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