img: add _IfdEntry.from_stream()

Steve Canny · Steve Canny · commit 57e86cbbb1d4 · 2014-02-27T12:08:36.000-08:00
diff --git a/docx/image/tiff.py b/docx/image/tiff.py
@@ -171,15 +171,34 @@ class _IfdEntry(object):
     Base class for IFD entry classes. Subclasses are differentiated by value
     type, e.g. ASCII, long int, etc.
     """
+    def __init__(self, tag_code, value):
+        super(_IfdEntry, self).__init__()
+        self._tag_code = tag_code
+        self._value = value
+
     @classmethod
     def from_stream(cls, stream_rdr, offset):
         """
-        Return an |_IfdEntry| subclass instance parsed from *stream_rdr* at
-        *offset*. Note this method is common to all subclasses. Override the
-        ``_parse_value()`` method to provide distinctive behavior based on
-        field type.
+        Return an |_IfdEntry| subclass instance containing the tag and value
+        of the tag parsed from *stream_rdr* at *offset*. Note this method is
+        common to all subclasses. Override the ``_parse_value()`` method to
+        provide distinctive behavior based on field type.
         """
-        raise NotImplementedError
+        tag_code = stream_rdr.read_short(offset, 0)
+        value_count = stream_rdr.read_long(offset, 4)
+        value_offset = stream_rdr.read_long(offset, 8)
+        value = cls._parse_value(
+            stream_rdr, offset, value_count, value_offset
+        )
+        return cls(tag_code, value)
+
+    @classmethod
+    def _parse_value(cls, stream_rdr, offset, value_count, value_offset):
+        """
+        Return the value of this field parsed from *stream_rdr* at *offset*.
+        Intended to be overridden by subclasses.
+        """
+        return 'UNIMPLEMENTED FIELD TYPE'
 
     @property
     def tag(self):
diff --git a/tests/image/test_tiff.py b/tests/image/test_tiff.py
@@ -17,7 +17,8 @@
 )
 
 from ..unitutil import (
-    function_mock, class_mock, initializer_mock, instance_mock, method_mock
+    function_mock, class_mock, initializer_mock, instance_mock, loose_mock,
+    method_mock
 )
 
 
@@ -328,3 +329,43 @@ def _RationalIfdEntry_(self, request, ifd_entry_):
     @pytest.fixture
     def offset_(self, request):
         return instance_mock(request, int)
+
+
+class Describe_IfdEntry(object):
+
+    def it_can_construct_from_a_stream_and_offset(self, from_stream_fixture):
+        (stream_rdr, offset, _parse_value_, value_count, value_offset,
+         _IfdEntry__init_, tag_code, value_) = from_stream_fixture
+        ifd_entry = _IfdEntry.from_stream(stream_rdr, offset)
+        _parse_value_.assert_called_once_with(
+            stream_rdr, offset, value_count, value_offset
+        )
+        _IfdEntry__init_.assert_called_once_with(tag_code, value_)
+        assert isinstance(ifd_entry, _IfdEntry)
+
+    # fixtures -------------------------------------------------------
+
+    @pytest.fixture
+    def from_stream_fixture(
+            self, _parse_value_, _IfdEntry__init_, value_):
+        bytes_ = b'\x00\x01\x66\x66\x00\x00\x00\x02\x00\x00\x00\x03'
+        stream_rdr = StreamReader(BytesIO(bytes_), BIG_ENDIAN)
+        offset, tag_code, value_count, value_offset = 0, 1, 2, 3
+        return (
+            stream_rdr, offset, _parse_value_, value_count, value_offset,
+            _IfdEntry__init_, tag_code, value_
+        )
+
+    @pytest.fixture
+    def _IfdEntry__init_(self, request):
+        return initializer_mock(request, _IfdEntry)
+
+    @pytest.fixture
+    def _parse_value_(self, request, value_):
+        return method_mock(
+            request, _IfdEntry, '_parse_value', return_value=value_
+        )
+
+    @pytest.fixture
+    def value_(self, request):
+        return loose_mock(request)
