pdb: import pdbcls lazily

Fixes https://github.com/pytest-dev/pytest/issues/2064.

changelog/2064.bugfix.rst

@@ -0,0 +1 @@
+The debugging plugin imports the wrapped ``Pdb`` class (``--pdbcls``) on-demand now.

src/_pytest/debugging.py

@@ -49,7 +49,57 @@ def pytest_addoption(parser):
     )
 
 
-def _import_pdbcls(modname, classname):
+def pytest_configure(config):
+    if config.getvalue("trace"):
+        config.pluginmanager.register(PdbTrace(), "pdbtrace")
+    if config.getvalue("usepdb"):
+        config.pluginmanager.register(PdbInvoke(), "pdbinvoke")
+
+    pytestPDB._saved.append(
+        (pdb.set_trace, pytestPDB._pluginmanager, pytestPDB._config)
+    )
+    pdb.set_trace = pytestPDB.set_trace
+    pytestPDB._pluginmanager = config.pluginmanager
+    pytestPDB._config = config
+
+    # NOTE: not using pytest_unconfigure, since it might get called although
+    #       pytest_configure was not (if another plugin raises UsageError).
+    def fin():
+        (
+            pdb.set_trace,
+            pytestPDB._pluginmanager,
+            pytestPDB._config,
+        ) = pytestPDB._saved.pop()
+
+    config._cleanup.append(fin)
+
+
+class pytestPDB(object):
+    """ Pseudo PDB that defers to the real pdb. """
+
+    _pluginmanager = None
+    _config = None
+    _saved = []
+    _recursive_debug = 0
+
+    @classmethod
+    def _is_capturing(cls, capman):
+        if capman:
+            return capman.is_capturing()
+        return False
+
+    @classmethod
+    def _import_pdb_cls(cls):
+        if not cls._config:
+            # Happens when using pytest.set_trace outside of a test.
+            return pdb.Pdb
+
+        pdb_cls = cls._config.getvalue("usepdb_cls")
+        if not pdb_cls:
+            return pdb.Pdb
+
+        modname, classname = pdb_cls
+
         try:
             __import__(modname)
             mod = sys.modules[modname]
@@ -65,55 +115,6 @@ def _import_pdbcls(modname, classname):
             value = ":".join((modname, classname))
             raise UsageError("--pdbcls: could not import {!r}: {}".format(value, exc))
 
-
-def pytest_configure(config):
-    pdb_cls = config.getvalue("usepdb_cls")
-    if pdb_cls:
-        pdb_cls = _import_pdbcls(*pdb_cls)
-    else:
-        pdb_cls = pdb.Pdb
-
-    if config.getvalue("trace"):
-        config.pluginmanager.register(PdbTrace(), "pdbtrace")
-    if config.getvalue("usepdb"):
-        config.pluginmanager.register(PdbInvoke(), "pdbinvoke")
-
-    pytestPDB._saved.append(
-        (pdb.set_trace, pytestPDB._pluginmanager, pytestPDB._config, pytestPDB._pdb_cls)
-    )
-    pdb.set_trace = pytestPDB.set_trace
-    pytestPDB._pluginmanager = config.pluginmanager
-    pytestPDB._config = config
-    pytestPDB._pdb_cls = pdb_cls
-
-    # NOTE: not using pytest_unconfigure, since it might get called although
-    #       pytest_configure was not (if another plugin raises UsageError).
-    def fin():
-        (
-            pdb.set_trace,
-            pytestPDB._pluginmanager,
-            pytestPDB._config,
-            pytestPDB._pdb_cls,
-        ) = pytestPDB._saved.pop()
-
-    config._cleanup.append(fin)
-
-
-class pytestPDB(object):
-    """ Pseudo PDB that defers to the real pdb. """
-
-    _pluginmanager = None
-    _config = None
-    _pdb_cls = pdb.Pdb
-    _saved = []
-    _recursive_debug = 0
-
-    @classmethod
-    def _is_capturing(cls, capman):
-        if capman:
-            return capman.is_capturing()
-        return False
-
     @classmethod
     def _init_pdb(cls, *args, **kwargs):
         """ Initialize PDB debugging, dropping any IO capturing. """
@@ -144,7 +145,9 @@ class pytestPDB(object):
                     else:
                         tw.sep(">", "PDB set_trace")
 
-            class PytestPdbWrapper(cls._pdb_cls, object):
+            pdb_cls = cls._import_pdb_cls()
+
+            class PytestPdbWrapper(pdb_cls, object):
                 _pytest_capman = capman
                 _continued = False
 
@@ -227,7 +230,8 @@ class pytestPDB(object):
             _pdb = PytestPdbWrapper(**kwargs)
             cls._pluginmanager.hook.pytest_enter_pdb(config=cls._config, pdb=_pdb)
         else:
-            _pdb = cls._pdb_cls(**kwargs)
+            pdb_cls = cls._import_pdb_cls()
+            _pdb = pdb_cls(**kwargs)
         return _pdb
 
     @classmethod

testing/test_pdb.py

@@ -1157,12 +1157,13 @@ def test_pdbcls_via_local_module(testdir):
     result = testdir.runpytest(
         str(p1), "--pdbcls=really.invalid:Value", syspathinsert=True
     )
-    result.stderr.fnmatch_lines(
+    result.stdout.fnmatch_lines(
         [
-            "ERROR: --pdbcls: could not import 'really.invalid:Value': No module named *really*"
+            "*= FAILURES =*",
+            "E * --pdbcls: could not import 'really.invalid:Value': No module named *really*",
         ]
     )
-    assert result.ret == 4
+    assert result.ret == 1
 
     result = testdir.runpytest(
         str(p1), "--pdbcls=mypdb:Wrapped.MyPdb", syspathinsert=True