Based on "Split a string by spaces in Python", which uses shlex.split to split a string with quotes smartly, I would be interested in hearing about other common tasks solved by non-obvious standard library functions.
If this turns into Module of The Week, that's fine too.
I found struct.unpack to be a godsend for unpacking binary data formats after I learned of it!
I've found sched module to be helpful in cron-like activities. It simplifies things a lot. Unfortunately I found it too late.
Oft overlooked modules, uses and tricks:
collections.defaultdict(): for when you want missing keys in a dict to have a default value.
functools.wraps(): for writing decorators that play nicely with introspection.
posixpath: the os.path module for POSIX systems. You can use it for manipulating POSIX paths (including URI elements) even on Windows and other non-POSIX systems.
ntpath: the os.path module for Windows; usable for manipulation of Windows paths on non-Windows systems.
(also: macpath, for MacOS 9 and earlier, os2emxpath for OS/2 EMX, but I'm not sure if anyone still cares.)
pprint: more structured printing of the repr() of containers makes debugging much easier.
imp: all the tools you need to write your own plugin system or make Python import modules from arbitrary archives.
rlcompleter: getting tab-completion in the normal interactive interpreter. Just do "import readline, rlcompleter; readline.parse_and_bind('tab: complete')"
the PYTHONSTARTUP environment variable: can be set to the path to a file that will be executed (in the main namespace) when entering the interactive interpreter; useful for putting things in like the rlcompleter recipe above.
getpass is useful for determining the login name of the current user.
grp allows you to lookup Unix group IDs by name, and vice versa.
dircache might be useful in situations where you're repeatedly polling the contents of a directory.
glob can find filenames matching wildcards like a Unix shell does.
shutil is useful when you need to copy, delete or rename a file.
csv can simplify parsing of delimited text files.
optparse provides a reliable way to parse command line options.
bz2 comes in handy when you need to manipulate a bzip2-compressed file.
urlparse will save you the hassle of breaking up a URL into component parts.
Most of the other examples are merely overlooked, not unexpected uses for module.
fnmatch, like shlex, can be applied in unexpected ways. fnmatch is a kind of poor-person's RE, and can be used for more than matching files, it can compare strings with the simplified wild-card patterns.
I use itertools (especially cycle, repeat, chain) to make python behave more like R and in other functional / vector applications. Often this lets me avoid the overhead and complication of Numpy.
# in R, shorter iterables are automatically cycled
# and all functions "apply" in a "map"-like way over lists
> 0:10 + 0:2
[1] 0 2 4 3 5 7 6 8 10 9 11
Python #Normal python In [1]: range(10) + range(3) Out[1]: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 1, 2]
## this code is terrible, but it demos the idea.
from itertools import cycle
def addR(L1,L2):
n = max( len(L1), len(L2))
out = [None,]*n
gen1,gen2 = cycle(L1), cycle(L2)
ii = 0
while ii < n:
out[ii] = gen1.next() + gen2.next()
ii += 1
return out
In [21]: addR(range(10), range(3))
Out[21]: [0, 2, 4, 3, 5, 7, 6, 8, 10, 9]
I was quite surprised to learn that you could use the bisect module to do a very fast binary search in a sequence. It's documentation doesn't say anything about it:
This module provides support for maintaining a list in sorted order without having to sort the list after each insertion.
The usage is very simple:
>>> import bisect
>>> lst = [4, 7, 10, 23, 25, 100, 103, 201, 333]
>>> bisect.bisect_left(lst, 23)
3
You have to remember though, that it's quicker to linearly look for something in a list goes item by item, than sorting the list and then doing a binary search on it. The first option is O(n), the second is O(nlogn).
One function I've come to appreciate is string.translate. Its very fast at what it does, and useful anywhere you want to alter or remove characters in a string. I've just used it in a seemingly inapplicable problem and found it beat all the other solutions handily.
The downside is that its API is a bit clunky, but this is improving in Py2.6 / Py3.0.
complex numbers. (The complexobject.c defines a class, so technically it's not a module). Great for 2d coordinates, with easy translation/rotations etc
eg.
TURN_LEFT_90= 1j
TURN_RIGHT_90= -1j
coord= 5+4j # x=5 y=4
print coord*TURN_LEFT_90