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Python emits ResourceWarning when an object using limited resource is destroyed without being explicitly closed: files, sockets, etc. The problem is that it's hard to find where the object comes from, since the warning can occur very late, in the garbage collector, etc.
I propose to reuse tracemalloc.get_object_traceback() to show were the object was allocated, when tracemalloc traces memory allocations. In practice, I propose to add a new "source" parameter to warnings.showwarning().
Attached patch:
* Add a new keyword-only source parameter to warnings.showwarning()
* Add C function PyErr_ResourceWarning() to pass source
* showwarning() uses tracemalloc.get_object_traceback() to get the traceback were the object was allocated
* Modify socket.socket, io.FileIO and os.scandir destructor to use PyErr_ResourceWarning()
Backward-compatibility problem: The C function PyErr_ResourceWarning() always call warnings.showwarning() with the keyword parameter source. If an application replaces the warnings.showwarning() function, it will probably fail because it doesn't know the source parameter.
I don't know how to handle this backward compatibility issue.
The patch is incomplete, it's not possible yet to emit a warning in pure Python with a source parameter.
x.py script used for examples below:
-----------------
import warnings
import os
import socket
def func2():
 #f=open("/etc/issue")
 #f=os.scandir('.')
 f=socket.socket()
 f=None
def func():
 func2()
func()
-----------------
Output with Python 3.5:
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x.py:9: ResourceWarning: unclosed <socket.socket fd=3, ...>
 f=None
-----
Output with -X tracemalloc=5 command line option and patched Python 3.6:
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x.py:9: ResourceWarning: unclosed <socket.socket fd=3, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=0, laddr=('0.0.0.0', 0)>
 f=None
Object allocated at (most recent call first):
 File "x.py", lineno 8
 f=socket.socket()
 File "x.py", lineno 12
 func2()
 File "x.py", lineno 14
 func()
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It's much easier to understand where the warning comes from, no? At x.py:8, line "f=socket.socket()".
Note: the traceback doesn't contain the function name, since tracemalloc only stores filename and line number.
See also the issue #26564 "Malloc debug hooks: display memory block traceback on error".
For Python < 3.6, I wrote "res_warn.py" script which monkey-patches io.FileIO and socket.socket to implement something similar. The script is a fragile hack. I would prefer to have the feature built-in Python.
https://bitbucket.org/haypo/misc/src/0a40f27360424145bad0f9b62c9e9148ffdbb169/python/res_warn.py