def _find_shortest_indentation(lines):
    """Return most shortest indentation."""
    assert not isinstance(lines, str)
    indentation = None
    for line in lines:
        if line.strip():
            non_whitespace_index = len(line) - len(line.lstrip())
            _indent = line[:non_whitespace_index]
            if indentation is None or len(_indent) < len(indentation):
                indentation = _indent
    return indentation or ''
