img: add _App0Marker.from_stream()

Steve Canny · Steve Canny · commit 16fa473619b1 · 2014-02-27T12:08:35.000-08:00
diff --git a/docx/image/jpeg.py b/docx/image/jpeg.py
@@ -266,13 +266,37 @@ class _App0Marker(_Marker):
     """
     Represents a JFIF APP0 marker segment.
     """
+    def __init__(
+            self, marker_code, offset, length, density_units, x_density,
+            y_density):
+        super(_App0Marker, self).__init__(marker_code, offset, length)
+        self._density_units = density_units
+        self._x_density = x_density
+        self._y_density = y_density
+
     @classmethod
     def from_stream(cls, stream, marker_code, offset):
         """
         Return an |_App0Marker| instance for the APP0 marker at *offset* in
         *stream*.
         """
-        raise NotImplementedError
+        # field               off  type   notes
+        # ------------------  ---  -----  -------------------
+        # segment length       0   short
+        # JFIF identifier      2   5 chr  'JFIF\x00'
+        # major JPEG version   7   byte   typically 1
+        # minor JPEG version   8   byte   typically 1 or 2
+        # density units        9   byte   1=inches, 2=cm
+        # horz dots per unit  10   short
+        # vert dots per unit  12   short
+        # ------------------  ---  -----  -------------------
+        length = stream.read_short(offset)
+        density_units = stream.read_byte(offset, 9)
+        x_density = stream.read_short(offset, 10)
+        y_density = stream.read_short(offset, 12)
+        return cls(
+            marker_code, offset, length, density_units, x_density, y_density
+        )
 
 
 class _SofMarker(_Marker):
diff --git a/tests/image/test_jpeg.py b/tests/image/test_jpeg.py
@@ -178,6 +178,35 @@ def _Marker__init_(self, request):
         return initializer_mock(request, _Marker)
 
 
+class Describe_App0Marker(object):
+
+    def it_can_construct_from_a_stream_and_offset(self, from_stream_fixture):
+        (stream, marker_code, offset, _App0Marker__init_, length,
+         density_units, x_density, y_density) = from_stream_fixture
+        app0_marker = _App0Marker.from_stream(stream, marker_code, offset)
+        _App0Marker__init_.assert_called_once_with(
+            marker_code, offset, length, density_units, x_density, y_density
+        )
+        assert isinstance(app0_marker, _App0Marker)
+
+    # fixtures -------------------------------------------------------
+
+    @pytest.fixture
+    def from_stream_fixture(self, request, _App0Marker__init_):
+        bytes_ = b'\x00\x10JFIF\x00\x01\x01\x01\x00\x2A\x00\x18'
+        stream_reader = StreamReader(BytesIO(bytes_), BIG_ENDIAN)
+        marker_code, offset, length = JPEG_MARKER_CODE.APP0, 0, 16
+        density_units, x_density, y_density = 1, 42, 24
+        return (
+            stream_reader, marker_code, offset, _App0Marker__init_, length,
+            density_units, x_density, y_density
+        )
+
+    @pytest.fixture
+    def _App0Marker__init_(self, request):
+        return initializer_mock(request, _App0Marker)
+
+
 class Describe_MarkerFactory(object):
 
     def it_constructs_the_appropriate_marker_object(self, call_fixture):
