Monday, July 25, 2011

py for-loop - string,file,dict,args,dir ...

Following the Unix philosophy, Python's for-in loop is a simple idea (iterator) pushed to the max. It supports
- iterating chars in a ..... string
- iterating lines in a .... file
- iterating integers in a range() or xrange()
- iterating .... KEYs in a dict -- values requires explicit look-up
- iterating .... key/value pairs in a dict
- iterating sys.argv on command line
- list, tuple
- retrieving pairs from list-of-pairs -- idiom ## this example also illustrates defaultdict(list)

>>> s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]
>>> d = defaultdict(list)
>>> for k, v in s:
...     d[k].append(v)
...
>>> d.items()
[('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])]
 

(read https://docs.python.org/2/library/collections.html#collections.defaultdict about  the "list" arg)

More generally, any class supporting iteration can use for-loop. Here's an example illustrating some of them.

import re, sys
from os import walk
printed={}
for (path, dirs, files) in walk("c:\\") :
 for filename in files :
  if not re.search("\.py$",filename) : continue
  if not printed.has_key(path):
      print " path = " + path
      printed[path] = True

  for line in open (path+'\\'+filename) :
      if re.search('^\s*class\s', line) : print filename + ':\t' + line,

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