'Return path to the virtualenv bin path, or a specific binary
:param filename: If specified, add this filename to the path return
:returns: Path to virtualenv bin or filename in virtualenv bin'

def venv_bin(self, filename=None):

    parts = [self.venv_path(), 'bin']
    if (filename is not None):
        parts.append(filename)
    return os.path.join(*parts)
