perf(expression): improve string lexing & error messages

Author: lovetheguitar

src/_pytest/mark/expression.py

@@ -99,18 +99,18 @@ def lex(self, input: str) -> Iterator[Token]:
             elif input[pos] == ",":
                 yield Token(TokenType.COMMA, ",", pos)
                 pos += 1
-            elif (quote_char := input[pos]) == "'" or input[pos] == '"':
-                quote_position = input[pos + 1 :].find(quote_char)
-                if quote_position == -1:
+            elif (quote_char := input[pos]) in ("'", '"'):
+                end_quote_pos = input.find(quote_char, pos + 1)
+                if end_quote_pos == -1:
                     raise ParseError(
                         pos + 1,
                         f'closing quote "{quote_char}" is missing',
                     )
-                value = input[pos : pos + 2 + quote_position]
-                if "\\" in value:
+                value = input[pos : end_quote_pos + 1]
+                if (backslash_pos := input.find("\\")) != -1:
                     raise ParseError(
-                        pos + 1,
-                        "escaping not supported in marker expression",
+                        backslash_pos + 1,
+                        r'escaping with "\" not supported in marker expression',
                     )
                 yield Token(TokenType.STRING, value, pos)
                 pos += len(value)
@@ -218,10 +218,15 @@ def not_expr(s: Scanner) -> ast.expr:
 
 def single_kwarg(s: Scanner) -> ast.keyword:
     keyword_name = s.accept(TokenType.IDENT, reject=True)
-    if not keyword_name.value.isidentifier() or keyword.iskeyword(keyword_name.value):
+    if not keyword_name.value.isidentifier():
+        raise ParseError(
+            keyword_name.pos + 1,
+            f"not a valid python identifier {keyword_name.value}",
+        )
+    if keyword.iskeyword(keyword_name.value):
         raise ParseError(
             keyword_name.pos + 1,
-            f'unexpected character/s "{keyword_name.value}"',
+            f"unexpected reserved python keyword `{keyword_name.value}`",
         )
     s.accept(TokenType.EQUAL, reject=True)
 

testing/test_mark_expression.py

@@ -207,25 +207,29 @@ def test_invalid_idents(ident: str) -> None:
 @pytest.mark.parametrize(
     "expr, expected_error_msg",
     (
-        ("mark(1=2)", 'unexpected character/s "1"'),
-        ("mark(/=2)", 'unexpected character/s "/"'),
-        ("mark(True=False)", 'unexpected character/s "True"'),
-        ("mark(def=False)", 'unexpected character/s "def"'),
-        ("mark(class=False)", 'unexpected character/s "class"'),
-        ("mark(if=False)", 'unexpected character/s "if"'),
-        ("mark(else=False)", 'unexpected character/s "else"'),
-        ("mark(1)", 'unexpected character/s "1"'),
-        ("mark(var:=False", 'unexpected character/s "var:"'),
-        ("mark(valid=False, def=1)", 'unexpected character/s "def"'),
+        ("mark(True=False)", "unexpected reserved python keyword `True`"),
+        ("mark(def=False)", "unexpected reserved python keyword `def`"),
+        ("mark(class=False)", "unexpected reserved python keyword `class`"),
+        ("mark(if=False)", "unexpected reserved python keyword `if`"),
+        ("mark(else=False)", "unexpected reserved python keyword `else`"),
+        ("mark(valid=False, def=1)", "unexpected reserved python keyword `def`"),
+        ("mark(1)", "not a valid python identifier 1"),
+        ("mark(var:=False", "not a valid python identifier var:"),
+        ("mark(1=2)", "not a valid python identifier 1"),
+        ("mark(/=2)", "not a valid python identifier /"),
         ("mark(var==", "expected identifier; got ="),
+        ("mark(var)", "expected =; got right parenthesis"),
         ("mark(var=none)", 'unexpected character/s "none"'),
         ("mark(var=1.1)", 'unexpected character/s "1.1"'),
-        ("mark(var)", "expected =; got right parenthesis"),
         ("mark(var=')", """closing quote "'" is missing"""),
         ('mark(var=")', 'closing quote """ is missing'),
         ("""mark(var="')""", 'closing quote """ is missing'),
         ("""mark(var='")""", """closing quote "'" is missing"""),
-        (r"mark(var='\hugo')", "escaping not supported in marker expression"),
+        (
+            r"mark(var='\hugo')",
+            r'escaping with "\\" not supported in marker expression',
+        ),
+        ("mark(empty_list=[])", r'unexpected character/s "\[\]"'),
     ),
 )
 def test_invalid_kwarg_name_or_value(  # TODO: move to `test_syntax_errors` ?