetModuleDict() in Python/import.c
[3] http://docs.python.org/dev/library/sys.html#sys.modules
----------
messages: 141068
nosy: ericsnow, ncoghlan
priority: normal
severity: normal
status: open
title: sys.modules gets special treatment
type: behavior
versions: Python 3.3
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue12633>
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