76 lines
2.5 KiB
Python
76 lines
2.5 KiB
Python
class UserComment:
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#
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# Names of encodings that we publicly support.
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#
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ASCII = "ascii"
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JIS = "jis"
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UNICODE = "unicode"
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ENCODINGS = (ASCII, JIS, UNICODE)
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#
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# The actual encodings accepted by the standard library differ slightly from
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# the above.
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#
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_JIS = "shift_jis"
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_UNICODE = "utf_16_be"
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_PREFIX_SIZE = 8
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#
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# From Table 9: Character Codes and their Designation
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#
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_ASCII_PREFIX = b"\x41\x53\x43\x49\x49\x00\x00\x00"
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_JIS_PREFIX = b"\x4a\x49\x53\x00\x00\x00\x00\x00"
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_UNICODE_PREFIX = b"\x55\x4e\x49\x43\x4f\x44\x45\x00"
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_UNDEFINED_PREFIX = b"\x00\x00\x00\x00\x00\x00\x00\x00"
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@classmethod
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def load(cls, data):
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"""
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Convert "UserComment" value in exif format to str.
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:param bytes data: "UserComment" value from exif
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:return: u"foobar"
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:rtype: str(Unicode)
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:raises: ValueError if the data does not conform to the EXIF specification,
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or the encoding is unsupported.
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"""
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if len(data) < cls._PREFIX_SIZE:
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raise ValueError("not enough data to decode UserComment")
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prefix = data[: cls._PREFIX_SIZE]
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body = data[cls._PREFIX_SIZE :]
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if prefix == cls._UNDEFINED_PREFIX:
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raise ValueError("prefix is UNDEFINED, unable to decode UserComment")
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try:
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encoding = {
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cls._ASCII_PREFIX: cls.ASCII,
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cls._JIS_PREFIX: cls._JIS,
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cls._UNICODE_PREFIX: cls._UNICODE,
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}[prefix]
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except KeyError as e:
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raise ValueError("unable to determine appropriate encoding") from e
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return body.decode(encoding, errors="replace")
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@classmethod
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def dump(cls, data, encoding="ascii"):
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"""
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Convert str to appropriate format for "UserComment".
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:param data: Like u"foobar"
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:param str encoding: "ascii", "jis", or "unicode"
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:return: b"ASCII\x00\x00\x00foobar"
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:rtype: bytes
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:raises: ValueError if the encoding is unsupported.
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"""
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if encoding not in cls.ENCODINGS:
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raise ValueError(
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"encoding %r must be one of %r" % (encoding, cls.ENCODINGS)
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)
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prefix = {
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cls.ASCII: cls._ASCII_PREFIX,
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cls.JIS: cls._JIS_PREFIX,
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cls.UNICODE: cls._UNICODE_PREFIX,
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}[encoding]
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internal_encoding = {cls.UNICODE: cls._UNICODE, cls.JIS: cls._JIS}.get(
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encoding, encoding
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)
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return prefix + data.encode(internal_encoding, errors="replace")
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