Capture and display warnings during collection

Fix #3251

changelog/3251.feture.rst

@@ -0,0 +1 @@
+Warnings are now captured and displayed during test collection.

src/_pytest/terminal.py

@@ -339,8 +339,9 @@ class TerminalReporter(object):
         fslocation = get_fslocation_from_item(item)
         message = warning_record_to_str(warning_message)
 
+        nodeid = item.nodeid if item is not None else ""
         warning_report = WarningReport(
-            fslocation=fslocation, message=message, nodeid=item.nodeid
+            fslocation=fslocation, message=message, nodeid=nodeid
         )
         warnings.append(warning_report)
 
@@ -707,7 +708,8 @@ class TerminalReporter(object):
 
             self.write_sep("=", "warnings summary", yellow=True, bold=False)
             for location, warning_records in grouped:
-                self._tw.line(str(location) if location else "<undetermined location>")
+                if location:
+                    self._tw.line(str(location))
                 for w in warning_records:
                     lines = w.message.splitlines()
                     indented = "\n".join("  " + x for x in lines)

src/_pytest/warnings.py

@@ -58,14 +58,16 @@ def pytest_configure(config):
 
 
 @contextmanager
-def catch_warnings_for_item(item):
+def catch_warnings_for_item(config, ihook, item):
     """
-    catches the warnings generated during setup/call/teardown execution
-    of the given item and after it is done posts them as warnings to this
-    item.
+    Context manager that catches warnings generated in the contained execution block.
+
+    ``item`` can be None if we are not in the context of an item execution.
+
+    Each warning captured triggers the ``pytest_warning_captured`` hook.
     """
-    args = item.config.getoption("pythonwarnings") or []
-    inifilters = item.config.getini("filterwarnings")
+    args = config.getoption("pythonwarnings") or []
+    inifilters = config.getini("filterwarnings")
     with warnings.catch_warnings(record=True) as log:
         for arg in args:
             warnings._setoption(arg)
@@ -73,14 +75,15 @@ def catch_warnings_for_item(item):
         for arg in inifilters:
             _setoption(warnings, arg)
 
-        for mark in item.iter_markers(name="filterwarnings"):
-            for arg in mark.args:
-                warnings._setoption(arg)
+        if item is not None:
+            for mark in item.iter_markers(name="filterwarnings"):
+                for arg in mark.args:
+                    warnings._setoption(arg)
 
         yield
 
         for warning_message in log:
-            item.ihook.pytest_warning_captured.call_historic(
+            ihook.pytest_warning_captured.call_historic(
                 kwargs=dict(warning_message=warning_message, when="runtest", item=item)
             )
 
@@ -119,5 +122,12 @@ def warning_record_to_str(warning_message):
 
 @pytest.hookimpl(hookwrapper=True)
 def pytest_runtest_protocol(item):
-    with catch_warnings_for_item(item):
+    with catch_warnings_for_item(config=item.config, ihook=item.ihook, item=item):
+        yield
+
+
+@pytest.hookimpl(hookwrapper=True)
+def pytest_collection(session):
+    config = session.config
+    with catch_warnings_for_item(config=config, ihook=config.hook, item=None):
         yield

testing/test_warnings.py

@@ -321,3 +321,28 @@ def test_warning_captured_hook(testdir, pyfile_with_warnings):
         (RuntimeWarning, "runtest", "test_func"),
     ]
     assert collected == expected
+
+
+@pytest.mark.filterwarnings("always")
+def test_collection_warnings(testdir):
+    """
+    """
+    testdir.makepyfile(
+        """
+        import warnings
+
+        warnings.warn(UserWarning("collection warning"))
+
+        def test_foo():
+            pass
+    """
+    )
+    result = testdir.runpytest()
+    result.stdout.fnmatch_lines(
+        [
+            "*== %s ==*" % WARNINGS_SUMMARY_HEADER,
+            "*collection_warnings.py:3: UserWarning: collection warning",
+            '    warnings.warn(UserWarning("collection warning"))',
+            "* 1 passed, 1 warnings*",
+        ]
+    )