designspaceLib: add loadSourceFonts method with custom opener

Allows to load the SourceDescriptor.font attribute from its path, using a custom callable (e.g. defcon.Font or ttLib.TTFont, etc.).

Lib/fontTools/designspaceLib/__init__.py

@@ -1262,3 +1262,41 @@ class DesignSpaceDocument(LogMixin, AsDictMixin):
                     newConditions.append(dict(name=cond['name'], minimum=minimum, maximum=maximum))
                 newConditionSets.append(newConditions)
             rule.conditionSets = newConditionSets
+
+    def loadSourceFonts(self, opener, **kwargs):
+        """Ensure SourceDescriptor.font attributes are loaded, and return list of fonts.
+
+        Takes a callable which initializes a new font object (e.g. TTFont, or
+        defcon.Font, etc.) from the SourceDescriptor.path, and sets the
+        SourceDescriptor.font attribute.
+        If the font attribute is already not None, it is not loaded again.
+        Fonts with the same path are only loaded once and shared among SourceDescriptors.
+
+        Args:
+            opener (Callable): takes one required positional argument, the source.path,
+                and an optional list of keyword arguments, and returns a new font object
+                loaded from the path.
+            **kwargs: extra options passed on to the opener function.
+
+        Returns:
+            List of font objects in the order they appear in the sources list.
+        """
+        # we load fonts with the same source.path only once
+        loaded = {}
+        fonts = []
+        for source in self.sources:
+            if source.font is not None:  # font already loaded
+                fonts.append(source.font)
+                continue
+            if source.path in loaded:
+                source.font = loaded[source.path]
+            else:
+                if source.path is None:
+                    raise DesignSpaceDocumentError(
+                        "Designspace source '%s' has no 'path' attribute"
+                        % (source.name or "<Unknown>")
+                    )
+                source.font = opener(source.path, **kwargs)
+                loaded[source.path] = source.font
+            fonts.append(source.font)
+        return fonts

Tests/designspaceLib/designspace_test.py

@@ -13,6 +13,7 @@ from fontTools.misc import plistlib
 from fontTools.designspaceLib import (
     DesignSpaceDocument, SourceDescriptor, AxisDescriptor, RuleDescriptor,
     InstanceDescriptor, evaluateRule, processRules, posix, DesignSpaceDocumentError)
+from fontTools import ttLib
 
 def _axesAsDict(axes):
     """
@@ -909,3 +910,42 @@ def test_findDefault_axis_mapping():
     designspace.axes[1].default = 0
 
     assert designspace.findDefault().filename == "Font-Regular.ufo"
+
+
+def test_loadSourceFonts():
+
+    def opener(path):
+        font = ttLib.TTFont()
+        font.importXML(path)
+        return font
+
+    # this designspace file contains .TTX source paths
+    path = os.path.join(
+        os.path.dirname(os.path.dirname(__file__)),
+        "varLib",
+        "data",
+        "SparseMasters.designspace"
+    )
+    designspace = DesignSpaceDocument.fromfile(path)
+
+    # force two source descriptors to have the same path
+    designspace.sources[1].path = designspace.sources[0].path
+
+    fonts = designspace.loadSourceFonts(opener)
+
+    assert len(fonts) == 3
+    assert all(isinstance(font, ttLib.TTFont) for font in fonts)
+    assert fonts[0] is fonts[1]  # same path, identical font object
+
+    fonts2 = designspace.loadSourceFonts(opener)
+
+    for font1, font2 in zip(fonts, fonts2):
+        assert font1 is font2
+
+
+def test_loadSourceFonts_no_required_path():
+    designspace = DesignSpaceDocument()
+    designspace.sources.append(SourceDescriptor())
+
+    with pytest.raises(DesignSpaceDocumentError, match="no 'path' attribute"):
+        designspace.loadSourceFonts(lambda p: p)