Ignore case in Python strings

Question

What is the easiest way to compare strings in Python  ignoring case   Of course one can do  str1 lower    lt   str2 lower     etc   but this created two additional temporary strings  with the obvious alloc g-c overheads    I guess I m looking for an equivalent to C s stricmp      Some more context requested  so I ll demonstrate with a trivial example    Suppose you want to sort a looong list of strings  You simply do theList sort    This is O n   log n   string comparisons and no memory management  since all strings and list elements are some sort of smart pointers   You are happy   Now  you want to do the same  but ignore the case  let s simplify and say all strings are ascii  so locale issues can be ignored   You can do theList sort key lambda s  s lower     but then you cause two new  allocations per comparison  plus burden the garbage-collector with the duplicated  lowered  strings   Each such memory-management noise is orders-of-magnitude slower than simple string comparison   Now  with an in-place stricmp  -like function  you do  theList sort cmp stricmp  and it is as fast and as memory-friendly as theList sort    You are happy again   The problem is any Python-based case-insensitive comparison involves implicit string  duplications  so I was expecting to find a C-based comparisons  maybe in module string    Could not find anything like that  hence the question here   Hope this clarifies the question

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This is how you d do it with re   import re p   re compile   hello    re I  p match  Hello   p match  hello   p match  HELLO

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I m pretty sure you either have to use  lower   or use a regular expression   I m not aware of a built-in case-insensitive string comparison function

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Are you using this compare in a very-frequently-executed path of a highly-performance-sensitive application   Alternatively  are you running this on strings which are megabytes in size   If not  then you shouldn t worry about the performance and just use the  lower   method   The following code demonstrates that doing a case-insensitive compare by calling  lower   on two strings which are each almost a megabyte in size takes about 0 009 seconds on my 1 8GHz desktop computer   from timeit import Timer  s1    1234567890    100000    a  s2    1234567890    100000    B   code    s1 lower    lt  s2 lower    time   Timer code   from   main   import s1  s2   timeit 1000  print time   1000     0 00920499992371 on my machine   If indeed this is an extremely significant  performance-critical section of code  then I recommend writing a function in C and calling it from your Python code  since that will allow you to do a truly efficient case-insensitive search   Details on writing C extension modules can be found here  https   docs python org extending extending html

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Just use the str   lower   method  unless high-performance is important - in which case write that sorting method as a C extension    How to write a Python Extension  seems like a decent intro    More interestingly  This guide compares using the ctypes library vs writing an external C module  the ctype is quite-substantially slower than the C extension

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There s no built in equivalent to that function you want   You can write your own function that converts to  lower   each character at a time to avoid  duplicating both strings  but I m sure it will very cpu-intensive and extremely inefficient    Unless you are working with extremely long strings  so long that can cause a memory problem if duplicated  then I would keep it simple and use   str1 lower      str2 lower     You ll be ok

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Your question implies that you don t need Unicode  Try the following code snippet  if it works for you  you re done   Python 2 5 2  r252 60911  Aug 22 2008  02 34 17   GCC 4 3 1  on linux2 Type  help    copyright    credits  or  license  for more information   gt  gt  gt  import locale  gt  gt  gt  locale setlocale locale LC COLLATE   en US    en US   gt  gt  gt  sorted  ABCabc   key locale strxfrm    a    A    b    B    c    C    gt  gt  gt  sorted  ABCabc   cmp locale strcoll    a    A    b    B    c    C     Clarification  in case it is not obvious at first sight  locale strcoll seems to be the function you need  avoiding the str lower or locale strxfrm  duplicate  strings

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import re if re match  tEXT    text   re IGNORECASE         is True

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You could subclass str and create your own case-insenstive string class but IMHO that would be extremely unwise and create far more trouble than it s worth

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I can t find any other built-in way of doing case-insensitive comparison  The python cook-book recipe uses lower     However you have to be careful when using lower for comparisons because of the Turkish I problem  Unfortunately Python s handling for Turkish Is is not good  i is converted to I  but I is not converted to i  I is converted to i  but i is not converted to I

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I can t find any other built-in way of doing case-insensitive comparison  The python cook-book recipe uses lower     However you have to be careful when using lower for comparisons because of the Turkish I problem  Unfortunately Python s handling for Turkish Is is not good  i is converted to I  but I is not converted to i  I is converted to i  but i is not converted to I

