img: add _App1Marker.from_stream()

Steve Canny · Steve Canny · commit 335dd46e35de · 2014-03-01T01:34:05.000-08:00
diff --git a/docx/image/jpeg.py b/docx/image/jpeg.py
@@ -397,6 +397,46 @@ class _App1Marker(_Marker):
     """
     Represents a JFIF APP1 (Exif) marker segment.
     """
+    @classmethod
+    def from_stream(cls, stream, marker_code, offset):
+        """
+        Extract the horizontal and vertical dots-per-inch value from the APP1
+        header at *offset* in *stream*.
+        """
+        # field                 off  len  type   notes
+        # --------------------  ---  ---  -----  ----------------------------
+        # segment length         0    2   short
+        # Exif identifier        2    6   6 chr  'Exif\x00\x00'
+        # TIFF byte order        8    2   2 chr  'II'=little 'MM'=big endian
+        # meaning of universe   10    2   2 chr  '*\x00' or '\x00*' depending
+        # IFD0 off fr/II or MM  10   16   long   relative to ...?
+        # --------------------  ---  ---  -----  ----------------------------
+        segment_length = stream.read_short(offset)
+        if cls._is_non_Exif_APP1_segment(stream, offset):
+            return cls(marker_code, offset, segment_length, 72, 72)
+        tiff = cls._tiff_from_exif_segment(stream, offset, segment_length)
+        return cls(
+            marker_code, offset, segment_length, tiff.horz_dpi, tiff.vert_dpi
+        )
+
+    @classmethod
+    def _is_non_Exif_APP1_segment(cls, stream, offset):
+        """
+        Return True if the APP1 segment at *offset* in *stream* is NOT an
+        Exif segment, as determined by the ``'Exif\x00\x00'`` signature at
+        offset 2 in the segment.
+        """
+        stream.seek(offset+2)
+        exif_signature = stream.read(6)
+        return exif_signature != b'Exif\x00\x00'
+
+    @classmethod
+    def _tiff_from_exif_segment(cls, stream, offset, segment_length):
+        """
+        Return a |Tiff| instance parsed from the Exif APP1 segment of
+        *segment_length* at *offset* in *stream*.
+        """
+        raise NotImplementedError
 
 
 class _SofMarker(_Marker):
diff --git a/tests/image/test_jpeg.py b/tests/image/test_jpeg.py
@@ -17,8 +17,11 @@
     _App0Marker, _App1Marker, Exif, Jfif, _JfifMarkers, Jpeg, _Marker,
     _MarkerFactory, _MarkerFinder, _MarkerParser, _SofMarker
 )
+from docx.image.tiff import Tiff
 
-from ..unitutil import class_mock, initializer_mock, instance_mock
+from ..unitutil import (
+    initializer_mock, class_mock, instance_mock, method_mock
+)
 
 
 class DescribeJpeg(object):
@@ -347,6 +350,66 @@ def from_stream_fixture(self, request, _App0Marker__init_):
         )
 
 
+class Describe_App1Marker(object):
+
+    def it_can_construct_from_a_stream_and_offset(self, from_stream_fixture):
+        (stream, marker_code, offset, _App1Marker__init_, length,
+         _tiff_from_exif_segment_, horz_dpi, vert_dpi) = from_stream_fixture
+        app1_marker = _App1Marker.from_stream(stream, marker_code, offset)
+        _tiff_from_exif_segment_.assert_called_once_with(
+            stream, offset, length
+        )
+        _App1Marker__init_.assert_called_once_with(
+            marker_code, offset, length, horz_dpi, vert_dpi
+        )
+        assert isinstance(app1_marker, _App1Marker)
+
+    def it_can_construct_from_non_Exif_APP1_segment(self, non_Exif_fixture):
+        stream, marker_code, offset = non_Exif_fixture[:3]
+        _App1Marker__init_, length = non_Exif_fixture[3:]
+        app1_marker = _App1Marker.from_stream(stream, marker_code, offset)
+        _App1Marker__init_.assert_called_once_with(
+            marker_code, offset, length, 72, 72
+        )
+        assert isinstance(app1_marker, _App1Marker)
+
+    # fixtures -------------------------------------------------------
+
+    @pytest.fixture
+    def _App1Marker__init_(self, request):
+        return initializer_mock(request, _App1Marker)
+
+    @pytest.fixture
+    def from_stream_fixture(
+            self, request, _App1Marker__init_, _tiff_from_exif_segment_):
+        bytes_ = b'\x00\x42Exif\x00\x00'
+        stream_reader = StreamReader(BytesIO(bytes_), BIG_ENDIAN)
+        marker_code, offset, length = JPEG_MARKER_CODE.APP1, 0, 66
+        horz_dpi, vert_dpi = 42, 24
+        return (
+            stream_reader, marker_code, offset, _App1Marker__init_, length,
+            _tiff_from_exif_segment_, horz_dpi, vert_dpi
+        )
+
+    @pytest.fixture
+    def non_Exif_fixture(self, request, _App1Marker__init_):
+        bytes_ = b'\x00\x42Foobar'
+        stream_reader = StreamReader(BytesIO(bytes_), BIG_ENDIAN)
+        marker_code, offset, length = JPEG_MARKER_CODE.APP1, 0, 66
+        return stream_reader, marker_code, offset, _App1Marker__init_, length
+
+    @pytest.fixture
+    def tiff_(self, request):
+        return instance_mock(request, Tiff, horz_dpi=42, vert_dpi=24)
+
+    @pytest.fixture
+    def _tiff_from_exif_segment_(self, request, tiff_):
+        return method_mock(
+            request, _App1Marker, '_tiff_from_exif_segment',
+            return_value=tiff_
+        )
+
+
 class Describe_SofMarker(object):
 
     def it_can_construct_from_a_stream_and_offset(self, from_stream_fixture):
