调用python版本检查的子进程在标准输出中返回空

Question

I have this code in Python

import subprocess
pp = subprocess.Popen(
   ['/bin/python', '-c', "'import sys; print(sys.version_info.major)'"],
   stdout=subprocess.PIPE, stderr=subprocess.PIPE)
print(pp.communicate())

print(
    subprocess.check_output(
      ['/bin/python', '-c', "'import sys; print(sys.version_info.major)'"])
)

Both versions end up with an empty string (or bytes). Tested in Py3 and Py2 as well. Why is that the case?

Answer 1

The problem is the second set of quotation marks. The command list ['python', '-c', "'import sys; print(sys.version_info.major)'"] translates to this shell command

python -c "'import sys; print(sys.version_info.major)'"

which is not what you intended as it just asks Python to "execute that string", and that doesn't result in any output to stdout .

So remove the extraneous quotation marks: ['python', '-c', 'import sys; print(sys.version_info.major)'] ['python', '-c', 'import sys; print(sys.version_info.major)'] . The subprocess module takes care of proper escaping. From the docs :

Note in particular that options (such as -input ) and arguments (such as eggs.txt ) that are separated by whitespace in the shell go in separate list elements, while arguments that need quoting or backslash escaping when used in the shell (such as filenames containing spaces) are single list elements.