I switched to virtualenvwrapper from plain old virtualenv a while back because I found out that virtualenvwrapper has a postactivate hook I can define. Now when I do

$ workon pitz

to start my work day, I get a customized work environment exactly the way I want it. In addition to activating a virtualenv named pitz, the postactivate script changes me into the top of my pitz checkout directory and then it starts up or reattaches a screen session customized just for pitz.

Setting this up was trivially easy. Here’s what my postactivate script looks like:
$ cat ~/.virtualenvs/pitz/bin/postactivate
cd /home/matt/projects/pitz
screen -S pitz -c ~/.screenrc-pitz -d -R

And here is that .screenrc-pitz file:
$ cat ~/.screenrc-pitz
# Matt's homemade .screenrc.

# Automatically start up a bunch of windows.
screen -t "shell" 0 pitz-shell
screen -t "code" 1
screen -t "tests" 2
chdir docs
screen -t "docs" 3


Everything from the top down to the comment # Automatically start up a bunch of windows. just tweaks how screen works. The last part creates a bunch of named windows. In the first wiindow, it doesn’t start a bash prompt. Instead it starts up the pitz-shell.

One tiny detail that may not be obvious is that if I do $ workon pitz in one terminal, and then maybe I ssh in and rerun $ workon pitz again later, I won’t cause a second screen to start up. Instead, I’ll remote-detach the original session and reconnect to it.

In practice, all my remote servers have highly customized screen sessions already waiting for me. I just need to log in and resume them, and I have all the interesting log files, database connections, and monitoring scripts already up. For example, I know my apache log is open in a pager in window 5. Window 2 has a psql session running. Window 1 has a supervisorctl client running.

This setup shaves off precious seconds when I’m trying to figure stuff out during an emergency.

This stuff isn’t rocket science, but I hope it helps somebody out.

nicety (plural niceties)

1. A small detail that is nice or polite to have but isn’t necessary.

Make a comma-separated string

>>> x = ['a', 'b', 'c']
>>> ', '.join(x)
'a, b, c'

Isn’t that prettier than some big loop?

If you want to join a list of elements that aren’t strings, you’ll have to add a step:>>> x = [1, 2, 3]
>>> ', '.join(map(str, x))
'1, 2, 3'

I’m using map to convert every element to a string first.

Choose a random element from a sequence

>>> from random import choice
>>> choice(x) # doctest: +SKIP
1

The doctest: +SKIP text is just there because the element returned is randomly chosen, and so I can’t predict what the value will be.

Use keyword parameters in your larger string templates

Well-named keywords make big string templates MUCH easier to deal with:>>> tmpl = """
... Dear %(display_name)s,
...
... Greetings from %(origin_country)s! I have millions of
... %(currency_type)s in my bank account and I can send it all to you if
... you first deposit %(amount)s %(currency_type)s into my account
... first.
...
... Sincerely,
... %(alias)s
... %(origin_country)s
... """


Seeing the key names makes it much easier to remember what gets interpolated. Also, it makes it easier to use the same values more than once.

Usually I pass in a dictionary to the template, but sometimes I do this instead:>>> display_name = 'Matt'
>>> origin_country = 'Nigeria',
>>> currency_type = 'US dollars'
>>> amount = '250.00'
>>> s = tmpl % locals()

I have a command line script that takes too long to run. I profiled the script and the problem is in some fairly complex third-party code. So instead of fixing that stuff, I’m going to distract users with a pretty ascii spinner.

Run this script to see the spinner in action (you’ll have to kill the process afterward):

$ cat spin_forever.py
# vim: set expandtab ts=4 sw=4 filetype=python:

while "forever":
draw_ascii_spinner()


Next I made a context manager (this is the name of the things you use with the new python “with” keyword) that uses the subprocess module to fork off another process to run the spinner:

$ cat distraction.py
# vim: set expandtab ts=4 sw=4 filetype=python:

@contextlib.contextmanager
def spinning_distraction():
p = subprocess.Popen(['python', 'spin_forever.py'])
yield
p.terminate()

Finally I added my new context manager to the code that takes so long to run:

def main():

if __name__ == '__main__':
main()


If you’re following along at home, you can run python distraction.py and you’ll see the spinner go until the main application finishes sleeping.

The main thing I don’t like is how I’m using an external file named spin_forever.py. I don’t want to worry about the path to where spin_forever.py lives. I would prefer to use some function defined in a module that I can import.

The vast majority of pythoners use None, but I prefer descriptive strings for default values, like this:
from datetime import datetime, timedelta

return initial_time.date() + timedelta(days=1)

Pylint doesn’t like it though:

$ pylint -e f.py
No config file found, using default configuration
************* Module f
E: 10:midnight_next_day: Instance of 'str' has no 'date' member (but some types could not be inferred)

I changed from using a string as the default to None, and then pylint
didn’t mind:

$ cat f.py
from datetime import datetime
def midnight_next_day(initial_time=None):

$ pylint -e f.py
No config file found, using default configuration


Checking where a variable is None is probably much faster than doing string comparison, but I find the string more informative.

Looking for comments. I already posted this to comp.lang.python here and got some good feedback.