py/lexer: Wrap in parenthesis all f-string arguments passed to format.

This is important for literal tuples, e.g.

    f"{a,b,}, {c}" --> "{}".format((a,b), (c),)

which would otherwise result in either a syntax error or the wrong result.

Fixes issue #9635.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

py/lexer.c

@@ -356,6 +356,9 @@ STATIC void parse_string_literal(mp_lexer_t *lex, bool is_raw, bool is_fstring)
                     vstr_add_byte(&lex->vstr, '{');
                     next_char(lex);
                 } else {
+                    // wrap each argument in (), e.g.
+                    // f"{a,b,}, {c}" --> "{}".format((a,b), (c),)
+                    vstr_add_byte(&lex->fstring_args, '(');
                     // remember the start of this argument (if we need it for f'{a=}').
                     size_t i = lex->fstring_args.len;
                     // extract characters inside the { until we reach the
@@ -382,7 +385,9 @@ STATIC void parse_string_literal(mp_lexer_t *lex, bool is_raw, bool is_fstring)
                         // remove the trailing '='
                         lex->fstring_args.len--;
                     }
-                    // comma-separate args
+                    // close the paren-wrapped arg to .format().
+                    vstr_add_byte(&lex->fstring_args, ')');
+                    // comma-separate args to .format().
                     vstr_add_byte(&lex->fstring_args, ',');
                 }
                 vstr_add_byte(&lex->vstr, '{');

tests/basics/string_fstring.py

@@ -56,3 +56,8 @@ try:
 except (ValueError, SyntaxError):
     # MicroPython incorrectly raises ValueError here.
     print('SyntaxError')
+
+# Allow literal tuples
+print(f"a {1,} b")
+print(f"a {x,y,} b")
+print(f"a {x,1} b")

tests/basics/string_fstring_debug.py

@@ -21,3 +21,7 @@ print(f"a {x=:08x} b {y} c")
 
 print(f'a {f() + g("foo") + h()=} b')
 print(f'a {f() + g("foo") + h()=:08x} b')
+
+print(f"a {1,=} b")
+print(f"a {x,y,=} b")
+print(f"a {x,1=} b")

tests/basics/string_fstring_debug.py.exp

@@ -4,3 +4,6 @@ a x=1 b 2 c
 a x=00000001 b 2 c
 a f() + g("foo") + h()=15 b
 a f() + g("foo") + h()=0000000f b
+a 1,=(1,) b
+a x,y,=(1, 2) b
+a x,1=(1, 1) b