Raise MissingFileSystem instead of AttributeError when diffing reports without source files

CHANGES.md

@@ -2,6 +2,8 @@
 
 ## Unreleased
 
+* Raise `Cobertura.MissingFileSystem` instead of `AttributeError` when diffing
+  reports without source files.
 * Add `--sort-by-uncovered-lines` to sort output, so the file with the most missing lines is at the
   top. Thanks @guettli
 * Fix: JSON and YAML output when using `--ignore-regex` in `show` command. Thanks @OidaTiftla

README.md

@@ -403,9 +403,14 @@ pycobertura/utils.py            12       0  100.00%
 TOTAL                          253       0  100.00%"""
 
 from pycobertura import TextReporterDelta
-
-coverage1 = Cobertura('coverage1.xml')
-coverage2 = Cobertura('coverage2.xml')
+from pycobertura.filesystem import filesystem_factory
+
+coverage1 = Cobertura(
+    'coverage1.xml', filesystem=filesystem_factory('source1')
+)
+coverage2 = Cobertura(
+    'coverage2.xml', filesystem=filesystem_factory('source2')
+)
 delta = TextReporterDelta(coverage1, coverage2)
 delta.generate() == """\
 Filename          Stmts    Miss    Cover     Missing

pycobertura/cobertura.py

@@ -212,6 +212,12 @@ def _raise_MissingFileSystem(self, filename):
             f"content of the file."
         )
 
+    def _filesystem_has_file(self, filename):
+        if self.filesystem is None:
+            self._raise_MissingFileSystem(filename)
+
+        return self.filesystem.has_file(filename)
+
     @memoize
     def file_source(self, filename):
         """
@@ -437,7 +443,7 @@ def file_source(self, filename: str):
 
         """
         nonexistent = True
-        if self.cobertura1.has_file(filename) and self.cobertura1.filesystem.has_file(
+        if self.cobertura1.has_file(filename) and self.cobertura1._filesystem_has_file(
             filename
         ):
             lines1 = self.cobertura1.source_lines(filename)
@@ -447,7 +453,7 @@ def file_source(self, filename: str):
             lines1 = []
             line_statuses1: Dict[int, LineStatus] = {}
 
-        if self.cobertura2.has_file(filename) and self.cobertura2.filesystem.has_file(
+        if self.cobertura2.has_file(filename) and self.cobertura2._filesystem_has_file(
             filename
         ):
             lines2 = self.cobertura2.source_lines(filename)

tests/test_cobertura_diff.py

@@ -1,6 +1,20 @@
+import pytest
+
 from .utils import make_cobertura
 
 
+@pytest.mark.parametrize("filename", ["dummy/dummy.py", "dummy/dummy3.py"])
+def test_diff_file_source_without_filesystem(filename):
+    from pycobertura.cobertura import Cobertura, CoberturaDiff
+
+    cobertura1 = Cobertura('tests/dummy.source1/coverage.xml')
+    cobertura2 = Cobertura('tests/dummy.source2/coverage.xml')
+    differ = CoberturaDiff(cobertura1, cobertura2)
+
+    with pytest.raises(Cobertura.MissingFileSystem):
+        differ.file_source(filename)
+
+
 def test_diff_class_source():
     from pycobertura.cobertura import CoberturaDiff
     from pycobertura.cobertura import Line