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

Question

I am trying to run this code in Python 3.7:

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:

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

Most major packages have fixed this issue by now, but one major package that hasn't is the clips/pattern project. It hasn't been updated since August 2018, so it never received a fix.

Since this is the top Google result for "python pattern stopiteration", here's a workaround:

def pattern_stopiteration_workaround():
    try:
        print(lexeme('gave'))
    except:
        pass

def main():
    pattern_stopiteration_workaround()
    #Add your other code here

Basically, the pattern-related code will only fail the first time you run it, so you first need to run it once and catch the Exception that it throws.

It's worked well enough for my own scripts, but I don't know if it fixes every possible issue.

Ideally though, somebody should fork the clips/pattern project since it's no longer maintained.

Answer 3

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 4

My solution was to upgrade these pips

mongoengine from 0.14.0 to 0.19.1 and

flask-mongoengine to 0.9.5

it worked.

Answer 5

I was facing the same issue for below command

python setup.py test

Error solved when I upgraded the pytest version

pip uninstall pytest
pip install pytest

Answer 6

This should be fixed in #577: https://github.com/webpy/webpy/pull/577

Answer 7

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.