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

@@ -272,10 +272,12 @@ _pattern_type = type(sre_compile.compile("", 0))
 _MAXCACHE = 512
 def _compile(pattern, flags):
     # internal: compile pattern
-    try:
-        return _cache[type(pattern), pattern, flags]
-    except KeyError:
-        pass
+    bypass_cache = flags & DEBUG
+    if not bypass_cache:
+        try:
+            return _cache[type(pattern), pattern, flags]
+        except KeyError:
+            pass
     if isinstance(pattern, _pattern_type):
         if flags:
             raise ValueError(
@@ -284,9 +286,10 @@ def _compile(pattern, flags):
     if not sre_compile.isstring(pattern):
         raise TypeError("first argument must be string or compiled pattern")
     p = sre_compile.compile(pattern, flags)
-    if len(_cache) >= _MAXCACHE:
-        _cache.clear()
-    _cache[type(pattern), pattern, flags] = p
+    if not bypass_cache:
+        if len(_cache) >= _MAXCACHE:
+            _cache.clear()
+        _cache[type(pattern), pattern, flags] = p
     return p
 
 def _compile_repl(repl, pattern):

Lib/test/test_re.py

@@ -1,5 +1,5 @@
 from test.support import verbose, run_unittest, gc_collect, bigmemtest, _2G, \
-        cpython_only
+        cpython_only, captured_stdout
 import io
 import re
 from re import Scanner
@@ -1193,6 +1193,18 @@ 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 '])
+
 
 class PatternReprTests(unittest.TestCase):
     def check(self, pattern, expected):

Misc/NEWS

@@ -16,6 +16,9 @@ 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.
   Improved error handling in variables-related commands.