Fix segfault due to an internal cache mechanism

Fix #86

CHANGELOG.md

@@ -1,5 +1,18 @@
 # PythonQwt Releases
 
+## Version 0.12.3
+
+- Fixed `Fatal Python error` issue reported in the `PlotPy` project:
+  - See [PlotPy's Issue #11](https://github.com/PlotPyStack/PlotPy/issues/11) for the
+    original issue, even if the problem is not directly pointed out in the issue
+    comments.
+  - The issue was caused by the `QwtAbstractScaleDraw` cache mechanism, which was
+    keeping references to `QSizeF` objects that were deleted by the garbage collector
+    at some point. This was causing a segmentation fault, but only on Linux, and
+    only when executing the `PlotPy` test suite in a specific order.
+  - Thanks to @yuzibo for helping to reproduce the issue and providing a test case,
+    that is the `PlotPy` Debian package build process.
+
 ## Version 0.12.2
 
 For this release, test coverage is 72%.

qwt/__init__.py

@@ -55,7 +55,7 @@
 from qwt.text import QwtText  # noqa: F401
 from qwt.toqimage import array_to_qimage as toQImage  # noqa: F401
 
-__version__ = "0.12.2"
+__version__ = "0.12.3"
 QWT_VERSION_STR = "6.1.5"
 
 

qwt/scale_draw.py

@@ -438,14 +438,13 @@ def tickLabel(self, font, value):
         :param float value: Value
         :return: Tuple (tick label, text size)
         """
-        lbl, tsize = self.__data.labelCache.get(value, (None, None))
+        lbl = self.__data.labelCache.get(value)
         if lbl is None:
             lbl = QwtText(self.label(value))
             lbl.setRenderFlags(0)
             lbl.setLayoutAttribute(QwtText.MinimumLayout)
-            tsize = lbl.textSize(font)
-            self.__data.labelCache[value] = lbl, tsize
-        return lbl, tsize
+            self.__data.labelCache[value] = lbl
+        return lbl, lbl.textSize(font)
 
     def invalidateCache(self):
         """