Can we use "default" instead of "always", and just one context manager that spans both compiler calls? I think that will do the de-duplication, if I understand the doc correctly.

https://docs.python.org/3/library/warnings.html#the-warnings-filter

From what I can tell it does not work in this case. The syntax warning uses_PyErr_EmitSyntaxWarning which in turn callsPyErr_WarnExplicitObject and sets theregistry (last argument) toNULL. Theregistry is used to dedup the warnings so when it's not used, all warnings are emitted regardless of the filter setting.

Here's an example that shows that:

importwarningsdef_compile():compile('1 is 1','input','eval')withwarnings.catch_warnings():warnings.simplefilter("default")_compile()_compile()

This is probably not something we want to change, so doing a manual deduplication in pyrepl seems like the way to go. What do you think?

Maybe we can pass a registry?

Maybe, though the registry would have to persist between separate invocations ofcompile. I'll check if there's a nice way to do that.

Ok so I spent some time going over the warning code and this is my understanding so far. The warning registries are dictionaries that are inserted as__warningregistry__ into the globals of a Python frame. Which frame this is, is controlled by the stack level. The registries are created on demand insetup_context:

This function is pretty complex and depends on other pieces in the warnings module so we cannot construct a registry manually in_PyErr_EmitSyntaxWarning (which is the one responsible for emitting warnings from ast_opt and currently sets the registry toNULL):

One thing we could do, though I don't know how desirable it is, is to add a new private function akin toPyErr_WarnExplicitObject which sets up the registry for us. Basically something like this:

int- PyErr_WarnExplicitObject(PyObject *category, PyObject *message,-                          PyObject *filename, int lineno,-                          PyObject *module, PyObject *registry)+ _PyErr_WarnExplicitObjectRegistry(PyObject *category, PyObject *message,+                                  PyObject *filename, int lineno){    PyObject *res;    if (category == NULL)        category = PyExc_RuntimeWarning;    PyThreadState *tstate = get_current_tstate();    if (tstate == NULL) {        return -1;    }+    PyObject *unused_filename, *module, *registry;+    int unused_lineno;++    if (!setup_context(1, NULL,+                       &unused_filename, &unused_lineno, &module, &registry))+        return -1;    warnings_lock(tstate->interp);    res = warn_explicit(tstate, category, message, filename, lineno,                        module, registry, NULL, NULL);    warnings_unlock(tstate->interp);    if (res == NULL)        return -1;    Py_DECREF(res);    return 0;}

This seems to eliminate the duplicated syntax warnings in the REPL. What do you think about this approach?

I like this. Can_PyErr_WarnExplicitObjectRegistry just create a registry and callPyErr_WarnExplicitObject with it?

I think so, basically something like this:

int_PyErr_WarnExplicitObjectRegistry(PyObject*category,PyObject*message,PyObject*filename,intlineno){PyObject*unused_filename,*module,*registry;intunused_lineno;intstack_level=1;if (!setup_context(stack_level,NULL,&unused_filename,&unused_lineno,&module,&registry))return-1;intrc=PyErr_WarnExplicitObject(category,message,filename,lineno,module,registry);Py_DECREF(module);returnrc;}

I'll update the PR to see if everything still works with this change.

You can use.count() here.

"'return' in a 'finally' block" may be an error in future versions. Maybe use some more long living warnings, likeassert(1, 'message') or1 is 1? BTW, they are currently emitted 3 times, not 2.

You can use .count() here.

Hmm I don't think I can, unless I want to match the warning message exactly?

"'return' in a 'finally' block" may be an error in future versions. Maybe use some more long living warnings, like assert(1, 'message') or 1 is 1?

I can't if I want to test the new repl. The PEP 765 warning is the only one emitted from the ast optimizer which is needed to trigger this behaviour in the repl. However this does change the behaviour of all warnings that use_PyErr_EmitSyntaxWarning so I also added tests totest_compile for it.

What happens if set it to "always"?

It emits twice, I added a test totest_compile for it