From 8e821d41131c1324a64eff4d446ac26de917fac3 Mon Sep 17 00:00:00 2001
From: Will McGugan <willmcgugan@gmail.com>
Date: Fri, 29 Apr 2022 10:34:35 +0100
Subject: [PATCH] lazy parse_rules

---
 src/textual/css/stylesheet.py | 14 +++++++++-----
 tests/css/test_parse.py       |  9 +++++++--
 tests/css/test_stylesheet.py  |  1 +
 3 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/src/textual/css/stylesheet.py b/src/textual/css/stylesheet.py
index 7b8606cc1..52993f02c 100644
--- a/src/textual/css/stylesheet.py
+++ b/src/textual/css/stylesheet.py
@@ -101,17 +101,19 @@ class StylesheetErrors:
 @rich.repr.auto
 class Stylesheet:
     def __init__(self, *, variables: dict[str, str] | None = None) -> None:
-        self._rules: list[RuleSet] | None = None
+        self._rules: list[RuleSet] = []
         self.variables = variables or {}
         self.source: dict[str, str] = {}
+        self._require_parse = False
 
     def __rich_repr__(self) -> rich.repr.Result:
         yield self.rules
 
     @property
     def rules(self) -> list[RuleSet]:
-        if self._rules is None:
+        if self._require_parse:
             self.parse()
+            self._require_parse = False
         assert self._rules is not None
         return self._rules
 
@@ -171,7 +173,7 @@ class Stylesheet:
         except Exception as error:
             raise StylesheetError(f"unable to read {filename!r}; {error}")
         self.source[path] = css
-        self._rules = None
+        self._require_parse = True
 
     def add_source(self, css: str, path: str | None = None) -> None:
         """Parse CSS from a string.
@@ -192,7 +194,7 @@ class Stylesheet:
             return
 
         self.source[path] = css
-        self._rules = None
+        self._require_parse = True
 
     def parse(self) -> None:
         """Parse the source in the stylesheet.
@@ -208,6 +210,7 @@ class Stylesheet:
                 raise StylesheetParseError(self.error_renderable)
             add_rules(css_rules)
         self._rules = rules
+        self._require_parse = False
 
     def reparse(self) -> None:
         """Re-parse source, applying new variables.
@@ -221,7 +224,8 @@ class Stylesheet:
         stylesheet = Stylesheet(variables=self.variables)
         for path, css in self.source.items():
             stylesheet.add_source(css, path)
-        self._rules = None
+        stylesheet.parse()
+        self.rules = stylesheet.rules
         self.source = stylesheet.source
 
     @classmethod
diff --git a/tests/css/test_parse.py b/tests/css/test_parse.py
index f0c05dc2d..ccf7a75f4 100644
--- a/tests/css/test_parse.py
+++ b/tests/css/test_parse.py
@@ -875,6 +875,7 @@ class TestParseLayout:
         stylesheet = Stylesheet()
         with pytest.raises(StylesheetParseError) as ex:
             stylesheet.add_source(css)
+            stylesheet.parse()
 
         assert ex.value.errors is not None
 
@@ -1033,8 +1034,10 @@ class TestParseTransition:
         stylesheet = Stylesheet()
         with pytest.raises(StylesheetParseError) as ex:
             stylesheet.add_source(css)
+            stylesheet.parse()
 
-        stylesheet_errors = stylesheet.rules[0].errors
+        rules = stylesheet._parse_rules(css, "foo")
+        stylesheet_errors = rules[0].errors
 
         assert len(stylesheet_errors) == 1
         assert stylesheet_errors[0][0].value == invalid_func_name
@@ -1067,7 +1070,9 @@ class TestParseOpacity:
 
         with pytest.raises(StylesheetParseError):
             stylesheet.add_source(css)
-        assert stylesheet.rules[0].errors
+            stylesheet.parse()
+        rules = stylesheet._parse_rules(css, "foo")
+        assert rules[0].errors
 
 
 class TestParseMargin:
diff --git a/tests/css/test_stylesheet.py b/tests/css/test_stylesheet.py
index b31633504..718446a43 100644
--- a/tests/css/test_stylesheet.py
+++ b/tests/css/test_stylesheet.py
@@ -43,6 +43,7 @@ def test_color_property_parsing(css_value, expectation, expected_color):
 
     with expectation:
         stylesheet.add_source(css)
+        stylesheet.parse()
 
     if expected_color:
         css_rule = stylesheet.rules[0]