=== modified file 'Lib/locale.py'
--- Lib/locale.py	2009-03-18 17:10:04 +0000
+++ Lib/locale.py	2009-03-29 17:05:35 +0000
@@ -13,6 +13,7 @@
 
 import sys, encodings, encodings.aliases
 import functools
+from re import compile
 
 # Try importing the _locale module.
 #
@@ -166,6 +167,10 @@
         amount -= 1
     return s[lpos:rpos+1]
 
+import re, operator
+_percent_re = re.compile(r'%(?:\((?P<key>.*?)\))?'
+                         r'(?P<modifiers>[-#0-9 +*.hlL]*?)[eEfFgGdiouxXcrs%]')
+
 def format(percent, value, grouping=False, monetary=False, *additional):
     """Returns the locale-aware substitution of a %? specifier
     (percent).
@@ -173,9 +178,13 @@
     additional is for format strings which contain one or more
     '*' modifiers."""
     # this is only for one-percent-specifier strings and this should be checked
-    if percent[0] != '%':
-        raise ValueError("format() must be given exactly one %char "
-                         "format specifier")
+    match = _percent_re.match(percent)
+    if not match or len(match.group())!= len(percent):
+        raise ValueError(("format() must be given exactly one %%char "
+                         "format specifier, %s not valid") % repr(percent))
+    return _format(percent, value, grouping, monetary, *additional)
+
+def _format(percent, value, grouping=False, monetary=False, *additional):
     if additional:
         formatted = percent % ((value,) + additional)
     else:
@@ -199,10 +208,6 @@
             formatted = _strip_padding(formatted, seps)
     return formatted
 
-import re, operator
-_percent_re = re.compile(r'%(?:\((?P<key>.*?)\))?'
-                         r'(?P<modifiers>[-#0-9 +*.hlL]*?)[eEfFgGdiouxXcrs%]')
-
 def format_string(f, val, grouping=False):
     """Formats a string in the same way that the % formatting would use,
     but takes the current locale into account.

=== modified file 'Lib/test/test_locale.py'
--- Lib/test/test_locale.py	2009-03-26 21:44:43 +0000
+++ Lib/test/test_locale.py	2009-03-29 16:56:07 +0000
@@ -221,6 +221,23 @@
                 (self.sep, self.sep))
 
 
+class TestFormatPatternArg(unittest.TestCase):
+    # From issue2522: if format doesn't enforce the contract that only
+    # one format string is passed for the first arg, then from the user
+    # point of view mysterious errors can result where decimal
+    # substitution does not take place but no error is generated.
+    # These test makes sure a ValueError gets generated for things
+    # that are not valid isolated format strings.
+
+    def test_onlyOnePattern(self):
+        self.assertRaises(ValueError, locale.format, "%f\n", 'foo')
+        self.assertRaises(ValueError, locale.format, "%f\r", 'foo')
+        self.assertRaises(ValueError, locale.format, "%f\r\n", 'foo')
+        self.assertRaises(ValueError, locale.format, " %f", 'foo')
+        self.assertRaises(ValueError, locale.format, "%fg", 'foo')
+        self.assertRaises(ValueError, locale.format, "%^g", 'foo')
+
+
 class TestNumberFormatting(BaseLocalizedTest, EnUSNumberFormatting):
     # Test number formatting with a real English locale.
 
@@ -351,6 +368,7 @@
 def test_main():
     tests = [
         TestMiscellaneous,
+        TestFormatPatternArg,
         TestEnUSNumberFormatting,
         TestCNumberFormatting,
         TestFrFRNumberFormatting,