Improve tracebacks for ImportErrors in conftest.py files

Fix #3332

Author: nicoddemus

changelog/3332.feature.rst

@@ -0,0 +1,4 @@
+Improve the error displayed when a ``conftest.py`` file could not be imported.
+
+In order to implement this, a new ``chain`` parameter was added to ``ExceptionInfo.getrepr``
+to show or hide chained tracebacks in Python 3 (defaults to ``True``).

src/_pytest/config/__init__.py

@@ -57,10 +57,24 @@ def main(args=None, plugins=None):
         try:
             config = _prepareconfig(args, plugins)
         except ConftestImportFailure as e:
+            from _pytest._code import ExceptionInfo
+
+            exc_info = ExceptionInfo(e.excinfo)
             tw = py.io.TerminalWriter(sys.stderr)
-            for line in traceback.format_exception(*e.excinfo):
+            tw.line(
+                "ImportError while loading conftest '{e.path}'.".format(e=e), red=True
+            )
+            from _pytest.python import filter_traceback
+
+            exc_info.traceback = exc_info.traceback.filter(filter_traceback)
+            exc_repr = (
+                exc_info.getrepr(style="short", chain=False)
+                if exc_info.traceback
+                else exc_info.exconly()
+            )
+            formatted_tb = safe_str(exc_repr)
+            for line in formatted_tb.splitlines():
                 tw.line(line.rstrip(), red=True)
-            tw.line("ERROR: could not load %s\n" % (e.path,), red=True)
             return 4
         else:
             try:

testing/acceptance_test.py

@@ -133,9 +133,16 @@ def test_not_collectable_arguments(self, testdir):
         assert result.ret
         result.stderr.fnmatch_lines(["*ERROR: not found:*{}".format(p2.basename)])
 
-    def test_issue486_better_reporting_on_conftest_load_failure(self, testdir):
+    def test_better_reporting_on_conftest_load_failure(self, testdir, request):
+        """Show a user-friendly traceback on conftest import failures (#486, #3332)"""
         testdir.makepyfile("")
-        testdir.makeconftest("import qwerty")
+        testdir.makeconftest(
+            """
+            def foo():
+                import qwerty
+            foo()
+        """
+        )
         result = testdir.runpytest("--help")
         result.stdout.fnmatch_lines(
             """
@@ -144,10 +151,23 @@ def test_issue486_better_reporting_on_conftest_load_failure(self, testdir):
         """
         )
         result = testdir.runpytest()
+        dirname = request.node.name + "0"
+        exc_name = (
+            "ModuleNotFoundError" if sys.version_info >= (3, 6) else "ImportError"
+        )
         result.stderr.fnmatch_lines(
-            """
-            *ERROR*could not load*conftest.py*
-        """
+            [
+                "ImportError while loading conftest '*{sep}{dirname}{sep}conftest.py'.".format(
+                    dirname=dirname, sep=os.sep
+                ),
+                "conftest.py:3: in <module>",
+                "    foo()",
+                "conftest.py:2: in foo",
+                "    import qwerty",
+                "E   {}: No module named {q}qwerty{q}".format(
+                    exc_name, q="'" if six.PY3 else ""
+                ),
+            ]
         )
 
     def test_early_skip(self, testdir):