Iterate over a string 2 (or n) characters at a time in Python

Question

Earlier today I needed to iterate over a string 2 characters at a time for parsing a string formatted like \"+c-R+D-E\" (there are a few extra letters).

Answer 1

Consider pip installing more_itertools, which already ships with a chunked implementation along with other helpful tools:

import more_itertools 

for op, code in more_itertools.chunked(s, 2):
    print(op, code)

Output:

+ c
- R
+ D
- e

Answer 2

I ran into a similar problem. Ended doing something like this:

ops = iter("+c-R+D-e")
for op in ops
    code = ops.next()

    print op, code

I felt it was the most readable.

Answer 3

Maybe not the most efficient, but if you like regexes...

import re
s = "+c-R+D-e"
for op, code in re.findall('(.)(.)', s):
    print op, code

Answer 4

Triptych inspired this more general solution:

def slicen(s, n, truncate=False):
    assert n > 0
    while len(s) >= n:
        yield s[:n]
        s = s[n:]
    if len(s) and not truncate:
        yield s

for op, code in slicen("+c-R+D-e", 2):
    print op,code

Answer 5

The other answers work well for n = 2, but for the general case you could try this:

def slicen(s, n, truncate=False):
    nslices = len(s) / n
    if not truncate and (len(s) % n):
        nslices += 1
    return (s[i*n:n*(i+1)] for i in range(nslices))

>>> s = '+c-R+D-e'
>>> for op, code in slicen(s, 2):
...     print op, code
... 
+ c
- R
+ D
- e

>>> for a, b, c in slicen(s, 3):
...     print a, b, c
... 
+ c -
R + D
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ValueError: need more than 2 values to unpack

>>> for a, b, c in slicen(s,3,True):
...     print a, b, c
... 
+ c -
R + D

Answer 6

Dunno about cleaner, but there's another alternative:

for (op, code) in zip(s[0::2], s[1::2]):
    print op, code

A no-copy version:

from itertools import izip, islice
for (op, code) in izip(islice(s, 0, None, 2), islice(s, 1, None, 2)):
    print op, code