allow empty css variables

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - Fixed a crash when a `SelectionList` had a prompt wider than itself https://github.com/Textualize/textual/issues/2900
 - Fixed a bug where `Click` events were bubbling up from `Switch` widgets https://github.com/Textualize/textual/issues/2366
+- Fixed a crash when using empty CSS variables https://github.com/Textualize/textual/issues/1849
 
 ## [0.30.0] - 2023-07-17
 

src/textual/css/parse.py

@@ -269,6 +269,7 @@ def substitute_references(
             break
         if token.name == "variable_name":
             variable_name = token.value[1:-1]  # Trim the $ and the :, i.e. "$x:" -> "x"
+            variable_tokens = variables.setdefault(variable_name, [])
             yield token
 
             while True:
@@ -284,7 +285,7 @@ def substitute_references(
                 if not token:
                     break
                 elif token.name == "whitespace":
-                    variables.setdefault(variable_name, []).append(token)
+                    variable_tokens.append(token)
                     yield token
                 elif token.name == "variable_value_end":
                     yield token
@@ -293,7 +294,6 @@ def substitute_references(
                 elif token.name == "variable_ref":
                     ref_name = token.value[1:]
                     if ref_name in variables:
-                        variable_tokens = variables.setdefault(variable_name, [])
                         reference_tokens = variables[ref_name]
                         variable_tokens.extend(reference_tokens)
                         ref_location = token.location
@@ -307,7 +307,7 @@ def substitute_references(
                     else:
                         _unresolved(ref_name, variables.keys(), token)
                 else:
-                    variables.setdefault(variable_name, []).append(token)
+                    variable_tokens.append(token)
                     yield token
                 token = next(iter_tokens, None)
         elif token.name == "variable_ref":

tests/css/test_parse.py

@@ -220,6 +220,22 @@ def test_undefined_variable(self):
         with pytest.raises(UnresolvedVariableError):
             list(substitute_references(tokenize(css, "")))
 
+    def test_empty_variable(self):
+        css = "$x:\n* { background:$x; }"
+        result = list(substitute_references(tokenize(css, "")))
+        assert [(t.name, t.value) for t in result] == [
+            ("variable_name", "$x:"),
+            ("variable_value_end", "\n"),
+            ("selector_start_universal", "*"),
+            ("whitespace", " "),
+            ("declaration_set_start", "{"),
+            ("whitespace", " "),
+            ("declaration_name", "background:"),
+            ("declaration_end", ";"),
+            ("whitespace", " "),
+            ("declaration_set_end", "}"),
+        ]
+
     def test_transitive_reference(self):
         css = "$x: 1\n$y: $x\n.thing { border: $y }"
         assert list(substitute_references(tokenize(css, ""))) == [