Short (and useful) python snippets [closed]

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Closed 7 years ago.

In spirit of the existing "what's your most useful C/C++ snippet" - thread:

Do you guys have short, monofunctional Python snippets that you use (often) and would like to share with the StackOverlow Community? Please keep the entries small (under 25 lines maybe?) and give only one example per post.

I'll start of with a short snippet i use from time to time to count sloc (source lines of code) in python projects:

# prints recursive count of lines of python source code from current directory
# includes an ignore_list. also prints total sloc

import os
cur_path = os.getcwd()
ignore_set = set(["__init__.py", "count_sourcelines.py"])

loclist = []

for pydir, _, pyfiles in os.walk(cur_path):
    for pyfile in pyfiles:
        if pyfile.endswith(".py") and pyfile not in ignore_set:
            totalpath = os.path.join(pydir, pyfile)
            loclist.append( ( len(open(totalpath, "r").read().splitlines()),
                               totalpath.split(cur_path)[1]) )

for linenumbercount, filename in loclist: 
    print "%05d lines in %s" % (linenumbercount, filename)

print "\nTotal: %s lines (%s)" %(sum([x[0] for x in loclist]), cur_path)

Answer 1

I actually just created this, but I think it's going to be a very useful debugging tool.

def dirValues(instance, all=False):
    retVal = {}
    for prop in dir(instance):
        if not all and prop[1] == "_":
            continue
        retVal[prop] = getattr(instance, prop)
    return retVal

I usually use dir() in a pdb context, but I think this will be much more useful:

(pdb) from pprint import pprint as pp
(pdb) from myUtils import dirValues
(pdb) pp(dirValues(someInstance))

Answer 2

When debugging, you sometimes want to see a string with a basic editor. For showing a string with notepad:

import os, tempfile, subprocess

def get_rand_filename(dir_=os.getcwd()):
    "Function returns a non-existent random filename."
    return tempfile.mkstemp('.tmp', '', dir_)[1]

def open_with_notepad(s):
    "Function gets a string and shows it on notepad"
    with open(get_rand_filename(), 'w') as f:
        f.write(s)
        subprocess.Popen(['notepad', f.name])

Answer 3

The only 'trick' I know that really wowed me when I learned it is enumerate. It allows you to have access to the indexes of the elements within a for loop.

>>> l = ['a','b','c','d','e','f']
>>> for (index,value) in enumerate(l):
...     print index, value
... 
0 a
1 b
2 c
3 d
4 e
5 f

Answer 4

For Python 2.4+ or earlier:

for x,y in someIterator:
  listDict.setdefault(x,[]).append(y)

In Python 2.5+ there is alternative using defaultdict.

Answer 5

Huge speedup for nested list and dictionaries with:

deepcopy = lambda x: cPickle.loads(cPickle.dumps(x))

Answer 6

Fire up a simple web server for files in the current directory:

python -m SimpleHTTPServer

Useful for sharing files.