Merge pull request #582 from Textualize/specificity-clash-order-fix

Tests around CSS specificity, and fix ordering in case of specificity clash

src/textual/css/stylesheet.py

@@ -288,7 +288,7 @@ class Stylesheet:
             rule_attributes[key].append((default_specificity, value))
 
         # Collect the rules defined in the stylesheet
-        for rule in self.rules:
+        for rule in reversed(self.rules):
             for specificity in _check_rule(rule, node):
                 for key, rule_specificity, value in rule.styles.extract_rules(
                     specificity
@@ -304,6 +304,7 @@ class Stylesheet:
                 for name, specificity_rules in rule_attributes.items()
             },
         )
+
         self.replace_rules(node, node_rules, animate=animate)
         if isinstance(node, Widget):
             node._refresh_scrollbars()

tests/css/test_stylesheet.py

@@ -7,6 +7,98 @@ from textual.color import Color
 from textual.css._help_renderables import HelpText
 from textual.css.stylesheet import Stylesheet, StylesheetParseError
 from textual.css.tokenizer import TokenizeError
+from textual.dom import DOMNode
+from textual.geometry import Spacing
+
+
+def _make_stylesheet(css: str) -> Stylesheet:
+    stylesheet = Stylesheet()
+    stylesheet.source["test.css"] = css
+    stylesheet.parse()
+    return stylesheet
+
+
+def test_stylesheet_apply_highest_specificity_wins():
+    """#ids have higher specificity than .classes"""
+    css = "#id {color: red;} .class {color: blue;}"
+    stylesheet = _make_stylesheet(css)
+    node = DOMNode(classes="class", id="id")
+    stylesheet.apply(node)
+
+    assert node.styles.color == Color(255, 0, 0)
+
+
+def test_stylesheet_apply_doesnt_override_defaults():
+    css = "#id {color: red;}"
+    stylesheet = _make_stylesheet(css)
+    node = DOMNode(id="id")
+    stylesheet.apply(node)
+
+    assert node.styles.margin == Spacing.all(0)
+    assert node.styles.box_sizing == "border-box"
+
+
+def test_stylesheet_apply_highest_specificity_wins_multiple_classes():
+    """When we use two selectors containing only classes, then the selector
+    `.b.c` has greater specificity than the selector `.a`"""
+    css = ".b.c {background: blue;} .a {background: red; color: lime;}"
+    stylesheet = _make_stylesheet(css)
+    node = DOMNode(classes="a b c")
+    stylesheet.apply(node)
+
+    assert node.styles.background == Color(0, 0, 255)
+    assert node.styles.color == Color(0, 255, 0)
+
+
+def test_stylesheet_many_classes_dont_overrule_id():
+    """#id is further to the left in the specificity tuple than class, and
+    a selector containing multiple classes cannot take priority over even a
+    single class."""
+    css = "#id {color: red;} .a.b.c.d {color: blue;}"
+    stylesheet = _make_stylesheet(css)
+    node = DOMNode(classes="a b c d", id="id")
+    stylesheet.apply(node)
+
+    assert node.styles.color == Color(255, 0, 0)
+
+
+def test_stylesheet_last_rule_wins_when_same_rule_twice_in_one_ruleset():
+    css = "#id {color: red; color: blue;}"
+    stylesheet = _make_stylesheet(css)
+    node = DOMNode(id="id")
+    stylesheet.apply(node)
+
+    assert node.styles.color == Color(0, 0, 255)
+
+
+def test_stylesheet_rulesets_merged_for_duplicate_selectors():
+    css = "#id {color: red; background: lime;} #id {color:blue;}"
+    stylesheet = _make_stylesheet(css)
+    node = DOMNode(id="id")
+    stylesheet.apply(node)
+
+    assert node.styles.color == Color(0, 0, 255)
+    assert node.styles.background == Color(0, 255, 0)
+
+
+def test_stylesheet_apply_takes_final_rule_in_specificity_clash():
+    """.a and .b both contain background and have same specificity, so .b wins
+    since it was declared last - the background should be blue."""
+    css = ".a {background: red; color: lime;} .b {background: blue;}"
+    stylesheet = _make_stylesheet(css)
+    node = DOMNode(classes="a b", id="c")
+    stylesheet.apply(node)
+
+    assert node.styles.color == Color(0, 255, 0)  # color: lime
+    assert node.styles.background == Color(0, 0, 255)  # background: blue
+
+
+def test_stylesheet_apply_empty_rulesets():
+    """Ensure that we don't crash when working with empty rulesets"""
+    css = ".a {} .b {}"
+    stylesheet = _make_stylesheet(css)
+    node = DOMNode(classes="a b")
+    stylesheet.apply(node)
 
 
 @pytest.mark.parametrize(