designspaceLib: add getSourcePath method

and remove BaseDocReader.getSourcePaths as it was broken anyway (it was assuming 'sources'
attribute of DesignSpaceDocument object was a dict, whereas it is now a list of
SourceDescriptor objects

Lib/fontTools/designspaceLib/__init__.py

@@ -673,12 +673,6 @@ class BaseDocReader(LogMixin):
         self.readInstances()
         self.readLib()
 
-    def getSourcePaths(self, makeGlyphs=True, makeKerning=True, makeInfo=True):
-        paths = []
-        for name in self.documentObject.sources.keys():
-            paths.append(self.documentObject.sources[name][0].path)
-        return paths
-
     def readRules(self):
         # we also need to read any conditions that are outside of a condition set.
         rules = []
@@ -1278,3 +1272,30 @@ class DesignSpaceDocument(LogMixin, AsDictMixin):
                     newConditions.append(dict(name=cond['name'], minimum=minimum, maximum=maximum))
                 newConditionSets.append(newConditions)
             rule.conditionSets = newConditionSets
+
+    def getSourcePath(self, sourceDescriptor):
+        """Return the absolute path of sourceDescriptor. This is either the
+        'path' attribute, or if the latter is not set, the concatenation of
+        the source's (relative) 'filename' attribute and the designspace's
+        directory.
+
+        Raises DesignSpaceDocumentError if sourceDescriptor.path is None and
+        self.path is None, or sourceDescriptor.filename is None.
+        """
+        if sourceDescriptor.path:
+            # prefer absolute path if present
+            return sourceDescriptor.path
+        if self.path is None:
+            raise DesignSpaceDocumentError(
+                "DesignSpaceDocument 'path' attribute is not defined; cannot "
+                "load source from relative filename"
+            )
+        if sourceDescriptor.filename is None:
+            raise DesignSpaceDocumentError(
+                "Designspace source '%s' has neither absolute 'path' nor "
+                "relative 'filename' defined"
+                % (sourceDescriptor.name or "<Unknown>")
+            )
+        return os.path.abspath(
+            os.path.join(os.path.dirname(self.path), sourceDescriptor.filename)
+        )

Lib/fontTools/varLib/__init__.py

@@ -809,26 +809,6 @@ def build(designspace, master_finder=lambda s:s, exclude=[], optimize=True):
 	return vf, model, master_ttfs
 
 
-# TODO: move to designspaceLib?
-def _get_master_path(designspace, master):
-	if master.path:
-		# prefer absolute path if present
-		return master.path
-	if designspace.path is None:
-		raise AttributeError(
-			"DesignSpaceDocument 'path' attribute is not defined; cannot "
-			"load master from relative filename"
-		)
-	if master.filename is None:
-		raise AttributeError(
-			"Designspace source '%s' has neither absolute 'path' nor "
-			"relative 'filename' defined"
-			% (master.name or "<Unknown>")
-		)
-	return os.path.join(os.path.dirname(designspace.path), master.filename)
-
-
-
 def load_masters(designspace, master_finder=lambda s: s):
 	"""Ensure that all SourceDescriptor.font attributes have an appropriate TTFont
 	object loaded.
@@ -855,7 +835,7 @@ def load_masters(designspace, master_finder=lambda s: s):
 			else:
 				# 2. A SourceDescriptor's filename might point to a UFO or an OpenType
 				# binary. Find out the hard way.
-				master_path = _get_master_path(designspace, master)
+				master_path = designspace.getSourcePath(master)
 				try:
 					font = TTFont(master_path)
 				except (IOError, TTLibError):

Tests/designspaceLib/designspace_test.py

@@ -785,3 +785,28 @@ def test_documentLib(tmpdir):
     assert dummyKey in new.lib
     assert new.lib[dummyKey] == dummyData
 
+
+def test_getSourcePath():
+    doc = DesignSpaceDocument()
+    s1 = SourceDescriptor()
+
+    with pytest.raises(
+        DesignSpaceDocumentError,
+        match="DesignSpaceDocument 'path' attribute is not defined",
+    ):
+        doc.getSourcePath(s1)
+
+    doc.path = "/tmp/foo/bar/MyDesignspace.designspace"
+
+    with pytest.raises(
+        DesignSpaceDocumentError,
+        match=(
+            "Designspace source '<Unknown>' has neither absolute 'path' nor "
+            "relative 'filename' defined"
+        ),
+    ):
+        doc.getSourcePath(s1)
+
+    s1.filename = "../masters/Source1.ufo"
+
+    assert doc.getSourcePath(s1) == "/tmp/foo/masters/Source1.ufo"