Fix variables with null default not being required (#1919) (#1920)

* Fix variables with null default not being required (#1919)

* Got rid of walrus operator to support Python 3.7

cookiecutter/prompt.py

@@ -20,7 +20,13 @@ def read_user_variable(var_name, default_value, prompts=None, prefix=""):
         if prompts and var_name in prompts.keys() and prompts[var_name]
         else var_name
     )
-    return Prompt.ask(f"{prefix}{question}", default=default_value)
+
+    while True:
+        variable = Prompt.ask(f"{prefix}{question}", default=default_value)
+        if variable is not None:
+            break
+
+    return variable
 
 
 class YesNoPrompt(Confirm):

tests/test_read_user_variable.py

@@ -1,18 +1,36 @@
 """test_read_user_variable."""
+import pytest
 from cookiecutter.prompt import read_user_variable
 
 VARIABLE = 'project_name'
 DEFAULT = 'Kivy Project'
 
 
-def test_click_invocation(mocker):
+@pytest.fixture
+def mock_prompt(mocker):
+    """Return a mocked version of the 'Prompt.ask' function."""
+    return mocker.patch('rich.prompt.Prompt.ask')
+
+
+def test_click_invocation(mock_prompt):
     """Test click function called correctly by cookiecutter.
 
     Test for string type invocation.
     """
-    prompt = mocker.patch('rich.prompt.Prompt.ask')
-    prompt.return_value = DEFAULT
+    mock_prompt.return_value = DEFAULT
 
     assert read_user_variable(VARIABLE, DEFAULT) == DEFAULT
 
-    prompt.assert_called_once_with(VARIABLE, default=DEFAULT)
+    mock_prompt.assert_called_once_with(VARIABLE, default=DEFAULT)
+
+
+def test_input_loop_with_null_default_value(mock_prompt):
+    """Test `Prompt.ask` is run repeatedly until a valid answer is provided.
+
+    Test for `default_value` parameter equal to None.
+    """
+    # Simulate user providing None input initially and then a valid input
+    mock_prompt.side_effect = [None, DEFAULT]
+
+    assert read_user_variable(VARIABLE, None) == DEFAULT
+    assert mock_prompt.call_count == 2