fixup! pdb: import pdbcls lazily

Author: blueyed

src/_pytest/debugging.py

@@ -90,29 +90,30 @@ def _is_capturing(cls, capman):
 
     @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
+        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]
+
+            # 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))
 
     @classmethod
     def _init_pdb(cls, *args, **kwargs):