This might be the wrong venue for this question, but WSGI is part of the
trouble, and Graham is always so helpful and responsive that it seems best
to start here.
I ran into a difficult-to-debug error in a Flask application, which I
eventually boiled down to a very short example. I'll give the complete
implementation with logs below, but let me start at the high level:
The high-level description is pretty simple: there's a data structure that
I had a "print" statement for. The print statement works perfectly in the
development version, but when I run the code in a "production" WSGI
environment, the print statement causes the server process to hang, with a
"500 Internal Server Error" in the browser. So, somehow, printing is
fundamentally different between development and WSGI, so I thought to start
here.
Here's the complete code for the application:
from flask import Flask
import pandas
import numpy
application = Flask(__name__)
@application.route('/')
def index():
print('about to load JSON')
df = pandas.read_json('/var/www/html/scott/prod/dfbad.json')
print('JSON loaded')
print('df is ',df)
return '<!doctype html><html><body>df is '+str(df)+'</body></html>'
if __name__ == '__main__':
port = 5000
application.run('0.0.0.0',port=port)
In development mode, the (slightly edited) console output looks like this:
about to load JSON
JSON loaded
('df is ', basal_amt bolus_volume carbs date_time rec_num
0 0.375 NaN NaN 2016-04-03 18:31:00 18539
1 0.000 NaN NaN 2016-04-03 18:37:00 18540
...
7 1.100 NaN NaN 2016-04-03 20:31:00 18546
8 0.000 NaN NaN 2016-04-03 21:13:00 18547
9 1.100 NaN NaN 2016-04-03 21:16:00 18548)
127.0.0.1 - - [26/Jul/2017 10:47:05] "GET / HTTP/1.1" 200 -
The GET response looks like this:
GET http://localhost:5000/
<!doctype html><html><body>df is basal_amt bolus_volume carbs
date_time rec_num
0 0.375 NaN NaN 2016-04-03 18:31:00 18539
1 0.000 NaN NaN 2016-04-03 18:37:00 18540
...
7 1.100 NaN NaN 2016-04-03 20:31:00 18546
8 0.000 NaN NaN 2016-04-03 21:13:00 18547
9 1.100 NaN NaN 2016-04-03 21:16:00
18548</body></html>
However, if I set up a WSGI production version (apache2, version
2.4.18-2ubuntu3.3) like this:
WSGIScriptAlias /df_not_printable
/var/www/html/scott/prod/df_not_printable.py
Then the response to the GET request is the following:
GET http://localhost/df_not_printable
read timeout at /usr/share/perl5/Net/HTTP/Methods.pm line 273, <STDIN> line
2.
The /var/log/apache2/error.log shows the following:
[Wed Jul 26 10:48:09.164138 2017] [wsgi:info] [pid 2291] mod_wsgi
(pid=2291): Create interpreter 'ip-132-148-72-77.ip.secureserver.net
|/df_not_printable'.
[Wed Jul 26 10:48:09.165605 2017] [wsgi:info] [pid 2291] [client
127.0.0.1:48072] mod_wsgi (pid=2291, process='', application='
ip-132-148-72-77.ip.secureserver.net|/df_not_printable'): Loading WSGI
script '/var/www/html/scott/prod/df_not_printable.py'.
[Wed Jul 26 10:48:09.658077 2017] [wsgi:error] [pid 2291] about to load JSON
[Wed Jul 26 10:48:09.701097 2017] [wsgi:error] [pid 2291] JSON loaded
So, you can see that the first two 'print' statements worked fine and went
to the log, but trying to print this pandas dataframe caused a
catastrophe. There is nothing more in the logs; that's the last line.
There's no stack backtrace, no exception, no hint about what could possibly
have gone wrong. Maybe some kind of infinite loop?
I know it's tempting just to blame pandas for the trouble, but I haven't a
clue about what went wrong or how to debug this. Obviously, for now I'll
just avoid printing any dataframes, but I worry about other unprintable
objects out there.
Thanks,
Scott
--
Scott D. Anderson
Computer Science Department
Wellesley College
scott.ander...@acm.org
scott.ander...@wellesley.edu
pronouns: he, him, his
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