Print empty char after updated code exercise (#80)

To enforce `clear_output` even when the update is not printing anything
we used `print("", end"")` this seems not to always work so we print an
empty char now. In the tests this introduces another output that makes
the tests fail checking the number of outputs. Because the number of
outputs is testing anything meaningful we remove these tests. There are
tests that check the joined texts of the outputs that cover the outputs
already good enough.

Author: agoscinski

src/scwidgets/exercise/_widget_code_exercise.py

@@ -727,8 +727,8 @@ def _on_click_update_action(self) -> bool:
 
             # The clear_output command at the beginning of the function waits till
             # something is printed. If nothing is printed, it is not cleared. We
-            # enforce it to be invoked by printing an empty string
-            print("", end="")
+            # enforce it to be invoked by printing an empty char
+            print("\0", end="")
 
         return not (raised_error)
 

tests/test_widgets.py

@@ -1234,7 +1234,6 @@ def test_code_exercise(
         if tunable_params:
             outputs = nb_cell.find_elements(By.CLASS_NAME, OUTPUT_CLASS_NAME)
             # In the code we print a text that adds another output
-            assert len(outputs) == 1 + include_code
             before_parameter_change_text = "".join([output.text for output in outputs])
 
             slider_input_box = nb_cell.find_element(By.CLASS_NAME, "widget-readout")
@@ -1257,15 +1256,13 @@ def test_code_exercise(
                 # Check if output has changed only after click when manual
                 outputs = nb_cell.find_elements(By.CLASS_NAME, OUTPUT_CLASS_NAME)
                 # In the code we print a text that adds another output
-                assert len(outputs) == 1 + include_code
                 after_parameter_change_text = "".join(
                     [output.text for output in outputs]
                 )
                 assert before_parameter_change_text == after_parameter_change_text
                 update_button.click()
 
             outputs = nb_cell.find_elements(By.CLASS_NAME, OUTPUT_CLASS_NAME)
-            assert len(outputs) == 1 + include_code
             after_parameter_change_text = "".join([output.text for output in outputs])
             assert before_parameter_change_text != after_parameter_change_text