Fastest way to insert these dashes in python string?

You are better off using string formatting than string concatenation

c['date'] = '{}-{}-{}'.format(c['date'][0:4], c['date'][4:6], c['date'][6:])

String concatenation is generally slower because as you said above strings are immutable.

s = '20110104'


def option_1():
    return '-'.join([s[:4], s[4:6], s[6:]])

def option_1a():
    return '-'.join((s[:4], s[4:6], s[6:]))

def option_2():
    return '{}-{}-{}'.format(s[:4], s[4:6], s[6:])

def option_3():
    return '%s-%s-%s' % (s[:4], s[4:6], s[6:])

def option_original():
    return s[:4] + "-" + s[4:6] + "-" + s[6:]

Running %timeit on each yields these results

• option_1: 35.9 ns per loop
• option_1a: 35.8 ns per loop
• option_2: 36 ns per loop
• option_3: 35.8 ns per loop
• option_original: 36 ns per loop

So... pick the most readable because the performance improvements are marginal

You could use .join() to clean it up a little bit:

d = c['date']
'-'.join([d[:4], d[4:6], d[6:]])

Dates are first class objects in Python, with a rich interface for manipulating them. The library is datetime.

> import datetime
> datetime.datetime.strptime('20110503','%Y%m%d').date().isoformat()
'2011-05-03'

Don't reinvent the wheel!