Question

Say I have a string that looks like this:

str = "The &yquick &cbrown &bfox &Yjumps over the &ulazy dog"

You’ll notice a lot of locations in the string where there is an ampersand, followed by a character (such as “&y” and “&c”). I need to replace these characters with an appropriate value that I have in a dictionary, like so:

dict = {"&y":"\033[0;30m",
        "&c":"\033[0;31m",
        "&b":"\033[0;32m",
        "&Y":"\033[0;33m",
        "&u":"\033[0;34m"}

What is the fastest way to do this? I could manually find all the ampersands, then loop through the dictionary to change them, but that seems slow. Doing a bunch of regex replaces seems slow as well (I will have a dictionary of about 30-40 pairs in my actual code).

Any suggestions are appreciated, thanks.

Edit:

As has been pointed out in comments throught this question, my dictionary is defined before runtime, and will never change during the course of the applications life cycle. It is a list of ANSI escape sequences, and will have about 40 items in it. My average string length to compare against will be about 500 characters, but there will be ones that are up to 5000 characters (although, these will be rare). I am also using Python 2.6 currently.

Edit #2
I accepted Tor Valamos answer as the correct one, as it not only gave a valid solution (although it wasn’t the best solution), but took all others into account and did a tremendous amount of work to compare all of them. That answer is one of the best, most helpful answers I have ever come across on StackOverflow. Kudos to you.

Answer 1

mydict = {"&y":"\033[0;30m",
          "&c":"\033[0;31m",
          "&b":"\033[0;32m",
          "&Y":"\033[0;33m",
          "&u":"\033[0;34m"}
mystr = "The &yquick &cbrown &bfox &Yjumps over the &ulazy dog"

for k, v in mydict.iteritems():
    mystr = mystr.replace(k, v)

print mystr
The ←[0;30mquick ←[0;31mbrown ←[0;32mfox ←[0;33mjumps over the ←[0;34mlazy dog

I took the liberty of comparing a few solutions:

mydict = dict([('&' + chr(i), str(i)) for i in list(range(65, 91)) + list(range(97, 123))])

# random inserts between keys
from random import randint
rawstr = ''.join(mydict.keys())
mystr = ''
for i in range(0, len(rawstr), 2):
    mystr += chr(randint(65,91)) * randint(0,20) # insert between 0 and 20 chars

from time import time

# How many times to run each solution
rep = 10000

print 'Running %d times with string length %d and ' \
      'random inserts of lengths 0-20' % (rep, len(mystr))

# My solution
t = time()
for x in range(rep):
    for k, v in mydict.items():
        mystr.replace(k, v)
    #print(mystr)
print '%-30s' % 'Tor fixed & variable dict', time()-t

from re import sub, compile, escape

# Peter Hansen
t = time()
for x in range(rep):
    sub(r'(&[a-zA-Z])', r'%(\1)s', mystr) % mydict
print '%-30s' % 'Peter fixed & variable dict', time()-t

# Claudiu
def multiple_replace(dict, text): 
    # Create a regular expression  from the dictionary keys
    regex = compile("(%s)" % "|".join(map(escape, dict.keys())))

    # For each match, look-up corresponding value in dictionary
    return regex.sub(lambda mo: dict[mo.string[mo.start():mo.end()]], text)

t = time()
for x in range(rep):
    multiple_replace(mydict, mystr)
print '%-30s' % 'Claudio variable dict', time()-t

# Claudiu - Precompiled
regex = compile("(%s)" % "|".join(map(escape, mydict.keys())))

t = time()
for x in range(rep):
    regex.sub(lambda mo: mydict[mo.string[mo.start():mo.end()]], mystr)
print '%-30s' % 'Claudio fixed dict', time()-t

# Andrew Y - variable dict
def mysubst(somestr, somedict):
  subs = somestr.split("&")
  return subs[0] + "".join(map(lambda arg: somedict["&" + arg[0:1]] + arg[1:], subs[1:]))

t = time()
for x in range(rep):
    mysubst(mystr, mydict)
print '%-30s' % 'Andrew Y variable dict', time()-t

# Andrew Y - fixed
def repl(s):
  return mydict["&"+s[0:1]] + s[1:]

t = time()
for x in range(rep):
    subs = mystr.split("&")
    res = subs[0] + "".join(map(repl, subs[1:]))
print '%-30s' % 'Andrew Y fixed dict', time()-t

Results in Python 2.6

Running 10000 times with string length 490 and random inserts of lengths 0-20
Tor fixed & variable dict      1.04699993134
Peter fixed & variable dict    0.218999862671
Claudio variable dict          2.48400020599
Claudio fixed dict             0.0940001010895
Andrew Y variable dict         0.0309998989105
Andrew Y fixed dict            0.0310001373291

Both claudiu’s and andrew’s solutions kept going into 0, so I had to increase it to 10 000 runs.

