Pythonic infinite for loop?

Fri Apr 15 05:32:31 EDT 2011

Chris Angelico wrote:
> On Fri, Apr 15, 2011 at 6:25 PM, Peter Otten <__peter__ at web.de> wrote:
>> The initial data structure seems less than ideal. You might be able to
>> replace it with a dictionary like
>>>> {"Keyword": [value_for_keyword_1, value_for_keyword_2, ...]}
>>>> if you try hard enough.
>> The initial data structure comes from a CSV file, and is not under my
> control.
>> ChrisA

Here's some code that might give you an idea. You can ignore the chunk 
before 'import csv'; it is there to make the demo self-contained.
from contextlib import contextmanager
@contextmanager
def open(filename):
 assert filename == "example.csv"
 from StringIO import StringIO
 yield StringIO("""\
beta3,alpha1,alpha2,beta1,beta2
b31,a11,a21,b11,b21
b32,a12,a22,b12,b22
b33,a13,a23,b13,b23
b34,a14,a24,b14,b24
""")
import csv
import re
def parse_name(s):
 name, index = re.match(r"(.+?)(\d+)$", s).groups()
 return name, int(index)-1
with open("example.csv") as instream:
 rows = csv.reader(instream)
 header = next(rows)
 dct = {}
 appends = []
 for h in header:
 name, index = parse_name(h)
 outer = dct.setdefault(name, {})
 inner = outer.setdefault(index, [])
 appends.append(inner.append)
 for row in rows:
 for value, append in zip(row, appends):
 append(value)
print dct