我在win 7下的cygwin上使用virtualenv 1.9.1。每个virtualenv都是在没有任何选项的情况下创建的（显然，除了目录名称之外）。没有花哨的定制。
所有 virtualenvs 都是新鲜的，因为我没有创建任何包装脚本，也没有编辑任何 virtualenv 脚本。我完全确定我没有做任何特别的事情（即除了滥用 virtualenv，而不是采取阻碍其操作的行动）这导致 PYTHONPATH 在多个 virtualenvs 中不存在。
每个新 virtualenv 的 virtualenv 激活脚本不会创建 PYTHONPATH。pip 在$VIRTUAL_ENV/lib/python2.7/site-packages/<packagedir>（例如lib/python2.7/site-packages/html5lib）下安装软件包。
pip 的行为似乎是可取的。但是，运行 python（在激活 virtualenv 之后）会导致无法加载 pip 安装的包（即“没有名为...的模块”）。
更新：添加 PYTHONPATHactivate确实“解决”了这个问题。显然，我不想手动维护它，也不想更新activate_this.py. 我仍然想知道为什么 virtualenvs 现在不能直接开箱即用。
解决方案是什么？virtualenv 从来没有这样做过。现在有什么问题？
更新 2：这是使用 -vv 运行 virtualenv 的输出：
$ virtualenv -vv newvirtualenv
Creating newvirtualenv/lib/python2.7
Symlinking Python bootstrap modules
Symlinking newvirtualenv/lib/python2.7/config
Symlinking newvirtualenv/lib/python2.7/lib-dynload
Symlinking newvirtualenv/lib/python2.7/os.py
Ignoring built-in bootstrap module: posix
Symlinking newvirtualenv/lib/python2.7/posixpath.py
Cannot import bootstrap module: nt
Symlinking newvirtualenv/lib/python2.7/ntpath.py
Symlinking newvirtualenv/lib/python2.7/genericpath.py
Symlinking newvirtualenv/lib/python2.7/fnmatch.py
Symlinking newvirtualenv/lib/python2.7/locale.py
Symlinking newvirtualenv/lib/python2.7/encodings
Symlinking newvirtualenv/lib/python2.7/codecs.py
Symlinking newvirtualenv/lib/python2.7/stat.py
Symlinking newvirtualenv/lib/python2.7/UserDict.py
File newvirtualenv/lib/python2.7/lib-dynload/readline.dll already exists
Symlinking newvirtualenv/lib/python2.7/copy_reg.py
Symlinking newvirtualenv/lib/python2.7/types.py
Symlinking newvirtualenv/lib/python2.7/re.py
Symlinking newvirtualenv/lib/python2.7/sre.py
Symlinking newvirtualenv/lib/python2.7/sre_parse.py
Symlinking newvirtualenv/lib/python2.7/sre_constants.py
Symlinking newvirtualenv/lib/python2.7/sre_compile.py
File newvirtualenv/lib/python2.7/lib-dynload/zlib.dll already exists
Symlinking newvirtualenv/lib/python2.7/warnings.py
Symlinking newvirtualenv/lib/python2.7/linecache.py
Symlinking newvirtualenv/lib/python2.7/_abcoll.py
Symlinking newvirtualenv/lib/python2.7/abc.py
Symlinking newvirtualenv/lib/python2.7/_weakrefset.py
Creating newvirtualenv/lib/python2.7/site-packages
Writing newvirtualenv/lib/python2.7/site.py
Writing newvirtualenv/lib/python2.7/orig-prefix.txt
Writing newvirtualenv/lib/python2.7/no-global-site-packages.txt
Creating parent directories for newvirtualenv/include
Symlinking newvirtualenv/include/python2.7
Creating newvirtualenv/bin
New python executable in newvirtualenv/bin/python2.7
Changed mode of newvirtualenv/bin/python2.7 to 0755
Also creating executable in newvirtualenv/bin/python
Changed mode of newvirtualenv/bin/python to 0755
Testing executable with newvirtualenv/bin/python2.7 -c "import sys;out=sys.stdout;getattr(out, "buffer", out).write(sys.prefix.encode("utf-8"))"
Got sys.prefix result: u'/cygdrive/c/Users/marcin/Documents/codeprojects/newvirtualenv'
Creating newvirtualenv/lib/python2.7/distutils
Writing newvirtualenv/lib/python2.7/distutils/__init__.py
Writing newvirtualenv/lib/python2.7/distutils/distutils.cfg
Using existing setuptools egg: /usr/lib/python2.7/site-packages/virtualenv_support/setuptools-0.6c11-py2.7.egg
Installing setuptools...
Running command /cygdrive/c/Users/ma...ualenv/bin/python2.7 -c "#!python
\"\"\"Bootstra...sys.argv[1:])
" -v /usr/lib/python2.7/s...ols-0.6c11-py2.7.egg
Processing setuptools-0.6c11-py2.7.egg
Copying setuptools-0.6c11-py2.7.egg to /cygdrive/c/Users/marcin/Documents/codeprojects/newvirtualenv/lib/python2.7/site-packages
Adding setuptools 0.6c11 to easy-install.pth file
Saving /cygdrive/c/Users/marcin/Documents/codeprojects/newvirtualenv/lib/python2.7/site-packages/easy-install.pth
Installing easy_install script to /cygdrive/c/Users/marcin/Documents/codeprojects/newvirtualenv/bin
changing mode of /cygdrive/c/Users/marcin/Documents/codeprojects/newvirtualenv/bin/easy_install to 755
Installing easy_install-2.7 script to /cygdrive/c/Users/marcin/Documents/codeprojects/newvirtualenv/bin
changing mode of /cygdrive/c/Users/marcin/Documents/codeprojects/newvirtualenv/bin/easy_install-2.7 to 755
Installed /cygdrive/c/Users/marcin/Documents/codeprojects/newvirtualenv/lib/python2.7/site-packages/setuptools-0.6c11-py2.7.egg
Processing dependencies for setuptools==0.6c11
Finished processing dependencies for setuptools==0.6c11
...Installing setuptools...done.
Installing existing pip-1.3.1.tar.gz distribution: /usr/lib/python2.7/site-packages/virtualenv_support/pip-1.3.1.tar.gz
Installing pip...
Running command /cygdrive/c/Users/ma...ualenv/bin/python2.7 -x /cygdrive/c/Users/ma...env/bin/easy_install /usr/lib/python2.7/s...ort/pip-1.3.1.tar.gz
Processing pip-1.3.1.tar.gz
Running pip-1.3.1/setup.py -q bdist_egg --dist-dir /tmp/easy_install-ZP_VIv/pip-1.3.1/egg-dist-tmp-H3XBh_
warning: no files found matching '*.html' under directory 'docs'
warning: no previously-included files matching '*.txt' found under directory 'docs/_build'
no previously-included directories found matching 'docs/_build/_sources'
Adding pip 1.3.1 to easy-install.pth file
Installing pip script to /cygdrive/c/Users/marcin/Documents/codeprojects/newvirtualenv/bin
Installing pip-2.7 script to /cygdrive/c/Users/marcin/Documents/codeprojects/newvirtualenv/bin
Installed /cygdrive/c/Users/marcin/Documents/codeprojects/newvirtualenv/lib/python2.7/site-packages/pip-1.3.1-py2.7.egg
Processing dependencies for pip==1.3.1
Finished processing dependencies for pip==1.3.1
...Installing pip...done.
Writing newvirtualenv/bin/activate
Writing newvirtualenv/bin/activate.fish
Writing newvirtualenv/bin/activate_this.py
Writing newvirtualenv/bin/activate.csh