How to fix shebang flags that are not recognized on some systems

For some reason, the -O (optimized) flag is not recognized in the shebang line on a Red Hat Enterprise Server (release 5.3) that I access. On other systems, the flag is recognized without any issue.

Executing the script below on OS X works fine. Recognition of the -O flag can be verified because it enables (when absent) or disables (when given) anything under the if __debug__ conditional:

#!/usr/bin/env python -O                                                                                                                                                                       

if __name__ == '__main__':

    if __debug__:
        print 'lots of debugging output on'

    print 'Fin'

Executing the same script on the RHE system result in:

/usr/bin/env: python -O: No such file or directory

Without the -O flag, the script executes normally on the RHE system (i.e., the __debug__ built-in variable will be set to True).

Is there a cross-platform way to fix this issue? Is there even a platform-specific way to fix the issue of flags on the shebang line to the python interpreter?

Edit: Any other workarounds to setting the __debug__ variable (without using shebang flags) interpreter-wide would also be interesting.

Asked by: Aida400 | Posted: 06-12-2021

Answer 1

Some systems do not allow multiple arguments on a #!-style line. The "env hack" is not an officially recommended way of solving the path problem in any case - the preferred way to deal with this is to have the install rewrite the #! line to refer to /bin/python, /usr/bin/python, as appropriate for the system.

Answered by: Emily838 | Posted: 07-01-2022

Answer 2

How about making a small shell script:


/usr/bin/env python -O "$@"

Then change your script to use:


Also note that setting the environment variable PYTHONOPTIMIZE to a non-empty string is the same as using the -O flag. From the man python man page:

          If this is set to a non-empty string it is equivalent to  specifying  the
          -O option. If set to an integer, it is equivalent to specifying -O multi‐
          ple times.

Answered by: John951 | Posted: 07-01-2022

Answer 3

To extend slightly what unutbu said, you have the option of initializing PYTHONOPTIMIZE at runtime. This works for all modern shells:


And for completeness:

lots of debugging output on

Answered by: Wilson361 | Posted: 07-01-2022

Answer 4

Please try this:

''''exec python -O -- "$0" ${1+"$@"} # '''
if __name__ == '__main__':

    if __debug__:
        print 'lots of debugging output on'

    print 'Fin'
# vi: syntax=python

Answered by: Catherine761 | Posted: 07-01-2022

Answer 5

Change the shebang to:

#!/usr/bin/python -O

This assumes, of course, that the Python interpreter is installed as /usr/bin/python; otherwise, adjust as needed.

See also this question and my answer to it.

Answered by: Alford597 | Posted: 07-01-2022

Answer 6

I believe the reason you are using #!/usr/bin/env python is so that you can leverage the PATH to find your favorite version of python.

If that is NOT the case and you are fine using /usr/bin/python (or some other hard coded path to the interpreter), then the most direct solution should be to use #!/usr/bin/python -O as suggested by @keith-thompson

However, assuming you want to continue relying on PATH to find python, you will need to invoke a shell in order to get control over the command line:

exec python -O - "$@" <<EOF

# ... your python code starts here ...
if __name__ == '__main__':

    if __debug__:
        print 'lots of debugging output on'

    print 'Fin'
# ... your python code ends here ...


This is similar to the pythono solution suggested by @unutbu except that it doesn't rely on the PATH to find the intermediate script which means it is slightly better performance and slightly more secure.

This solution has the drawback of mixing two languages which means you cannot simply say python to execute your code with __debug__ set to True. If it is your intention to be able to execute the code sometimes directly via the script (e.g. ~/bin/ with -O turned on but other times to run the script by hand in a debugging environment (e.g. python ~/bin/ with -O turned off, then I believe your best options are to use expert tricks like that proposed by @ade-yu or to use the simple pythono option suggested by @unutbu.

Hope this helps!

Answered by: Roman284 | Posted: 07-01-2022

Answer 7

Just make your shebang without a flag; then, execute the environment again with the flag passing it to a heredoc.

#!/usr/bin/env python

/usr/bin/env python -O <<EOF
code goes here...

Basically have your terminating EOF at the end of the file.

Answered by: Clark208 | Posted: 07-01-2022

Answer 8

Assuming you want to continue using #!/usr/bin/env, you can take advantage of the facts that

  1. python has environment variable equivalents for its command-line switches
  2. env can be used to set environment variables before running the command

Putting these together, you get the following solution:

#!/usr/bin/env PYTHONOPTIMIZE=1 python

You asked for "cross-platform". This only works for Unix (e.g. Linux, OS X, etc.). I can't speak for Windows. I would expect this to work under cygwin as well but YMMV.

Answered by: Arnold396 | Posted: 07-01-2022

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