Packaged basic Piexif module

openflexure_microscope/camera/piexif/helper.py

@@ -0,0 +1,66 @@
+class UserComment:
+    #
+    # Names of encodings that we publicly support.
+    #
+    ASCII = 'ascii'
+    JIS = 'jis'
+    UNICODE = 'unicode'
+    ENCODINGS = (ASCII, JIS, UNICODE)
+
+    #
+    # The actual encodings accepted by the standard library differ slightly from
+    # the above.
+    #
+    _JIS = 'shift_jis'
+    _UNICODE = 'utf_16_be'
+
+    _PREFIX_SIZE = 8
+    #
+    # From Table 9: Character Codes and their Designation
+    #
+    _ASCII_PREFIX = b'\x41\x53\x43\x49\x49\x00\x00\x00'
+    _JIS_PREFIX = b'\x4a\x49\x53\x00\x00\x00\x00\x00'
+    _UNICODE_PREFIX = b'\x55\x4e\x49\x43\x4f\x44\x45\x00'
+    _UNDEFINED_PREFIX = b'\x00\x00\x00\x00\x00\x00\x00\x00'
+
+    @classmethod
+    def load(cls, data):
+        """
+        Convert "UserComment" value in exif format to str.
+
+        :param bytes data: "UserComment" value from exif
+        :return: u"foobar"
+        :rtype: str(Unicode)
+        :raises: ValueError if the data does not conform to the EXIF specification,
+        or the encoding is unsupported.
+        """
+        if len(data) < cls._PREFIX_SIZE:
+            raise ValueError('not enough data to decode UserComment')
+        prefix = data[:cls._PREFIX_SIZE]
+        body = data[cls._PREFIX_SIZE:]
+        if prefix == cls._UNDEFINED_PREFIX:
+            raise ValueError('prefix is UNDEFINED, unable to decode UserComment')
+        try:
+            encoding = {
+                cls._ASCII_PREFIX: cls.ASCII, cls._JIS_PREFIX: cls._JIS, cls._UNICODE_PREFIX: cls._UNICODE,
+            }[prefix]
+        except KeyError:
+            raise ValueError('unable to determine appropriate encoding')
+        return body.decode(encoding, errors='replace')
+
+    @classmethod
+    def dump(cls, data, encoding="ascii"):
+        """
+        Convert str to appropriate format for "UserComment".
+
+        :param data: Like u"foobar"
+        :param str encoding: "ascii", "jis", or "unicode"
+        :return: b"ASCII\x00\x00\x00foobar"
+        :rtype: bytes
+        :raises: ValueError if the encoding is unsupported.
+        """
+        if encoding not in cls.ENCODINGS:
+            raise ValueError('encoding %r must be one of %r' % (encoding, cls.ENCODINGS))
+        prefix = {cls.ASCII: cls._ASCII_PREFIX, cls.JIS: cls._JIS_PREFIX, cls.UNICODE: cls._UNICODE_PREFIX}[encoding]
+        internal_encoding = {cls.UNICODE: cls._UNICODE, cls.JIS: cls._JIS}.get(encoding, encoding)
+        return prefix + data.encode(internal_encoding, errors='replace')