Unhelpful error when content of a Vyper file is not Vyper

[antazoey]

Version Information

• vyper Version (output of vyper --version): 0.4.0
• OS: macOS
• Python Version (output of python --version): 3.13.1

What's your issue about?

Weird setup so bear with me.
So I had a bug where I accidentally pasted JSON directly into a Vyper file... like replaced the file's contents, ok.
But that obviously was not compiling by Vyper, which is perfect... However, I had to hunt down the reason why (and eventually find the bug in my code), but the Vyper error was about as helpful as an umbrella on a squid...

Example "contract":

[{"inputs": [],"name": "retrieve","outputs": [{"internalType": "uint256", "name": "", "type": "uint256"}],"stateMutability": "view","type": "function"}]

Compile it:

➜  playground vyper contracts/NotVyper.vy 
AssertionError

How can it be fixed?

Make an actual error telling me I am an idiot

[antazoey]

I tracked down the error to this part:

breakpoint()
assert isinstance(file, FileInput)  # mypy hint

tis more than a mypy hint it seems, or validate earlier