Best way to replace multiple characters in a string

Question

I need to replace some characters as follows   amp      amp                I coded as follows  but I guess there should be some better way  Any hints   strs   strs replace   amp       amp    strs   strs replace

User · Answer

Late to the party  but I lost a lot of time with this issue until I found my answer   Short and sweet  translate is superior to replace  If you re more interested in funcionality over time optimization  do not use replace    Also use translate if you don t know if the set of characters to be replaced overlaps the set of characters used to replace    Case in point   Using replace you would naively expect the snippet  1234  replace  1    2   replace  2    3   replace  3    4   to return  2344   but it will return in fact  4444     Translation seems to perform what OP originally desired

User · Answer

Are you always going to prepend a backslash  If so  try  import re rx   re compile     amp                             fill in the characters here  strs   rx sub        1   strs    It may not be the most efficient method but I think it is the easiest

User · Answer

Simply chain the replace functions like this  strs    abc amp def ghi  print strs replace   amp       amp    replace              abc  amp def  ghi   If the replacements are going to be more in number  you can do this in this generic way  strs  replacements    abc amp def ghi      amp       amp               print    join  replacements get c  c  for c in strs     abc  amp def  ghi

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gt  gt  gt  string  abc amp def ghi   gt  gt  gt  for ch in    amp               if ch in string           string string replace ch      ch       gt  gt  gt  print string abc  amp def  ghi

User · Answer

Using reduce which is available in python2 7 and python3   you can easily replace mutiple substrings in a clean and pythonic way     Lets define a helper method to make it easy to use def replacer text  replacements       return reduce          lambda text  ptuple  text replace ptuple 0   ptuple 1             replacements  text        if   name         main         uncleaned str    abc amp def ghi      cleaned str   replacer uncleaned str      amp      amp                     print cleaned str     abc  amp def  ghi    In python2 7 you don t have to import reduce but in python3   you have to import it from the functools module

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gt  gt  gt  a     amp     gt  gt  gt  print a replace   amp    r   amp      amp    gt  gt  gt  print a replace      r       amp     gt  gt  gt     You want to use a  raw  string  denoted by the  r  prefixing the replacement string   since raw strings to not treat the backslash specially

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Here is a python3 method using str translate and str maketrans   s    abc amp def ghi  print s translate str maketrans    amp       amp                    The printed string is abc  amp def  ghi

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advanced way using regex import re text    quot hello  world  quot  replaces     quot hello quot    quot hi quot    quot world quot   quot  2020 quot    quot   quot   quot   quot   regex   re sub  quot   quot  join replaces keys     lambda match  replaces match string match start   match end      text  print regex

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For Python 3 8 and above  one can use assignment expressions  text    text replace s  f quot    i  quot   for s in  quot  amp   quot  if s in text   Although  I am quite unsure if this would be considered  quot appropriate use quot  of assignment expressions as described in PEP 572  but looks clean and reads quite well  to my eyes   This would be  quot appropriate quot  if you wanted all intermediate strings as well  For example   removing all lowercase vowels   text    quot Lorem ipsum dolor sit amet quot  intermediates    text    text replace i   quot  quot   for i in  quot aeiou quot  if i in text     Lorem ipsum dolor sit met     Lorm ipsum dolor sit mt     Lorm psum dolor st mt     Lrm psum dlr st mt     Lrm psm dlr st mt    On the plus side  it does seem  unexpectedly   faster than some of the faster methods in the accepted answer  and seems to perform nicely with both increasing strings length and an increasing number of substitutions   The code for the above comparison is below  I am using random strings to make my life a bit simpler  and the characters to replace are chosen randomly from the string itself   Note  I am using ipython s  timeit magic here  so run this in ipython jupyter   import random  string  def make txt length        quot makes a random string of a given length quot      return  quot  quot  join random choices string printable  k length    def get substring s  num        quot gets a substring quot      return  quot  quot  join random choices s  k num    def a text  replace     one of the better performing approaches from the accepted answer     for i in replace          if i in text               text   text replace i   quot  quot    def b text  replace            text    text replace i   quot  quot   for i in replace if i in text     def compare strlen  replace length        quot use ipython   jupyter for the  timeit functionality quot       times a  times b               for i in range  strlen           el   make txt i          et   get substring el  replace length           res a    timeit -n 1000 -o a el  et    ipython magic          el   make txt i          et   get substring el  replace length                   res b    timeit -n 1000 -o b el  et    ipython magic          times a append res a average   1e6          times b append res b average   1e6               return times a  times b   ----run t2   compare  2 2  1000  50   2  t10   compare  2 10  1000  50   10