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Are you using this compare in a very-frequently-executed path of a highly-performance-sensitive application   Alternatively  are you running this on strings which are megabytes in size   If not  then you shouldn t worry about the performance and just use the  lower   method   The following code demonstrates that doing a case-insensitive compare by calling  lower   on two strings which are each almost a megabyte in size takes about 0 009 seconds on my 1 8GHz desktop computer   from timeit import Timer  s1    1234567890    100000    a  s2    1234567890    100000    B   code    s1 lower    lt  s2 lower    time   Timer code   from   main   import s1  s2   timeit 1000  print time   1000     0 00920499992371 on my machine   If indeed this is an extremely significant  performance-critical section of code  then I recommend writing a function in C and calling it from your Python code  since that will allow you to do a truly efficient case-insensitive search   Details on writing C extension modules can be found here  https   docs python org extending extending html

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import re if re match  tEXT    text   re IGNORECASE         is True

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In response to your clarification     You could use ctypes to execute the c function  strcasecmp    Ctypes is included in Python 2 5   It provides the ability to call out to dll and shared libraries such as libc  Here is a quick example  Python on Linux  see link for Win32 help    from ctypes import   libc   CDLL  libc so 6       see link above for Win32 help libc strcasecmp  THIS    this      returns 0 libc strcasecmp  THIS    THAT      returns 8   may also want to reference strcasecmp documentation  Not really sure this is any faster or slower  have not tested   but it s a way to use a C function to do case insensitive string comparisons                    ActiveState Code - Recipe 194371  Case Insensitive Strings is a recipe for creating a case insensitive string class   It might be a bit over kill for something quick  but could provide you with a common way of handling case insensitive strings if you plan on using them often

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You could subclass str and create your own case-insenstive string class but IMHO that would be extremely unwise and create far more trouble than it s worth

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There s no built in equivalent to that function you want   You can write your own function that converts to  lower   each character at a time to avoid  duplicating both strings  but I m sure it will very cpu-intensive and extremely inefficient    Unless you are working with extremely long strings  so long that can cause a memory problem if duplicated  then I would keep it simple and use   str1 lower      str2 lower     You ll be ok

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Just use the str   lower   method  unless high-performance is important - in which case write that sorting method as a C extension    How to write a Python Extension  seems like a decent intro    More interestingly  This guide compares using the ctypes library vs writing an external C module  the ctype is quite-substantially slower than the C extension

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I m pretty sure you either have to use  lower   or use a regular expression   I m not aware of a built-in case-insensitive string comparison function

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You could subclass str and create your own case-insenstive string class but IMHO that would be extremely unwise and create far more trouble than it s worth

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You could translate each string to lowercase once --- lazily only when you need it  or as a prepass to the sort if you know you ll be sorting the entire collection of strings   There are several ways to attach this comparison key to the actual data being sorted  but these techniques should be addressed in a separate issue   Note that this technique can be used not only to handle upper lower case issues  but for other types of sorting such as locale specific sorting  or  Library-style  title sorting that ignores leading articles and otherwise normalizes the data before sorting it

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I m pretty sure you either have to use  lower   or use a regular expression   I m not aware of a built-in case-insensitive string comparison function

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This is how you d do it with re   import re p   re compile   hello    re I  p match  Hello   p match  hello   p match  HELLO

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Here is a benchmark showing that using str lower is faster than the accepted answer s proposed method  libc strcasecmp       usr bin env python2 7 import random import timeit  from ctypes import   libc   CDLL  libc dylib     change to  libc so 6  on linux  with open   usr share dict words    r   as wordlist      words   wordlist read   splitlines   random shuffle words  print   i words in list    len words   setup    from   main   import words  libc  gc enable    stmts           simple sort    sorted words           sort with key str lower    sorted words  key str lower           sort with cmp libc strcasecmp    sorted words  cmp libc strcasecmp        for  comment  stmt  in stmts      t   timeit Timer stmt stmt  setup setup      print   s    2f msec pass     comment   1000 t timeit 10  10     typical times on my machine   235886 words in list simple sort  483 59 msec pass sort with key str lower  1064 70 msec pass sort with cmp libc strcasecmp  5487 86 msec pass   So  the version with str lower is not only the fastest by far  but also the most portable and pythonic of all the proposed solutions here  I have not profiled memory usage  but the original poster has still not given a compelling reason to worry about it  Also  who says that a call into the libc module doesn t duplicate any strings   NB  The lower   string method also has the advantage of being locale-dependent  Something you will probably not be getting right when writing your own  optimised  solution  Even so  due to bugs and missing features in Python  this kind of comparison may give you wrong results in a unicode context

