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> But I observe that the same script(with proper modification of file names) 
> works very well under Linux. After I dive into the source code, I found 
> Python use execvpe to invoke the child process which _will_ use the PATH 
> variable to search the executable.
The posix execvpe function does not use PATH from the passed environment, but (surprisingly to me) os.execvpe does. subprocess in Python 3 no longer uses os.execvpe, but still goes out of its way to use envp['PATH'] on posix systems when searching for the executable.
So I think you have a legitimate complaint of inconsistency.
However, I don't think your suggested fix is a good idea since it (temporarily) changes the environment of the current process which might be problematic in a threaded program.
I am sure that Python 2.7 will not be patched to address this, but you might want to make a feature request for Python 3.4.