The duplicate finder that pops up works reasonably well on one site but it doesn't help you choose the best site to ask; for that you probably want to use the search engines.
Taking your query: "python postfix notation for map filter" gives this result:
python postfix notation for map filter -site:https://stackoverflow.com site:https://stackexchange.com
That returns just two results, but if you click on the last link, on the last page (in this case, with two results, it's on the first page) that says:
"If you like, you can repeat the search with the omitted results included."
You come up with 448 results, that appears to be a useful result that's relevant to your search terms. Now it's a matter of looking on the first few pages to see if there are sites most suitable for your question.
In some cases our Stack Overflow site is the better site to ask on; all too often, when using the https://stackexchange.com/ search bar, that one site overwhelms the returns. In that case it's often better to style your search as indicated in my first link (or on a different search engine, substituting their syntax). With the search terms for your example there are only two results returned, and one of them is this question. With those example words using an external search engine (and the last link) provides 448 results.
Once you find the best site typing your search terms (a fake title) into the [Ask Question] duplicate finder or using the site's search bar and our query modifiers helps check for duplicates and verify that the site is a good place to ask your question.
A second way to verify that the site is a good fit for your question is to search the site's tags, unfortunately that requires knowing exactly what tags you would want to use (or picking words that show up in the tag info). Sometimes that is done automatically by our search tool, but only for popular tags.
Once you can select a half dozen words that surely must be a part of your question or answer it's easy to whittle the results. Sometimes "quoting a few words together" along with several floating words is what's needed to shake loose what you are looking for.