Fix diff-parser: Copying parts of if else should not lead to the whole thing being copied

parso/python/diff.py

@@ -171,6 +171,7 @@ class DiffParser(object):
                 % (last_pos, line_length, parso.__version__, ''.join(diff))
             )
 
+        #assert self._module.get_code() == ''.join(new_lines)
         LOG.debug('diff parser end')
         return self._module
 
@@ -212,7 +213,9 @@ class DiffParser(object):
 
                     to = self._nodes_tree.parsed_until_line
 
-                    LOG.debug('actually copy %s to %s', from_, to)
+                    LOG.debug('copy old[%s:%s] new[%s:%s]',
+                              copied_nodes[0].start_pos[0],
+                              copied_nodes[-1].end_pos[0] - 1, from_, to)
             # Since there are potential bugs that might loop here endlessly, we
             # just stop here.
             assert last_until_line != self._nodes_tree.parsed_until_line \
@@ -228,7 +231,11 @@ class DiffParser(object):
             node = leaf
             while node.parent.type not in ('file_input', 'suite'):
                 node = node.parent
-            return node
+
+            # Make sure that if only the `else:` line of an if statement is
+            # copied that not the whole thing is going to be copied.
+            if node.start_pos[0] >= old_line:
+                return node
         # Must be on the same line. Otherwise we need to parse that bit.
         return None
 

test/test_diff_parser.py

@@ -142,7 +142,7 @@ def test_if_simple(differ):
     differ.initialize(src + 'a')
     differ.parse(src + else_ + "a", copies=0, parsers=1)
 
-    differ.parse(else_, parsers=1, expect_error_leaves=True)
+    differ.parse(else_, parsers=1, copies=1, expect_error_leaves=True)
     differ.parse(src + else_, parsers=1)
 
 
@@ -560,3 +560,34 @@ def test_invalid_to_valid_nodes(differ):
 
     differ.initialize(code1)
     differ.parse(code2, parsers=1, copies=3)
+
+
+def test_if_removal_and_reappearence(differ):
+    code1 = dedent('''\
+        la = 3
+        if foo:
+            latte = 3
+        else:
+            la
+        pass
+    ''')
+
+    code2 = dedent('''\
+        la = 3
+            latte = 3
+        else:
+            la
+        pass
+    ''')
+
+    code3 = dedent('''\
+        la = 3
+        if foo:
+            latte = 3
+        else:
+            la
+    ''')
+    differ.initialize(code1)
+    differ.parse(code2, parsers=1, copies=4, expect_error_leaves=True)
+    differ.parse(code1, parsers=1, copies=1)
+    differ.parse(code3, parsers=1, copies=1)