哪个程序正在运行？

Question

我在python2.7上运行自编译版本，--prefix = $ HOME，以免干扰python2.6的标准安装（在Ubuntu 10.04上）。 导入ssl模块时遇到一些问题，追踪问题，重新编译我的python2.7并进行测试：

$ ~/bin/python2.7 -c "import ssl"

看起来不错，再检查一下

$ python2.7 -c "import ssl"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/jab/lib/python2.7/ssl.py", line 60, in <module>
    import _ssl             # if we can't import it, let the error propagate
ImportError: No module named _ssl

嗯？这台机器上没有其他安装的python2.7

$ which python2.7
~/bin/python2.7
$ echo $PATH
/home/jab/bin:/usr/local/bin:/usr/bin:/bin:...
$ for dir in $(echo $PATH | tr ':' ' '); do ls $dir/python2.7 2>/dev/null; done
/home/jab/bin/python2.7

为了更加确定：

$ python2.7 -c "import sys; print sys.executable"
/home/jab/bin/python2.7

$ $(which python2.7) -c "import sys; print sys.executable"
/home/jab/bin/python2.7

上下文已经足够了。问题是：为什么这样做有效？

$(which python2.7) -c "import ssl"

当这不存在时？

$ python2.7 -c "import ssl"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/jab/lib/python2.7/ssl.py", line 60, in <module>
    import _ssl             # if we can't import it, let the error propagate
ImportError: No module named _ssl