pdb: try to import --pdbcls in pytest_configure only

Fixes pytest-dev#5039.

changelog/5039.bugfix.rst

@@ -0,0 +1 @@
+Fix regression with ``--pdbcls``, which stopped working with local modules in 4.0.0.

src/_pytest/debugging.py

@@ -10,31 +10,18 @@
 
 from _pytest import outcomes
 from _pytest.config import hookimpl
+from _pytest.config.exceptions import UsageError
 
 
 def _validate_usepdb_cls(value):
+    """Validate syntax of --pdbcls option."""
     try:
         modname, classname = value.split(":")
     except ValueError:
         raise argparse.ArgumentTypeError(
             "{!r} is not in the format 'modname:classname'".format(value)
         )
-
-    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:
-        raise argparse.ArgumentTypeError(
-            "could not get pdb class for {!r}: {}".format(value, exc)
-        )
+    return (modname, classname)
 
 
 def pytest_addoption(parser):
@@ -61,9 +48,28 @@ 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 not pdb_cls:
+    if pdb_cls:
+        pdb_cls = _import_pdbcls(*pdb_cls)
+    else:
         pdb_cls = pdb.Pdb
 
     if config.getvalue("trace"):

testing/test_pdb.py

@@ -1121,3 +1121,32 @@ def test_inner({fixture}):
     assert child.exitstatus == 0
     assert "= 1 passed in " in rest
     assert "> PDB continue (IO-capturing resumed for fixture %s) >" % (fixture) in rest
+
+
+def test_pdbcls_via_local_module(testdir):
+    """It should be imported in pytest_configure or later only."""
+    p1 = testdir.makepyfile(
+        """
+        def test():
+            print("before_settrace")
+            __import__("pdb").set_trace()
+        """,
+        mypdb="""
+        class MyPdb:
+            def set_trace(self, *args):
+                print("mypdb_called", args)
+        """,
+    )
+    result = testdir.runpytest(
+        str(p1), "--pdbcls=really.invalid:Value", syspathinsert=True
+    )
+    result.stderr.fnmatch_lines(
+        [
+            "ERROR: --pdbcls: could not import 'really.invalid:Value': No module named 'really'"
+        ]
+    )
+    assert result.ret == 4
+
+    result = testdir.runpytest(str(p1), "--pdbcls=mypdb:MyPdb", syspathinsert=True)
+    assert result.ret == 0
+    result.stdout.fnmatch_lines(["mypdb_called *", "* 1 passed in *"])