Allow for "pdb" module to be rewritten

src/_pytest/debugging.py

@@ -1,8 +1,6 @@
 """ interactive debugging with PDB, the Python Debugger. """
 import argparse
-import pdb
 import sys
-from doctest import UnexpectedException
 
 from _pytest import outcomes
 from _pytest.config import hookimpl
@@ -45,6 +43,8 @@ def pytest_addoption(parser):
 
 
 def pytest_configure(config):
+    import pdb
+
     if config.getvalue("trace"):
         config.pluginmanager.register(PdbTrace(), "pdbtrace")
     if config.getvalue("usepdb"):
@@ -87,6 +87,8 @@ class pytestPDB:
     @classmethod
     def _import_pdb_cls(cls, capman):
         if not cls._config:
+            import pdb
+
             # Happens when using pytest.set_trace outside of a test.
             return pdb.Pdb
 
@@ -113,6 +115,8 @@ class pytestPDB:
                     "--pdbcls: could not import {!r}: {}".format(value, exc)
                 )
         else:
+            import pdb
+
             pdb_cls = pdb.Pdb
 
         wrapped_cls = cls._get_pdb_wrapper_class(pdb_cls, capman)
@@ -313,6 +317,8 @@ def _enter_pdb(node, excinfo, rep):
 
 
 def _postmortem_traceback(excinfo):
+    from doctest import UnexpectedException
+
     if isinstance(excinfo.value, UnexpectedException):
         # A doctest.UnexpectedException is not useful for post_mortem.
         # Use the underlying exception instead:

testing/acceptance_test.py

@@ -1238,3 +1238,40 @@ def test_warn_on_async_gen_function(testdir):
     assert (
         result.stdout.str().count("async def functions are not natively supported") == 1
     )
+
+
+def test_pdb_can_be_rewritten(testdir):
+    testdir.makepyfile(
+        **{
+            "conftest.py": """
+                import pytest
+                pytest.register_assert_rewrite("pdb")
+                """,
+            "__init__.py": "",
+            "pdb.py": """
+                def check():
+                    assert 1 == 2
+                """,
+            "test_pdb.py": """
+                def test():
+                    import pdb
+                    assert pdb.check()
+                """,
+        }
+    )
+    # Disable debugging plugin itself to avoid:
+    # > INTERNALERROR> AttributeError: module 'pdb' has no attribute 'set_trace'
+    result = testdir.runpytest_subprocess("-p", "no:debugging", "-vv")
+    result.stdout.fnmatch_lines(
+        [
+            "    def check():",
+            ">       assert 1 == 2",
+            "E       assert 1 == 2",
+            "E         -1",
+            "E         +2",
+            "",
+            "pdb.py:2: AssertionError",
+            "*= 1 failed in *",
+        ]
+    )
+    assert result.ret == 1