Process substitution not allowed by Python's s

问题:

Here is a toy example of process substitution that works fine in Bash:

$ wc -l <(pwd)
1 /proc/self/fd/11

So why does the same command give a syntax error when invoked from Python's subprocess with shell=True?

>>> subprocess.check_call('wc -l <(pwd)', shell=True)
/bin/sh: 1: Syntax error: "(" unexpected
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/path/to/my/python/lib/python3.5/subprocess.py", line 581, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command 'wc -l <(pwd)' returned non-zero exit status 2

回答1:

/bin/sh: 1: Syntax error: "(" unexpected

You have a bashism. It is not valid according to POSIX, which is what /bin/sh implements.

回答2:

An alternate solution is to shift more of the shell code to Python itself. For example:

from subprocess import Popen, PIPE, check_call

p1 = Popen(["pwd"], stdout=PIPE)
p2 = check_call(["wc", "-l"], stdin=p1.stdout)

This could often be the first step towards eliminating the need to use subprocess at all, as it decomposes the work into smaller chunks for which you may more readily see how to do in Python itself.