Always validate input state and types before performing operations to prevent runtime errors and unexpected behavior. This includes checking data types before applying type-specific operations and preserving object state when performing validation checks. When working with streams, restore the original position after reading during validation to avoid side effects: ```python # Good: Preserve stream position cur_pos = file_stream.tell() data = file_stream.read() file_stream.seek(cur_pos) ``` When processing data, validate the type before applying operations: ```python # Good: Check type before text operations if isinstance(content, str): content = self._strip_leading_blanks(content) ``` This defensive approach prevents errors from assumptions about input state or type, similar to how null checks prevent null reference exceptions.