Fix handling of a blank prompt

Make sure prompt is correctly setup even if the prompt is literally a blank string, as could be set using input()

qtconsole/console_widget.py

@@ -2519,6 +2519,9 @@ def _show_prompt(self, prompt=None, html=False, newline=True,
 
         cursor = self._get_end_cursor()
 
+        # Memorize end to check if we actually add any prompt characters
+        prior_end_pos = cursor.position()
+
         # Save the current position to support _append*(before_prompt=True).
         # We can't leave the cursor at the end of the document though, because
         # that would cause any further additions to move the cursor. Therefore,
@@ -2557,7 +2560,16 @@ def _show_prompt(self, prompt=None, html=False, newline=True,
                 self._prompt_html = None
 
         self._flush_pending_stream()
-        self._prompt_cursor.setPosition(self._get_end_pos() - 1)
+
+        current_end_pos = self._get_end_pos()
+        if prior_end_pos != current_end_pos:
+            # Set the prompt cursor to end minus 1, so long as
+            # the prompt was not blank
+            self._prompt_cursor.setPosition(current_end_pos - 1)
+        else:
+            # The prompt didn't move end, i.e. the prompt inserted exactly 0 characters
+            # Move cursor to end
+            self._prompt_cursor.setPosition(current_end_pos)
 
         if move_forward:
             self._append_before_prompt_cursor.setPosition(

qtconsole/tests/test_00_console_widget.py

@@ -491,6 +491,15 @@ def test_prompt_cursors(self):
         self.assertEqual(w._append_before_prompt_pos,
                          w._prompt_pos - len(w._prompt))
 
+        # Test a blank prompt. Such as from input()
+        w._append_plain_text('\n')
+        w._show_prompt(prompt = '', separator = False)
+
+        w._append_plain_text('plain text\n')
+
+        self.assertEqual(w._prompt_pos, w._get_end_pos())
+        self.assertEqual(w._append_before_prompt_pos, w._prompt_pos)
+
     def test_select_all(self):
         w = ConsoleWidget()
         w._append_plain_text('Header\n')