Pyscript ftplib Cors issues in py-env

Technical Topics PyScript
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Hello Guys,
I’m running into an issue where ftplib can’t be used as a library inside of the py-env in the browser. It’s being blocked by cors.

“Access to fetch at ‘https://pypi.org/pypi/ftplib/json’ from origin ‘http://127.0.0.1:5500’ has been blocked by CORS policy: No ‘Access-Control-Allow-Origin’ header is present on the requested resource. If an opaque response serves your needs, set the request’s mode to ‘no-cors’ to fetch the resource with CORS disabled.”

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You can go to https://actualseomediareports.web.app and see that the pyscript runs well when ftplib is not specified to be used in the browser. However when i’m trying to use ftplib to grab files from a server it hits me with a cors error. It isn’t because of my own server. The reason why I know it isn’t. Is because i’m using jupyter’s notebook on my computer to access the same server over ftplib and I get no errors.

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Hi @coreytmayweather, and welcome to the forum! Interesting issue there - a couple of things to help us understand the issue better:

  • Can you copy and paste your python and HTML files inside of a codeblock in this thread? That’s text ```surrounded by three backticks like this```, which makes it show up as a block of code, which we can copy/paste and try for ourselves instead of retyping the screenshots
  • It looks like that firebase app hasn’t been published yet; this is what I see when I follow that link:
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Here’s the github of everything.

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Hey Guys,
You can load it up on vscode if you have live preview. The code in question is in the home.html. in the first “py-env”. If you go live with the server. and go to the console you can see that the ftplib will cause a cors issue.

Thank you for your help!

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Hello Jeff,
Apologies the website link is https://actualseomediareports.web.app, and it’s updated in the original post.

Thank you for your reply!

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CORS blocks direct requests because PyPI doesn’t have the necessary browser headers.

A quick solution for development is to proxy the request: https://corsproxy.io/?

But keep in mind: even if you bypass CORS, ftplib itself directly from the browser (Pyodide/PyScript) won’t work—browsers can’t open raw TCP sockets for FTP. FTP will still require its own backend server.