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Just use the str   lower   method  unless high-performance is important - in which case write that sorting method as a C extension    How to write a Python Extension  seems like a decent intro    More interestingly  This guide compares using the ctypes library vs writing an external C module  the ctype is quite-substantially slower than the C extension

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For occasional or even repeated comparisons  a few extra string objects shouldn t matter as long as this won t happen in the innermost loop of your core code or you don t have enough data to actually notice the performance impact  See if you do  doing things in a  stupid  way is much less stupid if you also do it less   If you seriously want to keep comparing lots and lots of text case-insensitively you could somehow keep the lowercase versions of the strings at hand to avoid finalization and re-creation  or normalize the whole data set into lowercase  This of course depends on the size of the data set  If there are a relatively few needles and a large haystack  replacing the needles with compiled regexp objects is one solution  If It s hard to say without seeing a concrete example

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For occasional or even repeated comparisons  a few extra string objects shouldn t matter as long as this won t happen in the innermost loop of your core code or you don t have enough data to actually notice the performance impact  See if you do  doing things in a  stupid  way is much less stupid if you also do it less   If you seriously want to keep comparing lots and lots of text case-insensitively you could somehow keep the lowercase versions of the strings at hand to avoid finalization and re-creation  or normalize the whole data set into lowercase  This of course depends on the size of the data set  If there are a relatively few needles and a large haystack  replacing the needles with compiled regexp objects is one solution  If It s hard to say without seeing a concrete example

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This question is asking 2 very different things    What is the easiest way to compare strings in Python  ignoring case  I guess I m looking for an equivalent to C s stricmp      Since  1 has been answered very well already  ie  str1 lower    lt  str2 lower     I will answer  2   def strincmp str1  str2  numchars None       result   0     len1   len str1      len2   len str2      if numchars is not None          minlen   min len1 len2 numchars      else          minlen   min len1 len2       end if     orda   ord  a       ordz   ord  z        i   0     while i  lt  minlen and 0    result          ord1   ord str1 i           ord2   ord str2 i           if ord1  gt   orda and ord1  lt   ordz              ord1   ord1-32          end if         if ord2  gt   orda and ord2  lt   ordz              ord2   ord2-32          end if         result   cmp ord1  ord2          i    1      end while      if 0    result and minlen    numchars          if len1  lt  len2              result   -1         elif len2  lt  len1              result   1          end if      end if      return result  end def   Only use this function when it makes sense to as in many instances the lowercase technique will be superior   I only work with ascii strings  I m not sure how this will behave with unicode

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Your question implies that you don t need Unicode  Try the following code snippet  if it works for you  you re done   Python 2 5 2  r252 60911  Aug 22 2008  02 34 17   GCC 4 3 1  on linux2 Type  help    copyright    credits  or  license  for more information   gt  gt  gt  import locale  gt  gt  gt  locale setlocale locale LC COLLATE   en US    en US   gt  gt  gt  sorted  ABCabc   key locale strxfrm    a    A    b    B    c    C    gt  gt  gt  sorted  ABCabc   cmp locale strcoll    a    A    b    B    c    C     Clarification  in case it is not obvious at first sight  locale strcoll seems to be the function you need  avoiding the str lower or locale strxfrm  duplicate  strings

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I m pretty sure you either have to use  lower   or use a regular expression   I m not aware of a built-in case-insensitive string comparison function

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In response to your clarification     You could use ctypes to execute the c function  strcasecmp    Ctypes is included in Python 2 5   It provides the ability to call out to dll and shared libraries such as libc  Here is a quick example  Python on Linux  see link for Win32 help    from ctypes import   libc   CDLL  libc so 6       see link above for Win32 help libc strcasecmp  THIS    this      returns 0 libc strcasecmp  THIS    THAT      returns 8   may also want to reference strcasecmp documentation  Not really sure this is any faster or slower  have not tested   but it s a way to use a C function to do case insensitive string comparisons                    ActiveState Code - Recipe 194371  Case Insensitive Strings is a recipe for creating a case insensitive string class   It might be a bit over kill for something quick  but could provide you with a common way of handling case insensitive strings if you plan on using them often

