Python antipattern: Close in finally

Don’t do this:

thing = Thing()
try:
    thing.do_stuff()
finally:
    thing.close()

Do do this:

from contextlib import closing

with closing(Thing()) as thing:
    thing.do_stuff()

Why is the second better? Using contextlib.closing() ties closing the item to its creation. These baby examples are about equally easy to reason about, with only a single line in the try block, but consider what happens ifwhen more lines get added in future? In the first example, the close moves away, potentially offscreen, but that doesn’t happen in the second.

Python atomic counter

Do you need a thread-safe atomic counter in Python? Use itertools.count():

>>> from itertools import count
>>> counter = count()
>>> next(counter)
0
>>> next(counter)
1
>>> next(counter)
2

I found this in the decorator package, labelled Atomic get-and-increment provided by the GIL. So simple! So cool!

Python debugger

By the way, if you’ve never used Python’s debugger, it really is as simple as adding a call to the built-in function breakpoint() at the point you want it to stop.

Flask on Elastic Beanstalk

I had a play with Elastic Beanstalk the other day. It’s one of those things people turn their noses up at, but it seems pretty good for prototyping and small things. My biggest issue so far has been that, for Python applications, it expects a WSGI callable called application in the file application.py… but I was using Flask, and every single Flask application ever has a WSGI callable called app in the file app.py. I tried to not care, but it got too much after about an hour so I went and found how to override it:

$ cat .ebextensions/01_wsgi.config
option_settings:
  aws:elasticbeanstalk:container:python:
    WSGIPath: "app:app"

Thank you Nik Tomazic for that! (=⌒‿‿⌒=)

Python hacking

Python‘s had this handy logging module since July 2003. A lot of things use it, so if you’re trying to understand or debug some Python code then a handy snippet to insert somewhere is:

import logging
logging.basicConfig(level=1)

Those two lines cause all loggers to log everything to the console. Check out the logging.basicConfig docs to see what else you could do.