bpo-28692: Deprecate using non-integer value for selecting a plural form in gettext. (#507)

Doc/whatsnew/3.7.rst

@@ -180,6 +180,10 @@ Deprecated
   both deprecated in Python 3.4 now emit :exc:`DeprecationWarning`. (Contributed
   by Matthias Bussonnier in :issue:`29576`)
 
+- Using non-integer value for selecting a plural form in :mod:`gettext` is
+  now deprecated.  It never correctly worked.
+  (Contributed by Serhiy Storchaka in :issue:`28692`.)
+
 
 Removed
 =======

Lib/gettext.py

@@ -164,6 +164,10 @@ def _as_int(n):
     except TypeError:
         raise TypeError('Plural value must be an integer, got %s' %
                         (n.__class__.__name__,)) from None
+    import warnings
+    warnings.warn('Plural value must be an integer, got %s' %
+                  (n.__class__.__name__,),
+                  DeprecationWarning, 4)
     return n
 
 def c2py(plural):

Lib/test/test_gettext.py

@@ -443,9 +443,12 @@ class PluralFormsTestCase(GettextBaseTest):
         f = gettext.c2py('n != 1')
         self.assertEqual(f(1), 0)
         self.assertEqual(f(2), 1)
-        self.assertEqual(f(1.0), 0)
+        with self.assertWarns(DeprecationWarning):
-        self.assertEqual(f(2.0), 1)
+            self.assertEqual(f(1.0), 0)
-        self.assertEqual(f(1.1), 1)
+        with self.assertWarns(DeprecationWarning):
+            self.assertEqual(f(2.0), 1)
+        with self.assertWarns(DeprecationWarning):
+            self.assertEqual(f(1.1), 1)
         self.assertRaises(TypeError, f, '2')
         self.assertRaises(TypeError, f, b'2')
         self.assertRaises(TypeError, f, [])

Misc/NEWS

@@ -270,6 +270,9 @@ Extension Modules
 Library
 -------
 
+- bpo-28692: Using non-integer value for selecting a plural form in gettext is
+  now deprecated.
+
 - bpo-26121: Use C library implementation for math functions:
   tgamma(), lgamma(), erf() and erfc().