User · Answer

How about this   def replace all dict  str       for key in dict          str   str replace key  dict key       return str   then  print replace all    amp      amp                 amp        output    amp      similar to answer

User · Answer

Maybe a simple loop for chars to replace   a     amp     to replace      amp          for char in to replace      a   a replace char       char   print a    gt  gt  gt    amp

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This will help someone looking for a simple solution  def replacemany our str  to be replaced tuple  replace with str       for nextchar in to be replaced          our str   our str replace nextchar  replace with      return our str  os    the rain in spain falls mainly on the plain ttttttttt sssssssssss nnnnnnnnnn  tbr     a   t   s   n   rw       print replacemany os tbr rw    Output   he ri i pi fll mily o he pli

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FYI  this is of little or no use to the OP but it may be of use to other readers  please do not downvote  I m aware of this    As a somewhat ridiculous but interesting exercise  wanted to see if I could use python functional programming to replace multiple chars  I m pretty sure this does NOT beat just calling replace   twice  And if performance was an issue  you could easily beat this in rust  C  julia  perl  java  javascript and maybe even awk  It uses an external  helpers  package called pytoolz  accelerated via cython  cytoolz  it s a pypi package    from cytoolz functoolz import compose from cytoolz itertoolz import chain sliding window from itertools import starmap imap ifilter from operator import itemgetter contains text   amp hello hi amp yo amp   char index iter compose partial imap  itemgetter 0    partial ifilter  compose partial contains     amp     itemgetter 1     enumerate  print      join imap text   getitem    starmap slice  sliding window 2  chain  0    char index iter text    len text           I m not even going to explain this because no one would bother using this to accomplish multiple replace  Nevertheless  I felt somewhat accomplished in doing this and thought it might inspire other readers or win a code obfuscation contest

User · Answer

You may consider writing a generic escape function   def mk esc esc chars       return lambda s     join         c if c in esc chars else c for c in s     gt  gt  gt  esc   mk esc   amp      gt  gt  gt  print esc  Learn  amp  be  1   Learn   amp  be   1   This way you can make your function configurable with a list of character that should be escaped

