When using subprocess.Popen.communicate(), it is natural to wonder how to get
the exit code of the subprocess. However, the documentation [1] says:

    Interact with process: Send data to stdin. Read data from stdout and
    stderr, until end-of-file is reached. Wait for process to terminate. The
    optional input argument should be data to be sent to the child process, or
    None, if no data should be sent to the child. If streams were opened in
    text mode, input must be a string. Otherwise, it must be bytes.

    communicate() returns a tuple (stdout_data, stderr_data). The data will be
    strings if streams were opened in text mode; otherwise, bytes.

If you can guess that communicate() might set returncode, then you can find
what you need in the documentation for that attribute [2]:

    The child return code, set by poll() and wait() (and indirectly by
    communicate()).

I suggest that the documentation for communicate() be updated to mention that
it sets the returncode attribute. This would be consistent with poll() and
wait(), which already mention this.

[1]: https://docs.python.org/3/library/subprocess.html#subprocess.Popen.communicate
[2]: https://docs.python.org/3/library/subprocess.html#subprocess.Popen.returncode

This is not a security issue.

Can you attach a script or post an interactive session, with both success and failure, showing that returncode is set appropriately after reading output?  Possible calls: '''python -c "print('Success')"''', "raise Exception('Failure')".  Or point to a test in test_subprocess.

The following test cases in test_subprocess.py call the communicate() method and then immediately assert that returncode attribute has the expected value:

* test_stdout_none
* test_stderr_redirect_with_no_stdout_redirect
* test_stdout_filedes_of_stdout
* test_communicate_stdin
* test_universal_newlines_communicate_stdin
* test_universal_newlines_communicate_input_none
* test_universal_newlines_communicate_stdin_stdout_stderr
* test_nonexisting_with_pipes
* test_wait_when_sigchild_ignored
* test_startupinfo_copy
* test_close_fds_with_stdio
* test_communicate_stdin

You'll see that some of these test for success (returncode == 0) and some for failure (returncode == 1). This seems like adequate test coverage to me, but if something is missing, let me know.

Is there anything I can do to move this forward?

Will merge after CI passes the addition of 'the'.

New changeset bf2e515fa43406d4bd9c4c53ecc9364034d8f9f6 by Gareth Rees in branch 'master':
bpo-40707: Document that Popen.communicate sets the returncode attribute (GH-20283)
https://github.com/python/cpython/commit/bf2e515fa43406d4bd9c4c53ecc9364034d8f9f6

New changeset 02d5c74f89709672baa866fcdd04278e59328351 by Miss Islington (bot) in branch '3.9':
bpo-40707: Document that Popen.communicate sets the returncode attribute (GH-20283)
https://github.com/python/cpython/commit/02d5c74f89709672baa866fcdd04278e59328351

New changeset 4f5dde463b588fc97cacb4a1905eb422b16daa37 by Miss Islington (bot) in branch '3.8':
bpo-40707: Document that Popen.communicate sets the returncode attribute (GH-20283)
https://github.com/python/cpython/commit/4f5dde463b588fc97cacb4a1905eb422b16daa37

Thanks for the fix.