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For occasional or even repeated comparisons  a few extra string objects shouldn t matter as long as this won t happen in the innermost loop of your core code or you don t have enough data to actually notice the performance impact  See if you do  doing things in a  stupid  way is much less stupid if you also do it less   If you seriously want to keep comparing lots and lots of text case-insensitively you could somehow keep the lowercase versions of the strings at hand to avoid finalization and re-creation  or normalize the whole data set into lowercase  This of course depends on the size of the data set  If there are a relatively few needles and a large haystack  replacing the needles with compiled regexp objects is one solution  If It s hard to say without seeing a concrete example

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When something isn t supported well in the standard library  I always look for a PyPI package  With virtualization and the ubiquity of modern Linux distributions  I no longer avoid Python extensions  PyICU seems to fit the bill  https   stackoverflow com a 1098160 3461  There now is also an option that is pure python  It s well tested  https   github com jtauber pyuca    Old answer   I like the regular expression solution  Here s a function you can copy and paste into any function  thanks to python s block structure support   def equals ignore case str1  str2       import re     return re match re escape str1    r  Z   str2  re I  is not None   Since I used match instead of search  I didn t need to add a caret     to the regular expression   Note  This only checks equality  which is sometimes what is needed  I also wouldn t go so far as to say that I like it

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This question is asking 2 very different things    What is the easiest way to compare strings in Python  ignoring case  I guess I m looking for an equivalent to C s stricmp      Since  1 has been answered very well already  ie  str1 lower    lt  str2 lower     I will answer  2   def strincmp str1  str2  numchars None       result   0     len1   len str1      len2   len str2      if numchars is not None          minlen   min len1 len2 numchars      else          minlen   min len1 len2       end if     orda   ord  a       ordz   ord  z        i   0     while i  lt  minlen and 0    result          ord1   ord str1 i           ord2   ord str2 i           if ord1  gt   orda and ord1  lt   ordz              ord1   ord1-32          end if         if ord2  gt   orda and ord2  lt   ordz              ord2   ord2-32          end if         result   cmp ord1  ord2          i    1      end while      if 0    result and minlen    numchars          if len1  lt  len2              result   -1         elif len2  lt  len1              result   1          end if      end if      return result  end def   Only use this function when it makes sense to as in many instances the lowercase technique will be superior   I only work with ascii strings  I m not sure how this will behave with unicode

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You could subclass str and create your own case-insenstive string class but IMHO that would be extremely unwise and create far more trouble than it s worth

User · Answer

The recommended idiom to sort lists of values using expensive-to-compute keys is to the so-called  decorated pattern   It consists simply in building a list of  key  value  tuples from the original list  and sort that list  Then it is trivial to eliminate the keys and get the list of sorted values    gt  gt  gt  original list     a    b    A    B    gt  gt  gt  decorated     s lower    s  for s in original list   gt  gt  gt  decorated sort    gt  gt  gt  sorted list    s 1  for s in decorated   gt  gt  gt  sorted list   A    a    B    b     Or if you like one-liners    gt  gt  gt  sorted list    s 1  for s in sorted  s lower    s  for s in original list    gt  gt  gt  sorted list   A    a    B    b     If you really worry about the cost of calling lower    you can just store tuples of  lowered string  original string  everywhere  Tuples are the cheapest kind of containers in Python  they are also hashable so they can be used as dictionary keys  set members  etc

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Your question implies that you don t need Unicode  Try the following code snippet  if it works for you  you re done   Python 2 5 2  r252 60911  Aug 22 2008  02 34 17   GCC 4 3 1  on linux2 Type  help    copyright    credits  or  license  for more information   gt  gt  gt  import locale  gt  gt  gt  locale setlocale locale LC COLLATE   en US    en US   gt  gt  gt  sorted  ABCabc   key locale strxfrm    a    A    b    B    c    C    gt  gt  gt  sorted  ABCabc   cmp locale strcoll    a    A    b    B    c    C     Clarification  in case it is not obvious at first sight  locale strcoll seems to be the function you need  avoiding the str lower or locale strxfrm  duplicate  strings

