pyls incorrectly requires that textDocument/didOpen be called before fufilling any requests

[ggilmore]

See this issue in the official LSP repo: microsoft/language-server-protocol#177

Per @debaumer's comment, language servers should be able to fulfill requests regardless of whether or not the text document is open.

For example, pyls will raise a KeyError if you call hover on a document that has not been opened yet. According to the above, this seems to be incorrect behavior - pyls should be able to read this document from disk and fulfill the hover request.