Issue #15609: Optimize str%args for integer argument

 - Use _PyLong_FormatWriter() instead of formatlong() when possible, to avoid
   a temporary buffer
 - Enable the fast path when width is smaller or equals to the length,
   and when the precision is bigger or equals to the length
 - Add unit tests!
 - formatlong() uses PyUnicode_Resize() instead of _PyUnicode_FromASCII()
   to resize the output string

1 files changed, 16 insertions, 0 deletions

diff --git a/Lib/test/test_format.py b/Lib/test/test_format.py
index b6e2540..e6b0d20 100644
--- a/Lib/test/test_format.py
+++ b/Lib/test/test_format.py
@@ -307,6 +307,22 @@ class FormatTest(unittest.TestCase):
         finally:
             locale.setlocale(locale.LC_ALL, oldloc)
 
+    @support.cpython_only
+    def test_optimisations(self):
+        text = "abcde" # 5 characters
+
+        self.assertIs("%s" % text, text)
+        self.assertIs("%.5s" % text, text)
+        self.assertIs("%.10s" % text, text)
+        self.assertIs("%1s" % text, text)
+        self.assertIs("%5s" % text, text)
+
+        self.assertIs("{0}".format(text), text)
+        self.assertIs("{0:s}".format(text), text)
+        self.assertIs("{0:.5s}".format(text), text)
+        self.assertIs("{0:.10s}".format(text), text)
+        self.assertIs("{0:1s}".format(text), text)
+        self.assertIs("{0:5s}".format(text), text)
 
 
 def test_main():