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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _compute(self):
 """ Processes this fields `src` from `ctx.src`. """

        src_path = self.ctx.src_path
        if not src_path.exists:
            return NONE
        if src_path.is_null:
            return None
        try:
            if self.parse:
                value = self.parse(src_path)
            else:
                value = self._parse(src_path)
            return value
        except (SourceError, ValueError), ex:
            self.ctx.errors.invalid(str(ex))
            return ERROR