Implement optional default for override_tag

docs/overview.md

@@ -134,6 +134,21 @@ The override process has two parts:
 1.  Override your template tag with a mock implementation
 2.  Define fake result for your tag in a `yaml` file
 
+
+### Providing a default value for template tags
+To provide a default for a template tag, you need to provide a keyword argument default when overriding your tag.
+
+```
+from pattern_library.monkey_utils import override_tag
+
+override_tag(register, 'a_tag_name', default="https://potato.com")
+```
+
+This default is used for any tag that's not passed its own context, allowing specificity for those elements that need it while preventing the tag from breaking when it's not structural to the component.
+
+#### limitation
+Currently the default override is limited to direct references only i.e. {% a_tag_name page.url %} and not {% a_tag_name page.url as variable_name %} 
+
 ### When do I need to override a template tag?
 
 Ideally your pattern library should be independent, so it doesn't fail when

pattern_library/monkey_utils.py

@@ -7,9 +7,10 @@
 )
 
 logger = logging.getLogger(__name__)
+UNSPECIFIED = object()
 
 
-def override_tag(register, name):
+def override_tag(register, name, default_html=None):
     """
     An utility that helps you override original tags for use in your pattern library.
 
@@ -75,9 +76,13 @@ def node_render(context):
 
                     # Render result instead of the tag
                     return result
+                elif default_html is not UNSPECIFIED:
+                    # Render provided default;
+                    # if no stub data supplied.
+                    return default_html
                 else:
                     logger.warning(
-                        'No stub data defined for the "%s" tag in the "%s" template',
+                        'No default or stub data defined for the "%s" tag in the "%s" template',
                         tag_name, current_template_name
                     )
 

tests/templates/patterns/atoms/tags_test_atom/tags_test_atom.html

@@ -3,3 +3,11 @@
 SAND{% error_tag empty_string %}WICH
 SAND{% error_tag none %}WICH
 SAND{% error_tag zero %}WICH
+
+POTA{% default_html_tag page.url %}TO 
+POTA{% default_html_tag page.child.url %}TO
+POTA{% default_html_tag None %}TO
+
+POTA{% default_html_tag_falsey empty_string %}TO1
+POTA{% default_html_tag_falsey none %}TO2
+POTA{% default_html_tag_falsey zero %}TO3

tests/templates/patterns/atoms/tags_test_atom/tags_test_atom.yaml

@@ -6,3 +6,17 @@ tags:
       raw: null
     zero:
       raw: 0
+  default_html_tag:
+    page.url:
+      raw: "example"
+    page.child.url:
+      raw: "another_example"
+    # None handled by default html value.
+  default_html_tag_falsey:
+    empty_string:
+      raw: ''
+    none:
+      raw: null
+    zero:
+      raw: 0
+

tests/templatetags/test_tags.py

@@ -5,10 +5,27 @@
 register = template.Library()
 
 
+# Basic template tag
 @register.simple_tag
 def error_tag(arg=None):
     "Just raise an exception, never do anything"
     raise Exception("error_tag raised an exception")
 
 
+# Template tag to to test setting a default html value.
+@register.simple_tag()
+def default_html_tag(arg=None):
+    "Just raise an exception, never do anything"
+    raise Exception("default_tag raised an exception")
+
+
+# Template tag to to test setting a default html value that is falsey.
+@register.simple_tag()
+def default_html_tag_falsey(arg=None):
+    "Just raise an exception, never do anything"
+    raise Exception("default_tag raised an exception")
+
+
 override_tag(register, 'error_tag')
+override_tag(register, 'default_html_tag', default_html="https://potato.com")
+override_tag(register, 'default_html_tag_falsey', default_html=None)

tests/tests/test_tags.py

@@ -3,7 +3,7 @@
 from .utils import reverse
 
 
-class TagsTesCase(SimpleTestCase):
+class TagsTestCase(SimpleTestCase):
     def test_falsey_raw_values_for_tag_output(self):
         response = self.client.get(reverse(
             'pattern_library:render_pattern',
@@ -12,3 +12,22 @@ def test_falsey_raw_values_for_tag_output(self):
         self.assertContains(response, "SANDWICH")
         self.assertContains(response, "SANDNoneWICH")
         self.assertContains(response, "SAND0WICH")
+
+    def test_default_html_override(self):
+        response = self.client.get(reverse(
+            'pattern_library:render_pattern',
+            kwargs={'pattern_template_name': 'patterns/atoms/tags_test_atom/tags_test_atom.html'},
+        ))
+
+        self.assertContains(response, "POTAexampleTO")
+        self.assertContains(response, "POTAanother_exampleTO")
+        self.assertContains(response, "POTAhttps://potato.comTO")
+
+    def test_falsey_default_html_overide(self):
+        response = self.client.get(reverse(
+            'pattern_library:render_pattern',
+            kwargs={'pattern_template_name': 'patterns/atoms/tags_test_atom/tags_test_atom.html'},
+        ))
+        self.assertContains(response, "POTATO1")
+        self.assertContains(response, "POTANoneTO2")
+        self.assertContains(response, "POTA0TO3")