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This is how you d do it with re   import re p   re compile   hello    re I  p match  Hello   p match  hello   p match  HELLO

User · Answer

Your question implies that you don t need Unicode  Try the following code snippet  if it works for you  you re done   Python 2 5 2  r252 60911  Aug 22 2008  02 34 17   GCC 4 3 1  on linux2 Type  help    copyright    credits  or  license  for more information   gt  gt  gt  import locale  gt  gt  gt  locale setlocale locale LC COLLATE   en US    en US   gt  gt  gt  sorted  ABCabc   key locale strxfrm    a    A    b    B    c    C    gt  gt  gt  sorted  ABCabc   cmp locale strcoll    a    A    b    B    c    C     Clarification  in case it is not obvious at first sight  locale strcoll seems to be the function you need  avoiding the str lower or locale strxfrm  duplicate  strings

User · Answer

Are you using this compare in a very-frequently-executed path of a highly-performance-sensitive application   Alternatively  are you running this on strings which are megabytes in size   If not  then you shouldn t worry about the performance and just use the  lower   method   The following code demonstrates that doing a case-insensitive compare by calling  lower   on two strings which are each almost a megabyte in size takes about 0 009 seconds on my 1 8GHz desktop computer   from timeit import Timer  s1    1234567890    100000    a  s2    1234567890    100000    B   code    s1 lower    lt  s2 lower    time   Timer code   from   main   import s1  s2   timeit 1000  print time   1000     0 00920499992371 on my machine   If indeed this is an extremely significant  performance-critical section of code  then I recommend writing a function in C and calling it from your Python code  since that will allow you to do a truly efficient case-insensitive search   Details on writing C extension modules can be found here  https   docs python org extending extending html

User · Answer

In response to your clarification     You could use ctypes to execute the c function  strcasecmp    Ctypes is included in Python 2 5   It provides the ability to call out to dll and shared libraries such as libc  Here is a quick example  Python on Linux  see link for Win32 help    from ctypes import   libc   CDLL  libc so 6       see link above for Win32 help libc strcasecmp  THIS    this      returns 0 libc strcasecmp  THIS    THAT      returns 8   may also want to reference strcasecmp documentation  Not really sure this is any faster or slower  have not tested   but it s a way to use a C function to do case insensitive string comparisons                    ActiveState Code - Recipe 194371  Case Insensitive Strings is a recipe for creating a case insensitive string class   It might be a bit over kill for something quick  but could provide you with a common way of handling case insensitive strings if you plan on using them often

User · Answer

The recommended idiom to sort lists of values using expensive-to-compute keys is to the so-called  decorated pattern   It consists simply in building a list of  key  value  tuples from the original list  and sort that list  Then it is trivial to eliminate the keys and get the list of sorted values    gt  gt  gt  original list     a    b    A    B    gt  gt  gt  decorated     s lower    s  for s in original list   gt  gt  gt  decorated sort    gt  gt  gt  sorted list    s 1  for s in decorated   gt  gt  gt  sorted list   A    a    B    b     Or if you like one-liners    gt  gt  gt  sorted list    s 1  for s in sorted  s lower    s  for s in original list    gt  gt  gt  sorted list   A    a    B    b     If you really worry about the cost of calling lower    you can just store tuples of  lowered string  original string  everywhere  Tuples are the cheapest kind of containers in Python  they are also hashable so they can be used as dictionary keys  set members  etc

User · Answer

The recommended idiom to sort lists of values using expensive-to-compute keys is to the so-called  decorated pattern   It consists simply in building a list of  key  value  tuples from the original list  and sort that list  Then it is trivial to eliminate the keys and get the list of sorted values    gt  gt  gt  original list     a    b    A    B    gt  gt  gt  decorated     s lower    s  for s in original list   gt  gt  gt  decorated sort    gt  gt  gt  sorted list    s 1  for s in decorated   gt  gt  gt  sorted list   A    a    B    b     Or if you like one-liners    gt  gt  gt  sorted list    s 1  for s in sorted  s lower    s  for s in original list    gt  gt  gt  sorted list   A    a    B    b     If you really worry about the cost of calling lower    you can just store tuples of  lowered string  original string  everywhere  Tuples are the cheapest kind of containers in Python  they are also hashable so they can be used as dictionary keys  set members  etc

