Switching from Python 3 to 2 in a runtime

Question

I try to check if current version is 3 and if so, switch to python2:

#!/usr/bin/python

import sys, os

if sys.version_info[0] != 2:
    os.execl("/usr/bin/", "python2", *sys.argv)

print(sys.version_info[:])

But this script returns this error:

Traceback (most recent call last):
  File "./a.py", line 6, in <module>
    os.execl("/usr/bin/", "python2", *sys.argv)
  File "/usr/lib/python3.3/os.py", line 531, in execl
    execv(file, args)
PermissionError: [Errno 13] Permission denied

What have I missed?

run this script as administartor, may help..

pradyunsg
– pradyunsg

2013-01-22 10:39:57 +00:00
Commented Jan 22, 2013 at 10:39 — pradyunsg
– pradyunsg, Commented Jan 22, 2013 at 10:39
Nope. And it wont have root privilages in the future.

ciembor
– ciembor

2013-01-22 10:43:03 +00:00
Commented Jan 22, 2013 at 10:43 — ciembor
– ciembor, Commented Jan 22, 2013 at 10:43

user9876 · Accepted Answer · 2013-01-22 10:46:55Z

2

os.execl("/usr/bin/", "python2", *sys.argv)

/usr/bin/ is a directory, you can't run it. Try:

os.execl("/usr/bin/python2", "/usr/bin/python2", *sys.argv[1:])

answered Jan 22, 2013 at 10:46

user9876

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Comments

Gareth Latty · Accepted Answer · 2013-01-22 10:45:16Z

0

I would argue what you are attempting is a bad idea - it is surprising behaviour and not needed, instead, simply use an explicit hashbang:

#!/usr/bin/python2

Or, preferably:

#!/usr/bin/env python2

As per PEP 394, any unix system should provide python2.

answered Jan 22, 2013 at 10:45

Gareth Latty

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2 Comments

ciembor Over a year ago

Well, that's the point - python doesn't have python2 symlink in OSX.

Gareth Latty Over a year ago

@ciembor Then I would recommend contacting the maintainer of that package and suggesting they fix that bug. The PEP is quite clear - "Unix-like software distributions (including systems like Mac OS X and Cygwin) should install the python2 command into the default path whenever a version of the Python 2 interpreter is installed, and the same for python3 and the Python 3 interpreter.".

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