Question

Couldn’t find an answer to this solution, so once I figured it out I thought I’d re-post my solution…

I was looking for a way of taking a user input (sys.argv[1]) and use this value to perform a lookup in a CSV file for another value in column x (e.g. 5). This would be part of a larger script and I would use the looked-up value as a test.

My example csv would be:

col0,col1,col2,col3,col4
a,foo,bar,blah,1
b,foo,bar,blah,2
c,foo,bar,blah,3
d,foo,bar,blah,4
e,foo,bar,blah,5
f,foo,bar,blah,6
g,foo,bar,blah,7
h,foo,bar,blah,8
i,foo,bar,blah,9
j,foo,bar,blah,10
k,foo,bar,blah,11
l,foo,bar,blah,12
m,foo,bar,blah,13
n,foo,bar,blah,14
o,foo,bar,blah,15
p,foo,bar,blah,16
q,foo,bar,blah,17
r,foo,bar,blah,18
s,foo,bar,blah,19
t,foo,bar,blah,20
u,foo,bar,blah,21
v,foo,bar,blah,22
w,foo,bar,blah,23
x,foo,bar,blah,24
y,foo,bar,blah,25
z,foo,bar,blah,26

Answer 1

I had a quick look at your Javascript question, and if you’re only ever mapping char->position in alphabet, what about the following?

def char_to_pos(char):
    from string import ascii_lowercase
    try:
        return ascii_lowercase.index(char) + 1
    except ValueError as e:
        pass # no match - do what's sensible here

And if you wanted to pre-generate a lookup table, then something like:

from string import ascii_lowercase
from itertools import count

lookup = dict(zip(ascii_lowercase, count(1)))
# or depending on taste
lookup = {letter: idx for idx, letter in enumerate(ascii_lowercase, start=1)}

Otherwise, since CSV files are generally relativey small you could just load the entire file to RAM to avoid repeated sequential lookups later (as long as the CSVs not so large it’ll put your machine in a coma)

with open('test1.csv') as fin:
    csvin = csv.reader(fin)
    lookup = {row[0]: row for row in csvin}

to_find = 'x'
try:
    print '{} = {[4]}'.format(to_find, lookup[to_find])
except (KeyError, IndexError) as e:
    pass # KeyError = no lookup match, IndexError is that CSV file didn't have 5th column...