User · Answer

There s no built in equivalent to that function you want   You can write your own function that converts to  lower   each character at a time to avoid  duplicating both strings  but I m sure it will very cpu-intensive and extremely inefficient    Unless you are working with extremely long strings  so long that can cause a memory problem if duplicated  then I would keep it simple and use   str1 lower      str2 lower     You ll be ok

User · Answer

This is how you d do it with re   import re p   re compile   hello    re I  p match  Hello   p match  hello   p match  HELLO

User · Answer

You could translate each string to lowercase once --- lazily only when you need it  or as a prepass to the sort if you know you ll be sorting the entire collection of strings   There are several ways to attach this comparison key to the actual data being sorted  but these techniques should be addressed in a separate issue   Note that this technique can be used not only to handle upper lower case issues  but for other types of sorting such as locale specific sorting  or  Library-style  title sorting that ignores leading articles and otherwise normalizes the data before sorting it

User · Answer

You could translate each string to lowercase once --- lazily only when you need it  or as a prepass to the sort if you know you ll be sorting the entire collection of strings   There are several ways to attach this comparison key to the actual data being sorted  but these techniques should be addressed in a separate issue   Note that this technique can be used not only to handle upper lower case issues  but for other types of sorting such as locale specific sorting  or  Library-style  title sorting that ignores leading articles and otherwise normalizes the data before sorting it

User · Answer

When something isn t supported well in the standard library  I always look for a PyPI package  With virtualization and the ubiquity of modern Linux distributions  I no longer avoid Python extensions  PyICU seems to fit the bill  https   stackoverflow com a 1098160 3461  There now is also an option that is pure python  It s well tested  https   github com jtauber pyuca    Old answer   I like the regular expression solution  Here s a function you can copy and paste into any function  thanks to python s block structure support   def equals ignore case str1  str2       import re     return re match re escape str1    r  Z   str2  re I  is not None   Since I used match instead of search  I didn t need to add a caret     to the regular expression   Note  This only checks equality  which is sometimes what is needed  I also wouldn t go so far as to say that I like it

User · Answer

I can t find any other built-in way of doing case-insensitive comparison  The python cook-book recipe uses lower     However you have to be careful when using lower for comparisons because of the Turkish I problem  Unfortunately Python s handling for Turkish Is is not good  i is converted to I  but I is not converted to i  I is converted to i  but i is not converted to I

User · Answer

You could translate each string to lowercase once --- lazily only when you need it  or as a prepass to the sort if you know you ll be sorting the entire collection of strings   There are several ways to attach this comparison key to the actual data being sorted  but these techniques should be addressed in a separate issue   Note that this technique can be used not only to handle upper lower case issues  but for other types of sorting such as locale specific sorting  or  Library-style  title sorting that ignores leading articles and otherwise normalizes the data before sorting it

User · Answer

I can t find any other built-in way of doing case-insensitive comparison  The python cook-book recipe uses lower     However you have to be careful when using lower for comparisons because of the Turkish I problem  Unfortunately Python s handling for Turkish Is is not good  i is converted to I  but I is not converted to i  I is converted to i  but i is not converted to I

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There s no built in equivalent to that function you want   You can write your own function that converts to  lower   each character at a time to avoid  duplicating both strings  but I m sure it will very cpu-intensive and extremely inefficient    Unless you are working with extremely long strings  so long that can cause a memory problem if duplicated  then I would keep it simple and use   str1 lower      str2 lower     You ll be ok

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Just use the str   lower   method  unless high-performance is important - in which case write that sorting method as a C extension    How to write a Python Extension  seems like a decent intro    More interestingly  This guide compares using the ctypes library vs writing an external C module  the ctype is quite-substantially slower than the C extension

