Fix parsing error for backslash continuations with no indentation

src/blib2to3/pgen2/tokenize.py

@@ -147,6 +147,10 @@ def tokenize(source: str, grammar: Grammar | None = None) -> Iterator[TokenInfo]
     line, column = 1, 0
 
     prev_token: pytokens.Token | None = None
+    in_backslash_continuation = False
+    # Track the balance of INDENT/DEDENT tokens skipped during continuation
+    # Positive means we skipped more DEDENTs than INDENTs
+    skipped_indent_balance = 0
     try:
         for token in pytokens.tokenize(source):
             token = transform_whitespace(token, source, prev_token)
@@ -163,6 +167,33 @@ def tokenize(source: str, grammar: Grammar | None = None) -> Iterator[TokenInfo]
                 prev_token = token
                 continue
 
+            # Track if we just saw a backslash continuation (NL token starting with \)
+            if token.type == TokenType.nl and token_str.startswith("\\"):
+                in_backslash_continuation = True
+            # Skip INDENT/DEDENT tokens that come during a backslash continuation
+            # (before the logical line ends with a NEWLINE token)
+            elif in_backslash_continuation and token.type == TokenType.dedent:
+                skipped_indent_balance += 1
+                prev_token = token
+                continue
+            elif in_backslash_continuation and token.type == TokenType.indent:
+                skipped_indent_balance -= 1
+                prev_token = token
+                continue
+            # NEWLINE ends the logical line and the continuation tracking
+            elif token.type == TokenType.newline:
+                in_backslash_continuation = False
+                # Don't reset balance here - we still need to skip matching tokens
+            # After continuation ends, skip tokens to balance what we skipped during continuation
+            elif skipped_indent_balance > 0 and token.type == TokenType.indent:
+                skipped_indent_balance -= 1
+                prev_token = token
+                continue
+            elif skipped_indent_balance < 0 and token.type == TokenType.dedent:
+                skipped_indent_balance += 1
+                prev_token = token
+                continue
+
             source_line = lines[token.start_line - 1]
 
             if token.type == TokenType.identifier and token_str in ("async", "await"):

tests/test_tokenize.py

@@ -98,6 +98,69 @@ def test_fstring() -> None:
     )
 
 
+def test_backslash_continuation() -> None:
+    """Test that backslash continuations with no indentation are handled correctly.
+
+    This is a regression test for https://github.com/psf/black/issues/XXXX
+    where Black failed to parse code with backslash continuations followed by
+    unindented lines.
+    """
+    # Simple case: backslash continuation with unindented number
+    assert_tokenizes(
+        "if True:\n    foo = 1+\\\n2\n    print(foo)\n",
+        [
+            Token(type="NAME", string="if", start=(1, 0), end=(1, 2)),
+            Token(type="NAME", string="True", start=(1, 3), end=(1, 7)),
+            Token(type="OP", string=":", start=(1, 7), end=(1, 8)),
+            Token(type="NEWLINE", string="\n", start=(1, 8), end=(1, 9)),
+            Token(type="INDENT", string="    ", start=(2, 0), end=(2, 4)),
+            Token(type="NAME", string="foo", start=(2, 4), end=(2, 7)),
+            Token(type="OP", string="=", start=(2, 8), end=(2, 9)),
+            Token(type="NUMBER", string="1", start=(2, 10), end=(2, 11)),
+            Token(type="OP", string="+", start=(2, 11), end=(2, 12)),
+            Token(type="NL", string="\\\n", start=(2, 12), end=(2, 14)),
+            Token(type="NUMBER", string="2", start=(3, 0), end=(3, 1)),
+            Token(type="NEWLINE", string="\n", start=(3, 1), end=(3, 2)),
+            Token(type="NAME", string="print", start=(4, 4), end=(4, 9)),
+            Token(type="OP", string="(", start=(4, 9), end=(4, 10)),
+            Token(type="NAME", string="foo", start=(4, 10), end=(4, 13)),
+            Token(type="OP", string=")", start=(4, 13), end=(4, 14)),
+            Token(type="NEWLINE", string="\n", start=(4, 14), end=(4, 15)),
+            Token(type="DEDENT", string="", start=(5, 0), end=(5, 0)),
+            Token(type="ENDMARKER", string="", start=(5, 0), end=(5, 0)),
+        ],
+    )
+
+    # Multiple backslash continuations
+    assert_tokenizes(
+        "if True:\n    result = 1+\\\n2+\\\n3\n    print(result)\n",
+        [
+            Token(type="NAME", string="if", start=(1, 0), end=(1, 2)),
+            Token(type="NAME", string="True", start=(1, 3), end=(1, 7)),
+            Token(type="OP", string=":", start=(1, 7), end=(1, 8)),
+            Token(type="NEWLINE", string="\n", start=(1, 8), end=(1, 9)),
+            Token(type="INDENT", string="    ", start=(2, 0), end=(2, 4)),
+            Token(type="NAME", string="result", start=(2, 4), end=(2, 10)),
+            Token(type="OP", string="=", start=(2, 11), end=(2, 12)),
+            Token(type="NUMBER", string="1", start=(2, 13), end=(2, 14)),
+            Token(type="OP", string="+", start=(2, 14), end=(2, 15)),
+            Token(type="NL", string="\\\n", start=(2, 15), end=(2, 17)),
+            Token(type="NUMBER", string="2", start=(3, 0), end=(3, 1)),
+            Token(type="OP", string="+", start=(3, 1), end=(3, 2)),
+            Token(type="NL", string="\\\n", start=(3, 2), end=(3, 4)),
+            Token(type="NUMBER", string="3", start=(4, 0), end=(4, 1)),
+            Token(type="NEWLINE", string="\n", start=(4, 1), end=(4, 2)),
+            Token(type="NAME", string="print", start=(5, 4), end=(5, 9)),
+            Token(type="OP", string="(", start=(5, 9), end=(5, 10)),
+            Token(type="NAME", string="result", start=(5, 10), end=(5, 16)),
+            Token(type="OP", string=")", start=(5, 16), end=(5, 17)),
+            Token(type="NEWLINE", string="\n", start=(5, 17), end=(5, 18)),
+            Token(type="DEDENT", string="", start=(6, 0), end=(6, 0)),
+            Token(type="ENDMARKER", string="", start=(6, 0), end=(6, 0)),
+        ],
+    )
+
+
 # Run "echo some code | python tests/test_tokenize.py" to generate test cases.
 if __name__ == "__main__":
     code = sys.stdin.read()