bpo-31389 Add an optional `header` argument to pdb.set_trace() (#3438)

* Give pdb.set_trace() an optional `header` argument

* What's new.

* Give pdb.set_trace() an optional `header` argument

* What's new.

2 files changed, 17 insertions, 2 deletions

diff --git a/Lib/pdb.py b/Lib/pdb.py
index 97618b0..8dd4ded 100755
--- a/Lib/pdb.py
+++ b/Lib/pdb.py
@@ -1581,8 +1581,11 @@ def runctx(statement, globals, locals):
 def runcall(*args, **kwds):
     return Pdb().runcall(*args, **kwds)
 
-def set_trace():
-    Pdb().set_trace(sys._getframe().f_back)
+def set_trace(*, header=None):
+    pdb = Pdb()
+    if header is not None:
+        pdb.message(header)
+    pdb.set_trace(sys._getframe().f_back)
 
 # Post-Mortem interface
 
diff --git a/Lib/test/test_pdb.py b/Lib/test/test_pdb.py
index 755d265..71d8203 100644
--- a/Lib/test/test_pdb.py
+++ b/Lib/test/test_pdb.py
@@ -9,9 +9,12 @@ import unittest
 import subprocess
 import textwrap
 
+from contextlib import ExitStack
+from io import StringIO
 from test import support
 # This little helper class is essential for testing pdb under doctest.
 from test.test_doctest import _FakeInput
+from unittest.mock import patch
 
 
 class PdbTestInput(object):
@@ -1107,6 +1110,15 @@ class PdbTestCase(unittest.TestCase):
             if save_home is not None:
                 os.environ['HOME'] = save_home
 
+    def test_header(self):
+        stdout = StringIO()
+        header = 'Nobody expects... blah, blah, blah'
+        with ExitStack() as resources:
+            resources.enter_context(patch('sys.stdout', stdout))
+            resources.enter_context(patch.object(pdb.Pdb, 'set_trace'))
+            pdb.set_trace(header=header)
+        self.assertEqual(stdout.getvalue(), header + '\n')
+
     def tearDown(self):
         support.unlink(support.TESTFN)