list.pop() in a list with 'undefined' returned from a function raises IndexError

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
48/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
javascript, python
Domain
compilers

Research direction

Start by running the supplied reproducer with Transcrypt 3.9.4 and compare its generated JavaScript behavior with Python's bare return semantics. Check the list.pop() path and the handling of undefined values; done means the example prints the list and removes the first element without raising IndexError.

Written by the indexing model from the issue text.

Description

IS: bug?

Hi,

I encountered an error in list.pop() in Transcrypt3.9.4. It had no problem in previous versions. When a list contains an element 'undefined' returned from a function, list.pop() raises IndexError. The return command with no argument returns None in Python. But, 'undefined' is returned in Transcrypt3.9.4.

test code:

sMsg= "list.pop() in a list with 'undefined' returned from a function raises IndexError in Transcrypt3.9.4"
def x():
    return  # Python returns None, but Transcrypt3.9.4 returns 'undefined'
def main():
    print('\n'+sMsg)
    l= [x(), x(), x()]
    print(l)
    print(l.pop(0))

if __name__ == '__main__':
    main()

output:

list.pop() in a list with 'undefined' returned from a function raises IndexError in Transcrypt3.9.4
msgWhileLoading.js:64 [None, None, None]
localhost/:1 Uncaught Function {__args__: Array(1), stack: 'Error\n    at Array.py_pop (https://localhost:8443/…/compiled.DEBUG/main_frontend.js:6:2', __class__: ƒ}

The exception can be caught by try... except IndexError.

Thanks.

Dominant language
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