string.py 文件源码

def _apply(self, value):
        value = self._filter(value, Type(binary_type)) # type: binary_type

        if self._has_errors:
            return None

        # Strip out whitespace.
        # Technically, whitespace is not part of the Base64 alphabet,
        # but virtually every implementation allows it.
        value = self.whitespace_re.sub(b'', value)

        # Check for invalid characters.
        # Note that Python 3's b64decode does this for us, but we also
        # have to support Python 2.
        # https://docs.python.org/3/library/base64.html#base64.b64decode
        if not self.base64_re.match(value):
            return self._invalid_value(
                value   = value,
                reason  = self.CODE_INVALID,
            )

        # Check to see if we are working with a URL-safe dialect.
        # https://en.wikipedia.org/wiki/Base64#URL_applications
        if (b'_' in value) or (b'-' in value):
            # You can't mix dialects, silly!
            if (b'+' in value) or (b'/' in value):
                return self._invalid_value(
                    value   = value,
                    reason  = self.CODE_INVALID,
                )

            url_safe = True
        else:
            url_safe = False

        # Normalize padding.
        # http://stackoverflow.com/a/9807138/
        value = value.rstrip(b'=')
        value += (b'=' * (4 - (len(value) % 4)))

        try:
            return (
                urlsafe_b64decode(value)
                    if url_safe
                    else standard_b64decode(value)
            )
        except TypeError:
            return self._invalid_value(value, self.CODE_INVALID, exc_info=True)


# noinspection SpellCheckingInspection