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Why is Flask hanging after importing the pandas lib or several other scientific libs? It still works, when running it locally via SSH, but when visiting the URL the browser loads and loads and nothing happens.

sunwarr10r
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  • There is another post explaining the answer http://stackoverflow.com/questions/42405125/issue-with-imports-when-using-wsgi-in-ec2-instance-to-host-flask-app/43303368#43303368 – Anant Gupta Apr 09 '17 at 05:35

3 Answers3

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The solution is for Ubuntu with Apache2 server. You have to configure the following file:

/etc/apache2/sites-available/your-flask-app-file.conf

paste the following line below WSGIScriptAlias:

WSGIApplicationGroup %{GLOBAL}
sunwarr10r
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  • what's the reason for flask hanging? – aash Dec 12 '16 at 11:07
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    @geeky_sh It seems that pandas, numpy, matplotlib etc. are using the simplified threading API for manipulation of the Python GIL and thus will not run correctly within any additional sub interpreters created by Python. That's why it is necessary to force the WSGI application to run within the first interpreter, which the above mentioned solution is doing. – sunwarr10r Dec 12 '16 at 11:39
  • This is curious because I've been running pandas on my Apache2 server for nearly a year but just upgraded to 0.19.1 - and my entire site was brought down for a day because I couldn't figure out the problem. Alas, this was the solution. Many thanks. – elPastor Dec 26 '16 at 07:02
  • My web app hung when trying to perform array algebra and this fixed it – public static void Mar 29 '17 at 15:22
  • Great! The solution did the trick on my end. – JoeyBright Jul 06 '22 at 12:27
5

After a long and painful exercise, I was able to finally get my app running .The issue is with the pandas 0.19.2 built when the application is getting imported in the .wsgi file

To resolve it Remove your imports from the global level and insert them at the function level

import pandas as pd
....
@app.route('/getFunction', methods=["GET"])
def sample_get_function():
    movieData=pd.read_csv('someData.csv')

to

....
@app.route('/getFunction', methods=["GET"])
def sample_get_function():
    import pandas as pd
    movieData=pd.read_csv('someData.csv')

This is not a very good solution but it is working

Anant Gupta
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  • This worked for me too...But now getting error for other imports. Not sure how importing something inside a function. I am on Windows 10 though. – Sid Feb 06 '20 at 08:36
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The other solution by @saitam was necessary, but not sufficient in my case.

As before edit:

/etc/apache2/sites-available/<your-flask-app>.conf

Add or update WSGIApplicationGroup with:

WSGIApplicationGroup %{GLOBAL}

and remove or edit WSGIProcessGroup such that you have:

WSGIProcessGroup %{GLOBAL}

When this was set to something else, the problem persisted.

Some people advocate setting the number of threads to 1, but this was not necessary for me. The following works fine:

WSGIDaemonProcess <...> processes=2 threads=6

Linux version: Ubuntu 14.04.3 LTS

Pandas version: 0.19.2

Python version: 3.4.3

Apache version: 2.4.7

Community
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André C. Andersen
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