Error when changing language to Python for unsaved file

[CrazyIvan359]

Environment data

• Language Server version: 2020.12.2
• OS and version: Ubuntu 20.04
• Python version: 3.8.5 64-bit

Expected behaviour

Language server should either support unsaved files (not sure how feasible that is) or not attempt to parse them. Perhaps a notification could indicate this?

Actual behaviour

Language server crashes when specifying the language of an unsaved file as Python.

Logs

(node:2548735) UnhandledPromiseRejectionWarning: Error: Debug Failure. Untitled-1 is not a source file
    at P._addToSourceFileListAndMap (/home/mike/.vscode/extensions/ms-python.vscode-pylance-2020.12.2/dist/pyright-internal/src/analyzer/program.ts:2075:13)
    at P.setFileOpened (/home/mike/.vscode/extensions/ms-python.vscode-pylance-2020.12.2/dist/pyright-internal/src/analyzer/program.ts:275:18)
    at O.setFileOpened (/home/mike/.vscode/extensions/ms-python.vscode-pylance-2020.12.2/dist/pyright-internal/src/analyzer/backgroundAnalysisProgram.ts:90:23)
    at d.setFileOpened (/home/mike/.vscode/extensions/ms-python.vscode-pylance-2020.12.2/dist/pyright-internal/src/analyzer/service.ts:192:41)
    at Y.<anonymous> (/home/mike/.vscode/extensions/ms-python.vscode-pylance-2020.12.2/dist/pyright-internal/src/languageServerBase.ts:789:39)
    at Generator.next (<anonymous>)
    at o (/home/mike/.vscode/extensions/ms-python.vscode-pylance-2020.12.2/dist/pyright.bundle.js:1:584031)
(node:2548735) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
Error: Debug Failure. Untitled-1 is not a source file
    at P._addToSourceFileListAndMap (/home/mike/.vscode/extensions/ms-python.vscode-pylance-2020.12.2/dist/pyright-internal/src/analyzer/program.ts:2075:13)
    at P.setFileOpened (/home/mike/.vscode/extensions/ms-python.vscode-pylance-2020.12.2/dist/pyright-internal/src/analyzer/program.ts:275:18)
    at B.onMessage (/home/mike/.vscode/extensions/ms-python.vscode-pylance-2020.12.2/dist/pyright-internal/src/backgroundAnalysisBase.ts:366:30)
    at B.onMessage (/home/mike/.vscode/extensions/ms-python.vscode-pylance-2020.12.2/dist/server.bundle.js:1:32759)
    at MessagePort.<anonymous> (/home/mike/.vscode/extensions/ms-python.vscode-pylance-2020.12.2/dist/pyright-internal/src/backgroundAnalysisBase.ts:273:66)
    at MessagePort.emit (events.js:223:5)
    at MessagePort.onmessage (internal/worker/io.js:70:8)
Background analysis message: setFileOpened

I enabled trace logging, but no additional messages were produced.

[erictraut]

I'm able to repro this with Pylance but not with the Pyright VS Code extension. I suspect it has to do with the interaction between the core Python extension and Pylance.

[jakebailey]

Marking this as a duplicate of #739, which is all the same issue.