
def git_versions_from_vcs(tag_prefix, root, verbose=False):
    # this runs 'git' from the root of the source tree. This only gets called
    # if the git-archive 'subst' keywords were *not* expanded, and
    # _version.py hasn't already been rewritten with a short version string,
    # meaning we're inside a checked out source tree.

    if not os.path.exists(os.path.join(root, ".git")):
        if verbose:
            print("no .git in %s" % root)
        return {}

    GITS = ["git"]
    if sys.platform == "win32":
        GITS = ["git.cmd", "git.exe"]
    stdout = run_command(GITS, ["describe", "--tags", "--dirty", "--always"],
                         cwd=root)
    if stdout is None:
        return {}
    if not stdout.startswith(tag_prefix):
        if verbose:
            fmt = "tag '%s' doesn't start with prefix '%s'"
            print(fmt % (stdout, tag_prefix))
        return {}
    tag = stdout[len(tag_prefix):]
    stdout = run_command(GITS, ["rev-parse", "HEAD"], cwd=root)
    if stdout is None:
        return {}
    full = stdout.strip()
    if tag.endswith("-dirty"):
        full += "-dirty"
    return {"version": tag, "full": full}

