Issue #20426: When passing the re.DEBUG flag, re.compile() displays the debug...

Issue #20426: When passing the re.DEBUG flag, re.compile() displays the debug output every time it is called, regardless of the compilation cache.

Lib/re.py

@@ -225,11 +225,13 @@ _MAXCACHE = 100
 
 def _compile(*key):
     # internal: compile pattern
+    pattern, flags = key
+    bypass_cache = flags & DEBUG
+    if not bypass_cache:
         cachekey = (type(key[0]),) + key
         p = _cache.get(cachekey)
         if p is not None:
             return p
-    pattern, flags = key
     if isinstance(pattern, _pattern_type):
         if flags:
             raise ValueError('Cannot process flags argument with a compiled pattern')
@@ -240,6 +242,7 @@ def _compile(*key):
         p = sre_compile.compile(pattern, flags)
     except error, v:
         raise error, v # invalid expression
+    if not bypass_cache:
         if len(_cache) >= _MAXCACHE:
             _cache.clear()
         _cache[cachekey] = p

Lib/test/test_re.py

 from test.test_support import verbose, run_unittest, import_module
 from test.test_support import precisionbigmemtest, _2G, cpython_only
+from test.test_support import captured_stdout
 import re
 from re import Scanner
 import sre_constants
@@ -920,6 +921,19 @@ class ReTests(unittest.TestCase):
                 self.assertEqual(m.group(1), "")
                 self.assertEqual(m.group(2), "y")
 
+    def test_debug_flag(self):
+        with captured_stdout() as out:
+            re.compile('foo', re.DEBUG)
+        self.assertEqual(out.getvalue().splitlines(),
+                         ['literal 102', 'literal 111', 'literal 111'])
+        # Debug output is output again even a second time (bypassing
+        # the cache -- issue #20426).
+        with captured_stdout() as out:
+            re.compile('foo', re.DEBUG)
+        self.assertEqual(out.getvalue().splitlines(),
+                         ['literal 102', 'literal 111', 'literal 111'])
+
+
 def run_re_tests():
     from test.re_tests import tests, SUCCEED, FAIL, SYNTAX_ERROR
     if verbose:

Misc/NEWS

@@ -38,6 +38,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #20426: When passing the re.DEBUG flag, re.compile() displays the
+  debug output every time it is called, regardless of the compilation cache.
+
 - Issue #20368: The null character now correctly passed from Tcl to Python (in
   unicode strings only).  Improved error handling in variables-related commands.