try:
    from shutil import which  # Python >= 3.3
except ImportError:
    import os, sys
    
    # This is copied from Python 3.4.1
    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