gh-144018: Preserve tab indentation across explicit line continuations

Lib/test/test_syntax.py

@@ -3273,6 +3273,30 @@ def fib(n):
         except SyntaxError:
             self.fail("Indented statement over multiple lines is valid")
 
+    @support.subTests("indent", ("\t", " \t", "\t ", "    "))
+    @support.subTests("continuations", (1, 3))
+    @support.subTests("newline", ("\n", "\r\n"))
+    @support.subTests("first_statement", (False, True))
+    def test_continuation_tab_indentation(self, indent, continuations,
+                                          newline, first_statement):
+        lines = ["if True:"]
+        if not first_statement:
+            lines.append(f"{indent}pass")
+        lines.extend([f"{indent}\\"] * continuations)
+        lines.extend(["pass", f"{indent}pass", "pass", ""])
+        compile(newline.join(lines), "<test>", "exec")
+
+    @support.subTests("indents", (
+        ("        ", "\t"), ("\t", "        "),
+    ))
+    @support.subTests("continuations", (1, 3))
+    def test_continuation_inconsistent_tabs(self, indents, continuations):
+        previous, continued = indents
+        lines = ["if True:", f"{previous}pass"]
+        lines.extend([f"{continued}\\"] * continuations)
+        lines.extend(["pass", ""])
+        self.assertRaises(TabError, compile, "\n".join(lines), "<test>", "exec")
+
     def test_continuation_bad_indentation(self):
         # Check that code that breaks indentation across multiple lines raises a syntax error
 

Misc/NEWS.d/next/Core_and_Builtins/2026-10-08-00-00-00.gh-issue-144018.tabs.rst

@@ -0,0 +1,3 @@
+Fix tab indentation checks for lines joined by explicit line continuation.
+Consistent tab indentation is accepted and ambiguous mixtures of tabs and
+spaces are rejected.

Parser/lexer/layout.c

@@ -84,6 +84,7 @@ _PyLexer_BeginLine(struct tok_state *tok)
     int altcol = 0;
     tok->layout.at_bol = 0;
     int cont_line_col = 0;
+    int cont_line_altcol = 0;
     for (;;) {
         c = tok_nextc(tok);
         if (c == ' ') {
@@ -101,6 +102,7 @@ _PyLexer_BeginLine(struct tok_state *tok)
             // using backslashes. This means that if we found a backslash
             // preceded by whitespace, **the first one we find** determines
             // the level of indentation of whatever comes next.
+            cont_line_altcol = cont_line_col ? cont_line_altcol : altcol;
             cont_line_col = cont_line_col ? cont_line_col : col;
             if ((c = _PyLexer_ContinueLine(tok)) == -1) {
                 return -1;
@@ -136,7 +138,7 @@ _PyLexer_BeginLine(struct tok_state *tok)
     }
     if (!blankline && tok->level == 0) {
         col = cont_line_col ? cont_line_col : col;
-        altcol = cont_line_col ? cont_line_col : altcol;
+        altcol = cont_line_col ? cont_line_altcol : altcol;
         if (update_indentation(tok, col, altcol) < 0) {
             return -1;
         }