make "import pdb ; pdb.set_trace()" work natively wrt capturing (no "-s" needed

anymore), turning ``pytest.set_trace()`` into a mere shortcut.

CHANGELOG

@@ -1,6 +1,9 @@
 Changes between 2.3.5 and 2.4.DEV
 -----------------------------------
 
+- make "import pdb ; pdb.set_trace()" work natively wrt capturing (no "-s" needed
+  anymore), making ``pytest.set_trace()`` a mere shortcut.
+
 - fix issue181: --pdb now also works on collect errors (and 
   on internal errors) .  This was implemented by a slight internal 
   refactoring and the introduction of a new hook 

_pytest/pdb.py

@@ -16,6 +16,12 @@ def pytest_configure(config):
     if config.getvalue("usepdb"):
         config.pluginmanager.register(PdbInvoke(), 'pdbinvoke')
 
+    old_trace = py.std.pdb.set_trace
+    def fin():
+        py.std.pdb.set_trace = old_trace
+    py.std.pdb.set_trace = pytest.set_trace
+    config._cleanup.append(fin)
+
 class pytestPDB:
     """ Pseudo PDB that defers to the real pdb. """
     item = None

testing/test_pdb.py

@@ -134,6 +134,22 @@ class TestPDB:
         if child.isalive():
             child.wait()
 
+    def test_pdb_set_trace_interception(self, testdir):
+        p1 = testdir.makepyfile("""
+            import pdb
+            def test_1():
+                pdb.set_trace()
+        """)
+        child = testdir.spawn_pytest(str(p1))
+        child.expect("test_1")
+        child.expect("(Pdb)")
+        child.sendeof()
+        rest = child.read()
+        assert "1 failed" in rest
+        assert "reading from stdin while output" not in rest
+        if child.isalive():
+            child.wait()
+
     def test_pdb_and_capsys(self, testdir):
         p1 = testdir.makepyfile("""
             import pytest