gh-142037: Improve error messages for printf-style formatting

Lib/test/test_bytes.py

@@ -781,16 +781,16 @@ def __int__(self):
         pi = PseudoFloat(3.1415)
 
         exceptions_params = [
-            ('%x format: an integer is required, not float', b'%x', 3.14),
-            ('%X format: an integer is required, not float', b'%X', 2.11),
-            ('%o format: an integer is required, not float', b'%o', 1.79),
-            ('%x format: an integer is required, not PseudoFloat', b'%x', pi),
-            ('%x format: an integer is required, not complex', b'%x', 3j),
-            ('%X format: an integer is required, not complex', b'%X', 2j),
-            ('%o format: an integer is required, not complex', b'%o', 1j),
-            ('%u format: a real number is required, not complex', b'%u', 3j),
-            ('%i format: a real number is required, not complex', b'%i', 2j),
-            ('%d format: a real number is required, not complex', b'%d', 2j),
+            ('an integer is required for format %x, not float', b'%x', 3.14),
+            ('an integer is required for format %X, not float', b'%X', 2.11),
+            ('an integer is required for format %o, not float', b'%o', 1.79),
+            (r'an integer is required for format %x, not .*\.PseudoFloat', b'%x', pi),
+            ('an integer is required for format %x, not complex', b'%x', 3j),
+            ('an integer is required for format %X, not complex', b'%X', 2j),
+            ('an integer is required for format %o, not complex', b'%o', 1j),
+            ('a real number is required for format %u, not complex', b'%u', 3j),
+            ('a real number is required for format %i, not complex', b'%i', 2j),
+            ('a real number is required for format %d, not complex', b'%d', 2j),
             (
                 r'%c requires an integer in range\(256\)'
                 r' or a single byte, not .*\.PseudoFloat',

Lib/test/test_peepholer.py

@@ -733,22 +733,27 @@ def test_format_errors(self):
         with self.assertRaisesRegex(TypeError,
                     'not all arguments converted during string formatting'):
             eval("'%s' % (x, y)", {'x': 1, 'y': 2})
-        with self.assertRaisesRegex(ValueError, 'incomplete format'):
+        with self.assertRaisesRegex(ValueError, 'stray % at position 2'):
             eval("'%s%' % (x,)", {'x': 1234})
-        with self.assertRaisesRegex(ValueError, 'incomplete format'):
+        with self.assertRaisesRegex(ValueError, 'stray % at position 4'):
             eval("'%s%%%' % (x,)", {'x': 1234})
         with self.assertRaisesRegex(TypeError,
                     'not enough arguments for format string'):
             eval("'%s%z' % (x,)", {'x': 1234})
-        with self.assertRaisesRegex(ValueError, 'unsupported format character'):
+        with self.assertRaisesRegex(ValueError,
+                    'unsupported format %z at position 2'):
             eval("'%s%z' % (x, 5)", {'x': 1234})
-        with self.assertRaisesRegex(TypeError, 'a real number is required, not str'):
+        with self.assertRaisesRegex(TypeError,
+                'format argument 1: a real number is required for format %d, not str'):
             eval("'%d' % (x,)", {'x': '1234'})
-        with self.assertRaisesRegex(TypeError, 'an integer is required, not float'):
+        with self.assertRaisesRegex(TypeError,
+                'format argument 1: an integer is required for format %x, not float'):
             eval("'%x' % (x,)", {'x': 1234.56})
-        with self.assertRaisesRegex(TypeError, 'an integer is required, not str'):
+        with self.assertRaisesRegex(TypeError,
+                'format argument 1: an integer is required for format %x, not str'):
             eval("'%x' % (x,)", {'x': '1234'})
-        with self.assertRaisesRegex(TypeError, 'must be real number, not str'):
+        with self.assertRaisesRegex(TypeError,
+                'format argument 1: a real number is required for format %f, not str'):
             eval("'%f' % (x,)", {'x': '1234'})
         with self.assertRaisesRegex(TypeError,
                     'not enough arguments for format string'):

Lib/test/test_str.py

@@ -1578,17 +1578,17 @@ def __int__(self):
         self.assertEqual('%X' % letter_m, '6D')
         self.assertEqual('%o' % letter_m, '155')
         self.assertEqual('%c' % letter_m, 'm')
-        self.assertRaisesRegex(TypeError, '%x format: an integer is required, not float', operator.mod, '%x', 3.14)
-        self.assertRaisesRegex(TypeError, '%X format: an integer is required, not float', operator.mod, '%X', 2.11)
-        self.assertRaisesRegex(TypeError, '%o format: an integer is required, not float', operator.mod, '%o', 1.79)
-        self.assertRaisesRegex(TypeError, '%x format: an integer is required, not PseudoFloat', operator.mod, '%x', pi)
-        self.assertRaisesRegex(TypeError, '%x format: an integer is required, not complex', operator.mod, '%x', 3j)
-        self.assertRaisesRegex(TypeError, '%X format: an integer is required, not complex', operator.mod, '%X', 2j)
-        self.assertRaisesRegex(TypeError, '%o format: an integer is required, not complex', operator.mod, '%o', 1j)
-        self.assertRaisesRegex(TypeError, '%u format: a real number is required, not complex', operator.mod, '%u', 3j)
-        self.assertRaisesRegex(TypeError, '%i format: a real number is required, not complex', operator.mod, '%i', 2j)
-        self.assertRaisesRegex(TypeError, '%d format: a real number is required, not complex', operator.mod, '%d', 1j)
-        self.assertRaisesRegex(TypeError, r'%c requires an int or a unicode character, not .*\.PseudoFloat', operator.mod, '%c', pi)
+        self.assertRaisesRegex(TypeError, 'an integer is required for format %x, not float', operator.mod, '%x', 3.14)
+        self.assertRaisesRegex(TypeError, 'an integer is required for format %X, not float', operator.mod, '%X', 2.11)
+        self.assertRaisesRegex(TypeError, 'an integer is required for format %o, not float', operator.mod, '%o', 1.79)
+        self.assertRaisesRegex(TypeError, r'an integer is required for format %x, not .*\.PseudoFloat', operator.mod, '%x', pi)
+        self.assertRaisesRegex(TypeError, 'an integer is required for format %x, not complex', operator.mod, '%x', 3j)
+        self.assertRaisesRegex(TypeError, 'an integer is required for format %X, not complex', operator.mod, '%X', 2j)
+        self.assertRaisesRegex(TypeError, 'an integer is required for format %o, not complex', operator.mod, '%o', 1j)
+        self.assertRaisesRegex(TypeError, 'a real number is required for format %u, not complex', operator.mod, '%u', 3j)
+        self.assertRaisesRegex(TypeError, 'a real number is required for format %i, not complex', operator.mod, '%i', 2j)
+        self.assertRaisesRegex(TypeError, 'a real number is required for format %d, not complex', operator.mod, '%d', 1j)
+        self.assertRaisesRegex(TypeError, r'%c requires an integer or a unicode character, not .*\.PseudoFloat', operator.mod, '%c', pi)
 
         class RaisingNumber:
             def __int__(self):

Misc/NEWS.d/next/Core_and_Builtins/2025-11-29-10-06-06.gh-issue-142037.OpIGzK.rst

@@ -0,0 +1,7 @@
+Improve error messages for printf-style formatting.
+For errors in the format string, always include the position of the
+start of the format unit.
+For errors related to the formatted arguments, always include the number
+or the name of the argument.
+Raise more specific errors and include more information (type and number
+of arguments, most probable causes of error).