img: add _Marker.from_stream()

Steve Canny · Steve Canny · commit ee8cbe5f8ce1 · 2014-02-27T12:08:35.000-08:00
diff --git a/docx/image/helpers.py b/docx/image/helpers.py
@@ -50,6 +50,14 @@ def read_long(self, base=None, offset=0):
         fmt = '<L' if self._byte_order is LITTLE_ENDIAN else '>L'
         return self._read_int(fmt, base, offset)
 
+    def read_short(self, base=None, offset=0):
+        """
+        Return the int value of the two bytes at the file position determined
+        by *base* and *offset*, similarly to ``read_long()`` above.
+        """
+        fmt = b'<H' if self._byte_order is LITTLE_ENDIAN else b'>H'
+        return self._read_int(fmt, base, offset)
+
     def read_str(self, char_count, base, offset=0):
         """
         Return a string containing the *char_count* bytes at the file
diff --git a/docx/image/jpeg.py b/docx/image/jpeg.py
@@ -228,28 +228,38 @@ class _Marker(object):
     Base class for JFIF marker classes. Represents a marker and its segment
     occuring in a JPEG byte stream.
     """
+    def __init__(self, marker_code, offset, segment_length):
+        super(_Marker, self).__init__()
+        self._marker_code = marker_code
+        self._offset = offset
+        self._segment_length = segment_length
+
     @classmethod
     def from_stream(cls, stream, marker_code, offset):
         """
         Return a generic |_Marker| instance for the marker at *offset* in
         *stream* having *marker_code*.
         """
-        raise NotImplementedError
+        if JPEG_MARKER_CODE.is_standalone(marker_code):
+            segment_length = 0
+        else:
+            segment_length = stream.read_short(offset)
+        return cls(marker_code, offset, segment_length)
 
     @property
     def marker_code(self):
         """
         The single-byte code that identifies the type of this marker, e.g.
         ``'\xE0'`` for start of image (SOI).
         """
-        raise NotImplementedError
+        return self._marker_code
 
     @property
     def segment_length(self):
         """
         The length in bytes of this marker's segment
         """
-        raise NotImplementedError
+        return self._segment_length
 
 
 class _App0Marker(_Marker):
diff --git a/tests/image/test_jpeg.py b/tests/image/test_jpeg.py
@@ -151,6 +151,33 @@ def stream_(self, request):
         return instance_mock(request, BytesIO)
 
 
+class Describe_Marker(object):
+
+    def it_can_construct_from_a_stream_and_offset(self, from_stream_fixture):
+        stream, marker_code, offset, _Marker__init_, length = (
+            from_stream_fixture
+        )
+        marker = _Marker.from_stream(stream, marker_code, offset)
+        _Marker__init_.assert_called_once_with(marker_code, offset, length)
+        assert isinstance(marker, _Marker)
+
+    # fixtures -------------------------------------------------------
+
+    @pytest.fixture(params=[
+        (JPEG_MARKER_CODE.SOI,  2,  0),
+        (JPEG_MARKER_CODE.APP0, 4, 16),
+    ])
+    def from_stream_fixture(self, request, _Marker__init_):
+        marker_code, offset, length = request.param
+        bytes_ = b'\xFF\xD8\xFF\xE0\x00\x10'
+        stream_reader = StreamReader(BytesIO(bytes_), BIG_ENDIAN)
+        return stream_reader, marker_code, offset, _Marker__init_, length
+
+    @pytest.fixture
+    def _Marker__init_(self, request):
+        return initializer_mock(request, _Marker)
+
+
 class Describe_MarkerFactory(object):
 
     def it_constructs_the_appropriate_marker_object(self, call_fixture):
