From e0a148060025a75f4051e5009d9dc3728cb2bc75 Mon Sep 17 00:00:00 2001
From: mcskatkat <mc_skatkat@hotmail.com>
Date: Tue, 11 Jul 2023 00:50:33 +0300
Subject: [PATCH] py/objstr: Fix `str % {}` edge case.

Eliminate `TypeError` when format string contains no named conversions.
This matches CPython behavior.

Signed-off-by: mcskatkat <mc_skatkat@hotmail.com>
---
 py/objstr.c                          |  4 +++-
 tests/basics/string_format_modulo.py | 11 ++++++++---
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/py/objstr.c b/py/objstr.c
index b966a70169..5dfe94ac4f 100644
--- a/py/objstr.c
+++ b/py/objstr.c
@@ -1645,7 +1645,9 @@ STATIC mp_obj_t str_modulo_format(mp_obj_t pattern, size_t n_args, const mp_obj_
         }
     }
 
-    if (arg_i != n_args) {
+    if (dict == MP_OBJ_NULL && arg_i != n_args) {
+        // NOTE: if `dict` exists, then `n_args` is 1 and the dict is always consumed; either
+        // positionally, or as a map of named args, even if none were actually referenced.
         mp_raise_TypeError(MP_ERROR_TEXT("format string didn't convert all arguments"));
     }
 
diff --git a/tests/basics/string_format_modulo.py b/tests/basics/string_format_modulo.py
index 14b4a6a484..7bddd96750 100644
--- a/tests/basics/string_format_modulo.py
+++ b/tests/basics/string_format_modulo.py
@@ -51,8 +51,9 @@ print('%c' % True)
 
 # Should be able to print dicts; in this case they aren't used
 # to lookup keywords in formats like %(foo)s
-print('%s' % {})
-print('%s' % ({},))
+print('%s' % {})      # dict treated as the single (positional) arg to %
+print('%s' % ({},))   # dict is the first (and only) arg in the positional arg tuple
+print('foo' % {})     # no error, dict treated as an empty map of named args
 
 # Cases when "*" used and there's not enough values total
 try:
@@ -65,7 +66,11 @@ except TypeError:
     print("TypeError")
 
 print("%(foo)s" % {"foo": "bar", "baz": False})
-print("%s %(foo)s %(foo)s" % {"foo": 1})
+print("%s %(foo)s %(foo)s" % {"foo": 1})  # dict consumed positionally, then used as map - ok
+try:
+    print("%(foo)s %s %(foo)s" % {"foo": 1})  # used as map, then positionally - not enough args
+except TypeError:
+    print("TypeError")
 try:
     print("%(foo)s" % {})
 except KeyError: