An app would hang if attempting "import nonexistent_module" from CDN

An app would hang if attempting “import nonexistent_module” from CDN, due to uncaught error in brython.js. Is it possible to have brython raise an ImportError instead?

Repro:

• Content in index.html

  ...
  <script src="https://cdnjs.cloudflare.com/ajax/libs/brython/3.9.1/brython.min.js"></script>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/brython/3.9.1/brython_stdlib.min.js"></script>
  <script type="text/python" src="main.py"></script>
  ...
  

• Content in main.py

  try:
      import optional_module
  except ImportError:
      print("That is fine")
  

Expects to see “That is fine” in the console.

What actually happened is that the app hangs, with the following output on the browser console. The first 2 or 3 lines are expected, but can Brython or javascript recover from a “Cross-Origin Request Blocked” request?

Error 404 means that Python module cowsay was not found at url http://localhost:8000/optional_module.py
Error 404 means that Python module cowsay was not found at url http://localhost:8000/__init__/optional_module.py
Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://cdnjs.cloudflare.com/ajax/libs/brython/3.9.1/Lib/site-packages/optional_module.py?v=1615571704659. (Reason: CORS header ‘Access-Control-Allow-Origin’ missing).
Javascript exception: DOMException: A network error occurred.
...
test is js exc is recursion error DOMException: A network error occurred. NetworkError: A network error occurred.
...
Uncaught Error: 
    $factory https://cdnjs.cloudflare.com/ajax/libs/brython/3.9.1/brython.min.js line 1 > eval:1
    loop https://cdnjs.cloudflare.com/ajax/libs/brython/3.9.1/brython.min.js:1
    onsuccess https://cdnjs.cloudflare.com/ajax/libs/brython/3.9.1/brython.min.js:1
    onsuccess https://cdnjs.cloudflare.com/ajax/libs/brython/3.9.1/brython.min.js:1
brython.min.js line 1 > eval:1:7500
    $factory https://cdnjs.cloudflare.com/ajax/libs/brython/3.9.1/brython.min.js line 1 > eval:1
    loop https://cdnjs.cloudflare.com/ajax/libs/brython/3.9.1/brython.min.js:1
    onsuccess https://cdnjs.cloudflare.com/ajax/libs/brython/3.9.1/brython.min.js:1
    onsuccess https://cdnjs.cloudflare.com/ajax/libs/brython/3.9.1/brython.min.js:1

[PierreQuentel]

I did try this Javascript program:

var url = "https://cdnjs.cloudflare.com/ajax/libs/brython/3.9.1/Lib/site-packages/optional_module.py?v=1615916897909"
var xhr = new XMLHttpRequest()
xhr.open('GET', url)
console.log("try")
try{
  xhr.send()
  console.log("no JS exception")
}catch(err){
  console.log("error", err)
}

The console prints this:

try
no JS exception
XHRGEThttps://cdnjs.cloudflare.com/ajax/libs/brython/3.9.1/Lib/site-packages/optional_module.py?v=1615916897909
CORS Missing Allow Origin

Blocage d’une requête multiorigines (Cross-Origin Request) : la politique « Same Origin » ne permet pas de consulter la ressource distante située sur https://cdnjs.cloudflare.com/ajax/libs/brython/3.9.1/Lib/site-packages/optional_module.py?v=1615916897909. Raison : l’en-tête CORS « Access-Control-Allow-Origin » est manquant.

[rayluo]

Interestingly, I notice that the ImportError can be caught now, and the except clause can be run. Last time @PierreQuentel and I both tested this issue 3 months ago, it did not work at that time. The only difference today is I run it with a newer version of browser. Just a FYI.