Expand warnings output for ResourceWarning

Fix pytest-dev#9644

changelog/9644.improvement.rst

@@ -0,0 +1,4 @@
+More information about the location of resources that led Python to raise :class:`ResourceWarning` can now
+be obtained by enabling :mod:`tracemalloc`.
+
+See :ref:`resource-warnings` for more information.

doc/en/how-to/capture-warnings.rst

@@ -441,3 +441,18 @@ Please read our :ref:`backwards-compatibility` to learn how we proceed about dep
 features.
 
 The full list of warnings is listed in :ref:`the reference documentation <warnings ref>`.
+
+
+.. _`resource-warnings`:
+
+Resource Warnings
+-----------------
+
+Additional information of the source of a :class:`ResourceWarning` can be obtained when captured by pytest if
+:mod:`tracemalloc` module is enabled.
+
+One convenient way to enable :mod:`tracemalloc` when running tests is to set the :envvar:`PYTHONTRACEMALLOC` to a large
+enough number of frames (say ``20``, but that number is application dependent).
+
+For more information, consult the `Python Development Mode <https://docs.python.org/3/library/devmode.html>`__
+section in the Python documentation.

src/_pytest/warnings.py

@@ -1,4 +1,5 @@
 import sys
+import tracemalloc
 import warnings
 from contextlib import contextmanager
 from typing import Generator
@@ -81,6 +82,20 @@ def warning_record_to_str(warning_message: warnings.WarningMessage) -> str:
         warning_message.lineno,
         warning_message.line,
     )
+    if warning_message.source is not None:
+        tb = tracemalloc.get_object_traceback(warning_message.source)
+        if tb is not None:
+            formatted_tb = "\n".join(tb.format())
+            # Use a leading new line to better separate the (large) output
+            # from the traceback to the previous warning text.
+            msg += f"\nObject allocated at (most recent call first):\n{formatted_tb}"
+        else:
+            # No need for a leading new line.
+            url = "https://docs.pytest.org/en/stable/how-to/capture-warnings.html#resource-warnings"
+            msg += (
+                "Enable tracemalloc to get traceback where the object was allocated.\n"
+            )
+            msg += f"See {url} for more info."
     return msg
 
 

testing/test_warnings.py

@@ -1,4 +1,5 @@
 import os
+import sys
 import warnings
 from typing import List
 from typing import Optional
@@ -774,3 +775,36 @@ def test_it():
                 "*Unknown pytest.mark.unknown*",
             ]
         )
+
+
+def test_resource_warning(pytester: Pytester, monkeypatch: pytest.MonkeyPatch) -> None:
+    pytester.makepyfile(
+        """
+        def open_file(p):
+            f = p.open("r")
+            assert p.read_text() == "hello"
+
+        def test_resource_warning(tmp_path):
+            p = tmp_path.joinpath("foo.txt")
+            p.write_text("hello")
+            open_file(p)
+        """
+    )
+    result = pytester.run(sys.executable, "-Xdev", "-m", "pytest")
+    result.stdout.fnmatch_lines(
+        [
+            "*ResourceWarning* unclosed file*",
+            "*Enable tracemalloc to get traceback where the object was allocated*",
+            "*See https* for more info.",
+        ]
+    )
+
+    monkeypatch.setenv("PYTHONTRACEMALLOC", "20")
+
+    result = pytester.run(sys.executable, "-Xdev", "-m", "pytest")
+    result.stdout.fnmatch_lines(
+        [
+            "*ResourceWarning* unclosed file*",
+            "*Object allocated at (most recent call first)*",
+        ]
+    )