New submission from Václav Šmilauer:
On windows, environment variables exist in two copies: one is manipulated using
win32 API (GetEnvironmentVariable, SetEnvironmentVariable), and another one is
maintained by the C runtime (getenv, _putenv). This is explained in more depth
in [1].
os.environ manipulates win32 environment variables, but *not* those seen by the
CRT. This means that if I set an environment variable using os.environ and
later read it, in the same process, using getenv in an extension module, it
will not give the expected result. Child processes *do* see those vars in CRT,
since it is copied over from the win32 version at process startup.
Setting env vars has legitimate uses, since it is one of the few ways to
influence initialization of extension modules: for instance, setting
OMP_NUM_THREADS sets number of threads for OpenMP runtime (which cannot be
changed once the module using it is loaded).
It would be ideal to keep both CRT and win32 env vars in sync transparently. If
that is not realistically achievable, this gotcha should be documented as a
warning in the os.environ documentation.
A workaround to this problem to set variables in both win32 and CRT using
something like
import ctypes, ctypes.util, os.environ
ctypes.cdll[ctypes.util.find_msvcrt()]._putenv("%s=%s"%(name,value))
os.environ[name]=value
[1]
http://msmvps.com/blogs/senthil/archive/2009/10/13/when-what-you-set-is-not-what-you-get-setenvironmentvariable-and-getenv.aspx
----------
assignee: docs@python
components: Documentation
messages: 177081
nosy: docs@python, eudoxos
priority: normal
severity: normal
status: open
title: os.environ updates only one copy of env vars under Windows
(GetEnvironmentVariable vs. getenv)
type: behavior
versions: Python 2.7, Python 3.3
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Python tracker <[email protected]>
<http://bugs.python.org/issue16633>
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