Flask CLI throws 'OSError: [Errno 8] Exec format error' when run through docker-compose

Looks like your api/manage.py doesn't have a shebang ([Wikipedia]: Shebang (Unix)), so the default (current) command processor (a shell - typically bash) is attempting to run it, which (obviously) fails.

To correct the problem, add a shebang (at the beginning of the file, making sure that your editor adds the Nix style line ending (\n, 0x0A, LF)):

• Default Python installation:

    #!/usr/bin/env python
  

  • Variant (specify Python 3 explicitly):

      #!/usr/bin/env python3
    

• Custom Python installation:

    #!/full/path/to/your/custom/python/executable
  

Note that you also need exec permissions on the file (chmod +x api/manage.py).

Example:

[cfati@cfati-5510-0:/cygdrive/e/Work/Dev/StackOverflow/q055271912]> ~/sopr.sh
*** Set shorter prompt to better fit when pasted in StackOverflow (or other) pages ***

[prompt]> ls
code00.py  code01.py
[prompt]>
[prompt]> cat code00.py
print("This is:", __file__)

[prompt]> python3 -c "import os, subprocess;subprocess.Popen(os.path.join(os.getcwd(), \"code00.py\")).communicate()"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python3.6/subprocess.py", line 709, in __init__
    restore_signals, start_new_session)
  File "/usr/lib/python3.6/subprocess.py", line 1344, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
OSError: [Errno 8] Exec format error: '/cygdrive/e/Work/Dev/StackOverflow/q055271912/code00.py'
[prompt]>
[prompt]> cat code01.py
#!/usr/bin/env python3

print("This is:", __file__)

[prompt]> python3 -c "import os, subprocess;subprocess.Popen(os.path.join(os.getcwd(), \"code01.py\")).communicate()"
This is: /cygdrive/e/Work/Dev/StackOverflow/q055271912/code01.py

Another way would be to run the interpreter followed by the file name, but I don't know how to do it from Flask - actually that would require patching Werkzeug (_reloader.py: _get_args_for_reloading), but that would be just a lame workaround (gainarie) - see below.

---

Update #0

Looking at @AxelGrytt's answer, it turns out it's a known issue: [GitHub]: pallets/werkzeug - 0.15.0 causes OSError: [Errno 8] Exec format error: in Docker for Windows (hmm, submitted in the same day as this question (and 2 days after the release) :) ).

So, what I have stated above is correct, but it is worth mentioning that there is another way of fixing it: removing the exec permission for the file:

chmod -x api/manage.py

According to Werkzeug authors, from now on, this is desired behavior (also applies to v0.15.2):

• A file with exec permission set, should also have a shebang
• A file without a shebang, shouldn't have the exec permission set

This is a new behavior in Werkzeug 0.15. Downgrading to Werkzeug 0.14.1 may work, but 0.14 is no longer supported, so you will be better off correcting the issue with your file as described in the other answers.

If you disable debug mode (don't pass debug=True or set FLASK_DEBUG=0), the reloader will not be used and so this issue will not happen. The tradeoff is that you no longer have the reloader.

if __name__ == "__main__":
    connexion_app.run(host="0.0.0.0", port=constants.API_PORT, debug=True)

It is preferrable to fix this by ensuring files marked as executable have an interpreter line, like #!/usr/bin/env python3 (from https://stackoverflow.com/a/55272071).