67 lines
2.5 KiB
Python
67 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, cls._JIS_PREFIX: cls._JIS, cls._UNICODE_PREFIX: cls._UNICODE,
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}[prefix]
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except KeyError:
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raise ValueError('unable to determine appropriate encoding')
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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('encoding {!r} must be one of {!r}'.format(encoding, cls.ENCODINGS))
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prefix = {cls.ASCII: cls._ASCII_PREFIX, cls.JIS: cls._JIS_PREFIX, cls.UNICODE: cls._UNICODE_PREFIX}[encoding]
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internal_encoding = {cls.UNICODE: cls._UNICODE, cls.JIS: cls._JIS}.get(encoding, encoding)
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return prefix + data.encode(internal_encoding, errors='replace')
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