User · Answer

In response to your clarification     You could use ctypes to execute the c function  strcasecmp    Ctypes is included in Python 2 5   It provides the ability to call out to dll and shared libraries such as libc  Here is a quick example  Python on Linux  see link for Win32 help    from ctypes import   libc   CDLL  libc so 6       see link above for Win32 help libc strcasecmp  THIS    this      returns 0 libc strcasecmp  THIS    THAT      returns 8   may also want to reference strcasecmp documentation  Not really sure this is any faster or slower  have not tested   but it s a way to use a C function to do case insensitive string comparisons                    ActiveState Code - Recipe 194371  Case Insensitive Strings is a recipe for creating a case insensitive string class   It might be a bit over kill for something quick  but could provide you with a common way of handling case insensitive strings if you plan on using them often

User · Answer

The recommended idiom to sort lists of values using expensive-to-compute keys is to the so-called  decorated pattern   It consists simply in building a list of  key  value  tuples from the original list  and sort that list  Then it is trivial to eliminate the keys and get the list of sorted values    gt  gt  gt  original list     a    b    A    B    gt  gt  gt  decorated     s lower    s  for s in original list   gt  gt  gt  decorated sort    gt  gt  gt  sorted list    s 1  for s in decorated   gt  gt  gt  sorted list   A    a    B    b     Or if you like one-liners    gt  gt  gt  sorted list    s 1  for s in sorted  s lower    s  for s in original list    gt  gt  gt  sorted list   A    a    B    b     If you really worry about the cost of calling lower    you can just store tuples of  lowered string  original string  everywhere  Tuples are the cheapest kind of containers in Python  they are also hashable so they can be used as dictionary keys  set members  etc

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Are you using this compare in a very-frequently-executed path of a highly-performance-sensitive application   Alternatively  are you running this on strings which are megabytes in size   If not  then you shouldn t worry about the performance and just use the  lower   method   The following code demonstrates that doing a case-insensitive compare by calling  lower   on two strings which are each almost a megabyte in size takes about 0 009 seconds on my 1 8GHz desktop computer   from timeit import Timer  s1    1234567890    100000    a  s2    1234567890    100000    B   code    s1 lower    lt  s2 lower    time   Timer code   from   main   import s1  s2   timeit 1000  print time   1000     0 00920499992371 on my machine   If indeed this is an extremely significant  performance-critical section of code  then I recommend writing a function in C and calling it from your Python code  since that will allow you to do a truly efficient case-insensitive search   Details on writing C extension modules can be found here  https   docs python org extending extending html

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For occasional or even repeated comparisons  a few extra string objects shouldn t matter as long as this won t happen in the innermost loop of your core code or you don t have enough data to actually notice the performance impact  See if you do  doing things in a  stupid  way is much less stupid if you also do it less   If you seriously want to keep comparing lots and lots of text case-insensitively you could somehow keep the lowercase versions of the strings at hand to avoid finalization and re-creation  or normalize the whole data set into lowercase  This of course depends on the size of the data set  If there are a relatively few needles and a large haystack  replacing the needles with compiled regexp objects is one solution  If It s hard to say without seeing a concrete example

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Here is a benchmark showing that using str lower is faster than the accepted answer s proposed method  libc strcasecmp       usr bin env python2 7 import random import timeit  from ctypes import   libc   CDLL  libc dylib     change to  libc so 6  on linux  with open   usr share dict words    r   as wordlist      words   wordlist read   splitlines   random shuffle words  print   i words in list    len words   setup    from   main   import words  libc  gc enable    stmts           simple sort    sorted words           sort with key str lower    sorted words  key str lower           sort with cmp libc strcasecmp    sorted words  cmp libc strcasecmp        for  comment  stmt  in stmts      t   timeit Timer stmt stmt  setup setup      print   s    2f msec pass     comment   1000 t timeit 10  10     typical times on my machine   235886 words in list simple sort  483 59 msec pass sort with key str lower  1064 70 msec pass sort with cmp libc strcasecmp  5487 86 msec pass   So  the version with str lower is not only the fastest by far  but also the most portable and pythonic of all the proposed solutions here  I have not profiled memory usage  but the original poster has still not given a compelling reason to worry about it  Also  who says that a call into the libc module doesn t duplicate any strings   NB  The lower   string method also has the advantage of being locale-dependent  Something you will probably not be getting right when writing your own  optimised  solution  Even so  due to bugs and missing features in Python  this kind of comparison may give you wrong results in a unicode context

[python] Ignore case in Python strings

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