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Replacing two characters  I timed all the methods in the current answers along with one extra   With an input string of abc amp def ghi and replacing  amp  -    amp  and   -      the fastest way was to chain together the replacements like this  text replace   amp       amp    replace              Timings for each function    a  1000000 loops  best of 3  1 47   s per loop b  1000000 loops  best of 3  1 51   s per loop c  100000 loops  best of 3  12 3   s per loop d  100000 loops  best of 3  12   s per loop e  100000 loops  best of 3  3 27   s per loop f  1000000 loops  best of 3  0 817   s per loop g  100000 loops  best of 3  3 64   s per loop h  1000000 loops  best of 3  0 927   s per loop i  1000000 loops  best of 3  0 814   s per loop   Here are the functions   def a text       chars     amp        for c in chars          text   text replace c         c    def b text       for ch in    amp                 if ch in text              text   text replace ch      ch    import re def c text       rx   re compile     amp           text   rx sub r    1   text    RX   re compile     amp       def d text       text   RX sub r    1   text    def mk esc esc chars       return lambda s     join         c if c in esc chars else c for c in s   esc   mk esc   amp     def e text       esc text    def f text       text   text replace   amp       amp    replace              def g text       replacements      amp       amp                   text      join  replacements get c  c  for c in text     def h text       text   text replace   amp    r   amp        text   text replace      r        def i text       text   text replace   amp    r   amp    replace      r        Timed like this   python -mtimeit -s import time functions   time functions a  abc amp def ghi    python -mtimeit -s import time functions   time functions b  abc amp def ghi    python -mtimeit -s import time functions   time functions c  abc amp def ghi    python -mtimeit -s import time functions   time functions d  abc amp def ghi    python -mtimeit -s import time functions   time functions e  abc amp def ghi    python -mtimeit -s import time functions   time functions f  abc amp def ghi    python -mtimeit -s import time functions   time functions g  abc amp def ghi    python -mtimeit -s import time functions   time functions h  abc amp def ghi    python -mtimeit -s import time functions   time functions i  abc amp def ghi        Replacing 17 characters  Here s similar code to do the same but with more characters to escape           -       def a text       chars                gt   -         for c in chars          text   text replace c         c    def b text       for ch in                                             gt            -                             if ch in text              text   text replace ch      ch    import re def c text       rx   re compile     amp           text   rx sub r    1   text    RX   re compile                gt   -        def d text       text   RX sub r    1   text    def mk esc esc chars       return lambda s     join         c if c in esc chars else c for c in s   esc   mk esc              gt   -      def e text       esc text    def f text       text   text replace               replace            replace            replace            replace            replace            replace            replace            replace            replace            replace   gt       gt    replace            replace            replace  -     -   replace            replace            replace              def g text       replacements                                                                                                                                                                                                                gt       gt                                                   -     -                                                                      text      join  replacements get c  c  for c in text     def h text       text   text replace       r          text   text replace      r          text   text replace      r          text   text replace      r          text   text replace      r          text   text replace      r          text   text replace      r          text   text replace      r          text   text replace      r          text   text replace      r          text   text replace   gt    r   gt        text   text replace      r          text   text replace      r          text   text replace  -   r  -       text   text replace      r          text   text replace      r          text   text replace      r        def i text       text   text replace       r      replace      r      replace      r      replace      r      replace      r      replace      r      replace      r      replace      r      replace      r      replace      r      replace   gt    r   gt    replace      r      replace      r      replace  -   r  -   replace      r      replace      r      replace      r        Here s the results for the same input string abc amp def ghi    a  100000 loops  best of 3  6 72   s per loop b  100000 loops  best of 3  2 64   s per loop c  100000 loops  best of 3  11 9   s per loop d  100000 loops  best of 3  4 92   s per loop e  100000 loops  best of 3  2 96   s per loop f  100000 loops  best of 3  4 29   s per loop g  100000 loops  best of 3  4 68   s per loop h  100000 loops  best of 3  4 73   s per loop i  100000 loops  best of 3  4 24   s per loop   And with a longer input string      Something  and  another  thing in a longer sentence with  more  things to replace      a  100000 loops  best of 3  7 59   s per loop b  100000 loops  best of 3  6 54   s per loop c  100000 loops  best of 3  16 9   s per loop d  100000 loops  best of 3  7 29   s per loop e  100000 loops  best of 3  12 2   s per loop f  100000 loops  best of 3  5 38   s per loop g  10000 loops  best of 3  21 7   s per loop h  100000 loops  best of 3  5 7   s per loop i  100000 loops  best of 3  5 13   s per loop   Adding a couple of variants   def ab text       for ch in                                             gt            -                             text   text replace ch      ch    def ba text       chars                gt   -         for c in chars          if c in text              text   text replace c         c    With the shorter input    ab  100000 loops  best of 3  7 05   s per loop ba  100000 loops  best of 3  2 4   s per loop   With the longer input    ab  100000 loops  best of 3  7 71   s per loop ba  100000 loops  best of 3  6 08   s per loop   So I m going to use ba for readability and speed   Addendum  Prompted by haccks in the comments  one difference between ab and ba is the if c in text  check  Let s test them against two more variants   def ab with check text       for ch in                                             gt            -                             if ch in text              text   text replace ch      ch   def ba without check text       chars                gt   -         for c in chars          text   text replace c         c    Times in   s per loop on Python 2 7 14 and 3 6 3  and on a different machine from the earlier set  so cannot be compared directly    -------------------------------------------------------------     Py  input      ab     ab with check     ba     ba without check      ------------ ------ --------------- ------ ------------------      Py2  short    8 81       4 22          3 45       8 01                Py3  short    5 54       1 34          1 46       5 34              ------------ ------ --------------- ------ ------------------      Py2  long     9 3        7 15          6 85       8 55                Py3  long     7 43       4 38          4 41       7 02              -------------------------------------------------------------    We can conclude that    Those with the check are up to 4x faster than those without the check ab with check is slightly in the lead on Python 3  but ba  with check  has a greater lead on Python 2 However  the biggest lesson here is Python 3 is up to 3x faster than Python 2  There s not a huge difference between the slowest on Python 3 and fastest on Python 2

[python] Best way to replace multiple characters in a string?

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