I ran it in Python 3 (because of unicode) with replacements of chars from 39 to 1024 (38 is ampersand, so I didn’t wanna include it). String length up to 10.000 including about 980 replacements with variable random inserts of length 0-20. The unicode values from 39 to 1024 causes characters of both 1 and 2 bytes length, which could affect some solutions.

mydict = dict([('&' + chr(i), str(i)) for i in range(39,1024)])

# random inserts between keys
from random import randint
rawstr = ''.join(mydict.keys())
mystr = ''
for i in range(0, len(rawstr), 2):
    mystr += chr(randint(65,91)) * randint(0,20) # insert between 0 and 20 chars

from time import time

# How many times to run each solution
rep = 10000

print('Running %d times with string length %d and ' \
      'random inserts of lengths 0-20' % (rep, len(mystr)))

# Tor Valamo - too long
#t = time()
#for x in range(rep):
#    for k, v in mydict.items():
#        mystr.replace(k, v)
#print('%-30s' % 'Tor fixed & variable dict', time()-t)

from re import sub, compile, escape

# Peter Hansen
t = time()
for x in range(rep):
    sub(r'(&[a-zA-Z])', r'%(\1)s', mystr) % mydict
print('%-30s' % 'Peter fixed & variable dict', time()-t)

# Peter 2
def dictsub(m):
    return mydict[m.group()]

t = time()
for x in range(rep):
    sub(r'(&[a-zA-Z])', dictsub, mystr)
print('%-30s' % 'Peter fixed dict', time()-t)

# Claudiu - too long
#def multiple_replace(dict, text): 
#    # Create a regular expression  from the dictionary keys
#    regex = compile("(%s)" % "|".join(map(escape, dict.keys())))
#
#    # For each match, look-up corresponding value in dictionary
#    return regex.sub(lambda mo: dict[mo.string[mo.start():mo.end()]], text)
#
#t = time()
#for x in range(rep):
#    multiple_replace(mydict, mystr)
#print('%-30s' % 'Claudio variable dict', time()-t)

# Claudiu - Precompiled
regex = compile("(%s)" % "|".join(map(escape, mydict.keys())))

t = time()
for x in range(rep):
    regex.sub(lambda mo: mydict[mo.string[mo.start():mo.end()]], mystr)
print('%-30s' % 'Claudio fixed dict', time()-t)

# Separate setup for Andrew and gnibbler optimized dict
mydict = dict((k[1], v) for k, v in mydict.items())

# Andrew Y - variable dict
def mysubst(somestr, somedict):
  subs = somestr.split("&")
  return subs[0] + "".join(map(lambda arg: somedict[arg[0:1]] + arg[1:], subs[1:]))

def mysubst2(somestr, somedict):
  subs = somestr.split("&")
  return subs[0].join(map(lambda arg: somedict[arg[0:1]] + arg[1:], subs[1:]))

t = time()
for x in range(rep):
    mysubst(mystr, mydict)
print('%-30s' % 'Andrew Y variable dict', time()-t)
t = time()
for x in range(rep):
    mysubst2(mystr, mydict)
print('%-30s' % 'Andrew Y variable dict 2', time()-t)

# Andrew Y - fixed
def repl(s):
  return mydict[s[0:1]] + s[1:]

t = time()
for x in range(rep):
    subs = mystr.split("&")
    res = subs[0] + "".join(map(repl, subs[1:]))
print('%-30s' % 'Andrew Y fixed dict', time()-t)

# gnibbler
t = time()
for x in range(rep):
    myparts = mystr.split("&")
    myparts[1:]=[mydict[x[0]]+x[1:] for x in myparts[1:]]
    "".join(myparts)
print('%-30s' % 'gnibbler fixed & variable dict', time()-t)

Results:

Running 10000 times with string length 9491 and random inserts of lengths 0-20
Tor fixed & variable dict      0.0 # disqualified 329 secs
Peter fixed & variable dict    2.07799983025
Peter fixed dict               1.53100013733 
Claudio variable dict          0.0 # disqualified, 37 secs
Claudio fixed dict             1.5
Andrew Y variable dict         0.578000068665
Andrew Y variable dict 2       0.56299996376
Andrew Y fixed dict            0.56200003624
gnibbler fixed & variable dict 0.530999898911

(** Note that gnibbler’s code uses a different dict, where keys don’t have the ‘&’ included. Andrew’s code also uses this alternate dict, but it didn’t make much of a difference, maybe just 0.01x speedup.)