img: #124 default to inches on no TIFF res unit

HISTORY.rst

@@ -3,6 +3,12 @@
 Release History
 ---------------
 
+NEXT
+++++++++++++++++++
+
+- Fix #124: default to inches on no TIFF resolution unit
+
+
 0.8.3 (2015-02-19)
 ++++++++++++++++++
 

docx/image/tiff.py

@@ -116,14 +116,22 @@ def _dpi(self, resolution_tag):
         calculation is based on the values of both that tag and the
         TIFF_TAG.RESOLUTION_UNIT tag in this parser's |_IfdEntries| instance.
         """
-        if resolution_tag not in self._ifd_entries:
+        ifd_entries = self._ifd_entries
+
+        if resolution_tag not in ifd_entries:
             return 72
-        resolution_unit = self._ifd_entries[TIFF_TAG.RESOLUTION_UNIT]
+
+        # resolution unit defaults to inches (2)
+        resolution_unit = (
+            ifd_entries[TIFF_TAG.RESOLUTION_UNIT]
+            if TIFF_TAG.RESOLUTION_UNIT in ifd_entries else 2
+        )
+
         if resolution_unit == 1:  # aspect ratio only
             return 72
         # resolution_unit == 2 for inches, 3 for centimeters
         units_per_inch = 1 if resolution_unit == 2 else 2.54
-        dots_per_unit = self._ifd_entries[resolution_tag]
+        dots_per_unit = ifd_entries[resolution_tag]
         return int(round(dots_per_unit * units_per_inch))
 
     @classmethod

tests/image/test_tiff.py

@@ -116,16 +116,19 @@ def it_knows_the_horz_and_vert_dpi_after_parsing(self, dpi_fixture):
     # fixtures -------------------------------------------------------
 
     @pytest.fixture(params=[
-        (1,  150,  240,  72,  72),
-        (2,   42,   24,  42,  24),
-        (3,  100,  200, 254, 508),
-        (6, None, None,  72,  72),
+        (1,     150,  240,  72,  72),
+        (2,      42,   24,  42,  24),
+        (3,     100,  200, 254, 508),
+        (2,    None, None,  72,  72),
+        (None,   96,  100,  96, 100),
     ])
     def dpi_fixture(self, request):
         resolution_unit, x_resolution, y_resolution = request.param[:3]
         expected_horz_dpi, expected_vert_dpi = request.param[3:]
 
-        entries = {TIFF_TAG.RESOLUTION_UNIT: resolution_unit}
+        entries = {}
+        if resolution_unit is not None:
+            entries[TIFF_TAG.RESOLUTION_UNIT] = resolution_unit
         if x_resolution is not None:
             entries[TIFF_TAG.X_RESOLUTION] = x_resolution
         if y_resolution is not None: