switching to python3

[Skaperen]

it seems lots or even most of the suggestions here tend to increase complexity, work only in a different context, and/or meet a different goal. in many cases i wish it were as simple as:

1 2	import switchtopython3 switchtopython3.switch_now()

[Gribouillis]

Then here is your module, implementing the suggestion above. It needs to be tested in Windows and OSX

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# switchtopython3.py from __future__ import print_function import os import sys   # which() is copied from shutil.which() in python 3.5.2 def which(cmd, mode=os.F_OK | os.X_OK, path=None):     """Given a command, mode, and a PATH string, return the path which     conforms to the given mode on the PATH, or None if there is no such     file.       `mode` defaults to os.F_OK | os.X_OK. `path` defaults to the result     of os.environ.get("PATH"), or can be overridden with a custom search     path.       """     # Check that a given file can be accessed with the correct mode.     # Additionally check that `file` is not a directory, as on Windows     # directories pass the os.access check.     def _access_check(fn, mode):         return (os.path.exists(fn) and os.access(fn, mode)                 and not os.path.isdir(fn))       # If we're given a path with a directory part, look it up directly rather     # than referring to PATH directories. This includes checking relative to the     # current directory, e.g. ./script     if os.path.dirname(cmd):         if _access_check(cmd, mode):             return cmd         return None       if path is None:         path = os.environ.get("PATH", os.defpath)     if not path:         return None     path = path.split(os.pathsep)       if sys.platform == "win32":         # The current directory takes precedence on Windows.         if not os.curdir in path:             path.insert(0, os.curdir)           # PATHEXT is necessary to check on Windows.         pathext = os.environ.get("PATHEXT", "").split(os.pathsep)         # See if the given file matches any of the expected path extensions.         # This will allow us to short circuit when given "python.exe".         # If it does match, only test that one, otherwise we have to try         # others.         if any(cmd.lower().endswith(ext.lower()) for ext in pathext):             files = [cmd]         else:             files = [cmd + ext for ext in pathext]     else:         # On other platforms you don't have things like PATHEXT to tell you         # what file suffixes are executable, so just pass on cmd as-is.         files = [cmd]       seen = set()     for dir in path:         normdir = os.path.normcase(dir)         if not normdir in seen:             seen.add(normdir)             for thefile in files:                 name = os.path.join(dir, thefile)                 if _access_check(name, mode):                     return name     return None   def switch_now(verbose=False):     if sys.version_info.major > 2:         return     python3 = which('python3')     if python3 is None:         raise RuntimeError('Cannot find python3 executable')     if verbose:         print('Switching to new interpreter:', python3)     os.execl(python3, python3, *sys.argv)

You don't need to maintain which() because it is copied verbatim from python's source tree.

In Windows, I think it won't work that easily because there is usually no python3.exe executable. One can perhaps change the code
to detect the 'py' launcher, eg

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if sys.platform = "win32":     py = which('py')     if py is not None:         os.execl(py, py, '-3', *sys.argv)

This is only the beginning if you want a general solution, because several versions of python 3 can exist on the same machine. The complexity increases if one also wants to cover virtualenvs.

[Skaperen]

i do want it to just continue if python3 is not available. my use cases are code that can run in python2 but would do better if python3 were being used. a return value may not even be needed since code that needs to know can check sys.version_info.major or the like.

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if sys.version_info.major < 3:     switchpython.switch_now(3)     print('Python3 is not available - expect degraded results')

if the switch is successful, execution restarts back at the top.