Fix bug in _insert_thousands_sep: too much zero padding could be

added for 'n' formats with non-repeating thousands-separator.

Lib/decimal.py

@@ -5621,8 +5621,6 @@ def _insert_thousands_sep(digits, spec, min_width=1):
 
     groups = []
     for l in _group_lengths(grouping):
-        if groups:
-            min_width -= len(sep)
         if l <= 0:
             raise ValueError("group length should be positive")
         # max(..., 1) forces at least 1 digit to the left of a separator
@@ -5632,6 +5630,7 @@ def _insert_thousands_sep(digits, spec, min_width=1):
         min_width -= l
         if not digits and min_width <= 0:
             break
+        min_width -= len(sep)
     else:
         l = max(len(digits), min_width, 1)
         groups.append('0'*(l - len(digits)) + digits[-l:])

Lib/test/test_decimal.py

@@ -809,6 +809,28 @@ class DecimalFormatTest(unittest.TestCase):
         self.assertEqual(get_fmt(123456789, ru_RU, '.6n'), '1,23457e+8')
         self.assertEqual(get_fmt(123456789, crazy, '.6n'), '1&23457e+8')
 
+        # zero padding
+        self.assertEqual(get_fmt(1234, fr_FR, '03n'), '1234')
+        self.assertEqual(get_fmt(1234, fr_FR, '04n'), '1234')
+        self.assertEqual(get_fmt(1234, fr_FR, '05n'), '01234')
+        self.assertEqual(get_fmt(1234, fr_FR, '06n'), '001234')
+
+        self.assertEqual(get_fmt(12345, en_US, '05n'), '12,345')
+        self.assertEqual(get_fmt(12345, en_US, '06n'), '12,345')
+        self.assertEqual(get_fmt(12345, en_US, '07n'), '012,345')
+        self.assertEqual(get_fmt(12345, en_US, '08n'), '0,012,345')
+        self.assertEqual(get_fmt(12345, en_US, '09n'), '0,012,345')
+        self.assertEqual(get_fmt(12345, en_US, '010n'), '00,012,345')
+
+        self.assertEqual(get_fmt(123456, crazy, '06n'), '1-2345-6')
+        self.assertEqual(get_fmt(123456, crazy, '07n'), '1-2345-6')
+        self.assertEqual(get_fmt(123456, crazy, '08n'), '1-2345-6')
+        self.assertEqual(get_fmt(123456, crazy, '09n'), '01-2345-6')
+        self.assertEqual(get_fmt(123456, crazy, '010n'), '0-01-2345-6')
+        self.assertEqual(get_fmt(123456, crazy, '011n'), '0-01-2345-6')
+        self.assertEqual(get_fmt(123456, crazy, '012n'), '00-01-2345-6')
+        self.assertEqual(get_fmt(123456, crazy, '013n'), '000-01-2345-6')
+
 
 class DecimalArithmeticOperatorsTest(unittest.TestCase):
     '''Unit tests for all arithmetic operators, binary and unary.'''