pdb: import pdbcls lazily

Fixes pytest-dev#2064.

src/_pytest/debugging.py

@@ -49,42 +49,18 @@ def pytest_addoption(parser):
     )
 
 
-def _import_pdbcls(modname, classname):
-    try:
-        __import__(modname)
-        mod = sys.modules[modname]
-
-        # Handle --pdbcls=pdb:pdb.Pdb (useful e.g. with pdbpp).
-        parts = classname.split(".")
-        pdb_cls = getattr(mod, parts[0])
-        for part in parts[1:]:
-            pdb_cls = getattr(pdb_cls, part)
-
-        return pdb_cls
-    except Exception as exc:
-        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._pluginmanager, pytestPDB._config)
     )
     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).
@@ -93,7 +69,6 @@ def fin():
             pdb.set_trace,
             pytestPDB._pluginmanager,
             pytestPDB._config,
-            pytestPDB._pdb_cls,
         ) = pytestPDB._saved.pop()
 
     config._cleanup.append(fin)
@@ -104,7 +79,6 @@ class pytestPDB(object):
 
     _pluginmanager = None
     _config = None
-    _pdb_cls = pdb.Pdb
     _saved = []
     _recursive_debug = 0
 
@@ -114,6 +88,32 @@ def _is_capturing(cls, capman):
             return capman.is_capturing()
         return False
 
+    @classmethod
+    def _import_pdb_cls(cls):
+        if cls._config:
+            pdb_cls = cls._config.getvalue("usepdb_cls")
+            if pdb_cls:
+                modname, classname = pdb_cls
+
+                try:
+                    __import__(modname)
+                    mod = sys.modules[modname]
+
+                    # Handle --pdbcls=pdb:pdb.Pdb (useful e.g. with pdbpp).
+                    parts = classname.split(".")
+                    pdb_cls = getattr(mod, parts[0])
+                    for part in parts[1:]:
+                        pdb_cls = getattr(pdb_cls, part)
+
+                    return pdb_cls
+                except Exception as exc:
+                    value = ":".join((modname, classname))
+                    raise UsageError(
+                        "--pdbcls: could not import {!r}: {}".format(value, exc)
+                    )
+
+        return pdb.Pdb
+
     @classmethod
     def _init_pdb(cls, *args, **kwargs):
         """ Initialize PDB debugging, dropping any IO capturing. """
@@ -144,7 +144,9 @@ def _init_pdb(cls, *args, **kwargs):
                     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 +229,8 @@ def get_stack(self, f, t):
             _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 runcall(self, *args, **kwds):
     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