allow saving a file with a new name

babi/screen.py

@@ -494,17 +494,17 @@ class Screen:
             else:
                 self.file.filename = filename
 
-        if os.path.isfile(self.file.filename):
+        if not os.path.isfile(self.file.filename):
+            sha256: Optional[str] = None
+        else:
             with open(self.file.filename, encoding='UTF-8', newline='') as f:
                 *_, sha256 = get_lines(f)
-        else:
-            sha256 = hashlib.sha256(b'').hexdigest()
 
         contents = self.file.nl.join(self.file.buf)
         sha256_to_save = hashlib.sha256(contents.encode()).hexdigest()
 
         # the file on disk is the same as when we opened it
-        if sha256 not in (self.file.sha256, sha256_to_save):
+        if sha256 not in (None, self.file.sha256, sha256_to_save):
             self.status.update('(file changed on disk, not implemented)')
             return PromptResult.CANCELLED
 

tests/features/save_test.py

@@ -162,6 +162,18 @@ def test_save_via_ctrl_o_set_filename(run, tmpdir):
     assert f.read() == 'hello world\n'
 
 
+def test_save_via_ctrl_o_new_filename(run, tmpdir):
+    f = tmpdir.join('f')
+    f.write('wat\n')
+    with run(str(f)) as h, and_exit(h):
+        h.press('^O')
+        h.await_text('enter filename: ')
+        h.press_and_enter('new')
+        h.await_text('saved! (1 line written)')
+    assert f.read() == 'wat\n'
+    assert tmpdir.join('fnew').read() == 'wat\n'
+
+
 @pytest.mark.parametrize('key', ('^C', 'Enter'))
 def test_save_via_ctrl_o_cancelled(run, key):
     with run() as h, and_exit(h):