“RuntimeError: generator raised StopIteration” every time I try to run app

Question

I am trying to run this code:

import web

urls = (
    '/', 'index'
)

if __name__ == "__main__":
    app = web.application(urls, globals())
    app.run()

But it gives me this error everytime

C:\Users\aidke\Desktop>python app.py
Traceback (most recent call last):
  File "C:\Users\aidke\AppData\Local\Programs\Python\Python37-32\lib\site-packages\web\utils.py", line 526, in take
    yield next(seq)
StopIteration

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "app.py", line 14, in <module>
    app = web.application(urls, globals())
  File "C:\Users\aidke\AppData\Local\Programs\Python\Python37-32\lib\site-packages\web\application.py", line 62, in __init__
    self.init_mapping(mapping)
  File "C:\Users\aidke\AppData\Local\Programs\Python\Python37-32\lib\site-packages\web\application.py", line 130, in init_mapping
    self.mapping = list(utils.group(mapping, 2))
  File "C:\Users\aidke\AppData\Local\Programs\Python\Python37-32\lib\site-packages\web\utils.py", line 531, in group
    x = list(take(seq, size))
RuntimeError: generator raised StopIteration

I tried someone else's code and the exact same thing happened. Additionally I tried reinstalling web.py(experimental) but it still didn't work.

Answer 1

To judge from the file paths, it looks like you're running Python 3.7. If so, you're getting caught by new-in-3.7 behavior described here:

PEP 479 is enabled for all code in Python 3.7, meaning that StopIteration exceptions raised directly or indirectly in coroutines and generators are transformed into RuntimeError exceptions. (Contributed by Yury Selivanov in bpo-32670.)

Before this change, a StopIteration raised by, or passing through, a generator simply ended the generator's useful life (the exception was silently swallowed). The module you're using will have to be recoded to work as intended with 3.7.

Chances are they'll need to change:

yield next(seq)

to:

try:
    yield next(seq)
except StopIteration:
    return

Answer 2

So during my recent self-learning on Python, a course required me to install Web.py and I was getting this error and as one of the answer stated, it had to be updated to be compatible with Python 3.7.

I installed the package with pip3 install web.py==0.40-dev1 ran into this error and started searching the web for a solution.

What I did was search through webpy git and find the utils.py file that was more recent in https://github.com/webpy/webpy/tree/master/web, downloaded it, and used it to replace the one that was in my Lib/site-packages/web folder (I'm a Windows user) and it just worked.

Hope this help someone.

Answer 3

They fixed this issue, just uninstall your current web.py version and I got an error when running pip install web.py from windows 10. So I run the pip install -e git+https://github.com/webpy/webpy.git#egg=webpy command to get the latest version from master branch. This won't execute RuntimeError: generator raised StopIteration error as question mentioned.