pdb: support kwargs with `pdb.set_trace`

This handles `header` similar to Python 3.7 does it, and forwards any
other keyword arguments to the Pdb constructor.

This allows for `__import__("pdb").set_trace(skip=["foo.*"])`.

Fixes https://github.com/pytest-dev/pytest/issues/4416.

changelog/4416.feature.rst

@@ -0,0 +1,6 @@
+pdb: support keyword arguments with ``pdb.set_trace``
+
+It handles ``header`` similar to Python 3.7 does it, and forwards any
+other keyword arguments to the ``Pdb`` constructor.
+
+This allows for ``__import__("pdb").set_trace(skip=["foo.*"])``.

src/_pytest/debugging.py

@@ -77,18 +77,21 @@ class pytestPDB(object):
     _saved = []
 
     @classmethod
-    def set_trace(cls, set_break=True):
-        """ invoke PDB set_trace debugging, dropping any IO capturing. """
+    def _init_pdb(cls, *args, **kwargs):
+        """ Initialize PDB debugging, dropping any IO capturing. """
         import _pytest.config
 
-        frame = sys._getframe().f_back
         if cls._pluginmanager is not None:
             capman = cls._pluginmanager.getplugin("capturemanager")
             if capman:
                 capman.suspend_global_capture(in_=True)
             tw = _pytest.config.create_terminal_writer(cls._config)
             tw.line()
-            if capman and capman.is_globally_capturing():
+            # Handle header similar to pdb.set_trace in py37+.
+            header = kwargs.pop("header", None)
+            if header is not None:
+                tw.sep(">", header)
+            elif capman and capman.is_globally_capturing():
                 tw.sep(">", "PDB set_trace (IO-capturing turned off)")
             else:
                 tw.sep(">", "PDB set_trace")
@@ -129,13 +132,18 @@ class pytestPDB(object):
                             self._pytest_capman.suspend_global_capture(in_=True)
                     return ret
 
-            _pdb = _PdbWrapper()
+            _pdb = _PdbWrapper(**kwargs)
             cls._pluginmanager.hook.pytest_enter_pdb(config=cls._config, pdb=_pdb)
         else:
-            _pdb = cls._pdb_cls()
+            _pdb = cls._pdb_cls(**kwargs)
+        return _pdb
 
-        if set_break:
-            _pdb.set_trace(frame)
+    @classmethod
+    def set_trace(cls, *args, **kwargs):
+        """Invoke debugging via ``Pdb.set_trace``, dropping any IO capturing."""
+        frame = sys._getframe().f_back
+        _pdb = cls._init_pdb(*args, **kwargs)
+        _pdb.set_trace(frame)
 
 
 class PdbInvoke(object):
@@ -161,9 +169,9 @@ class PdbTrace(object):
 
 
 def _test_pytest_function(pyfuncitem):
-    pytestPDB.set_trace(set_break=False)
+    _pdb = pytestPDB._init_pdb()
     testfunction = pyfuncitem.obj
-    pyfuncitem.obj = pdb.runcall
+    pyfuncitem.obj = _pdb.runcall
     if pyfuncitem._isyieldedfunction():
         arg_list = list(pyfuncitem._args)
         arg_list.insert(0, testfunction)

testing/test_pdb.py

@@ -390,6 +390,28 @@ class TestPDB(object):
         assert "hello17" in rest  # out is captured
         self.flush(child)
 
+    def test_pdb_set_trace_kwargs(self, testdir):
+        p1 = testdir.makepyfile(
+            """
+            import pytest
+            def test_1():
+                i = 0
+                print("hello17")
+                pytest.set_trace(header="== my_header ==")
+                x = 3
+        """
+        )
+        child = testdir.spawn_pytest(str(p1))
+        child.expect("== my_header ==")
+        assert "PDB set_trace" not in child.before.decode()
+        child.expect("Pdb")
+        child.sendeof()
+        rest = child.read().decode("utf-8")
+        assert "1 failed" in rest
+        assert "def test_1" in rest
+        assert "hello17" in rest  # out is captured
+        self.flush(child)
+
     def test_pdb_set_trace_interception(self, testdir):
         p1 = testdir.makepyfile(
             """
@@ -634,6 +656,12 @@ class TestPDB(object):
         testdir.makepyfile(
             custom_pdb="""
             class CustomPdb(object):
+                def __init__(self, *args, **kwargs):
+                    skip = kwargs.pop("skip")
+                    assert skip == ["foo.*"]
+                    print("__init__")
+                    super(CustomPdb, self).__init__(*args, **kwargs)
+
                 def set_trace(*args, **kwargs):
                     print('custom set_trace>')
          """
@@ -643,12 +671,13 @@ class TestPDB(object):
             import pytest
 
             def test_foo():
-                pytest.set_trace()
+                pytest.set_trace(skip=['foo.*'])
         """
         )
         monkeypatch.setenv("PYTHONPATH", str(testdir.tmpdir))
         child = testdir.spawn_pytest("--pdbcls=custom_pdb:CustomPdb %s" % str(p1))
 
+        child.expect("__init__")
         child.expect("custom set_trace>")
         self.flush(child)