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 ###

[064bit prompt]> ls
code00.py  code01.py
[064bit prompt]>
[064bit prompt]> cat code00.py
print("This is:", __file__)

[064bit 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'
[064bit prompt]>
[064bit prompt]> cat code01.py
#!/usr/bin/env python3

print("This is:", __file__)

[064bit 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.

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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