How would you make a comma-separated string from a list of strings

Question

What would be your preferred way to concatenate strings from a sequence such that between every two consecutive pairs a comma is added  That is  how do you map  for instance    a    b    c   to  a b c    The cases   s   and    should be mapped to  s  and     respectively    I usually end up using something like    join map lambda x  x     l    -1   but also feeling somewhat unsatisfied

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l   a   1   b   2   print str l  1 -1   Output    a   1   b   2

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If you want to do the shortcut way          join  str word  for word in wordList    But if you want to show off with logic      wordList     USD    EUR    JPY    NZD    CHF    CAD   stringText       for word in wordList      stringText    word        stringText   stringText  -2      get rid of last comma print stringText

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My two cents  I like simpler an one-line code in python    gt  gt  gt  from itertools import imap  ifilter  gt  gt  gt  l     a        b   1  None   gt  gt  gt      join imap str  ifilter lambda x  x  l    a b 1  gt  gt  gt  m     a       None   gt  gt  gt      join imap str  ifilter lambda x  x  m     a    It s pythonic  works for strings  numbers  None and empty string  It s short and satisfies the requirements  If the list is not going to contain numbers  we can use this simpler variation    gt  gt  gt      join ifilter lambda x  x  l     Also this solution doesn t create a new list  but uses an iterator  like  Peter Hoffmann pointed  thanks

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join l  will not work for all cases  I d suggest using the csv module with StringIO  import StringIO import csv  l     list   of       crazy quotes and    123  other things    line   StringIO StringIO   writer   csv writer line  writer writerow l  csvcontent   line getvalue      list of         crazy  quotes  and    123 other things r n

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gt  gt  gt  my list     A            D    E     gt  gt  gt      join  str i  for i in my list if i    A D E    my list may contain any type of variables  This avoid the result  A   D E

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Here is a alternative solution in Python 3 0 which allows non-string list items    gt  gt  gt  alist     a   1   2   b       a standard way   gt  gt  gt       join map str  alist    a  1   2   b     the alternative solution   gt  gt  gt  import io  gt  gt  gt  s   io StringIO    gt  gt  gt  print  alist  file s  sep       end      gt  gt  gt  s getvalue    a  1   2   b       NOTE  The space after comma is intentional

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Why the map lambda magic  Doesn t this work    gt  gt  gt  foo     a    b    c    gt  gt  gt  print     join foo   a b c  gt  gt  gt  print     join        gt  gt  gt  print     join   a     a   In case if there are numbers in the list  you could use list comprehension    gt  gt  gt      join  str x  for x in foo     or a generator expression    gt  gt  gt      join str x  for x in foo

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Here is an example with list   gt  gt  gt  myList      Apple     Orange     gt  gt  gt  myList       join map str   i 0  for i in myList      gt  gt  gt  print  Output    myList Output  Apple Orange   More Accurate -   gt  gt  gt  myList      Apple     Orange     gt  gt  gt  myList       join map str   type i     list and i 0  for i in myList      gt  gt  gt  print  Output    myList Output  Apple Orange   Example 2 -  myList     Apple   Orange   myList       join map str  myList    print  Output    myList Output  Apple Orange

User · Answer

I would say the csv library is the only sensible option here  as it was built to cope with all csv use cases such as commas in a string  etc   To output a list l to a  csv file   import csv with open  some csv    w   newline     as f      writer   csv writer f      writer writerow l     this will output l as a single row      It is also possible to use writer writerows iterable  to output multiple rows to csv   This example is compatible with Python 3  as the other answer here used StringIO which is Python 2

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my list     a    b    c    d   my string       join my list     a b c d    This won t work if the list contains integers    And if the list contains non-string types  such as integers  floats  bools  None  then do   my string       join map str  my list

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Don t you just want       join l    Obviously it gets more complicated if you need to quote escape commas etc in the values  In that case I would suggest looking at the csv module in the standard library   https   docs python org library csv html

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Unless I m missing something      join foo  should do what you re asking for    gt  gt  gt      join           gt  gt  gt      join   s     s   gt  gt  gt      join   a   b   c     a b c     edit   and as jmanning2k points out        join  str x  for x in foo     is safer and quite Pythonic  though the resulting string will be difficult to parse if the elements can contain commas -- at that point  you need the full power of the csv module  as Douglas points out in his answer

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jmanning2k using a list comprehension has the downside of creating a new temporary list  The better solution would be using itertools imap which returns an iterator  from itertools import imap l    1   foo   4   bar       join imap str  l

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Peter Hoffmann  Using generator expressions has the benefit of also producing an iterator but saves importing itertools  Furthermore  list comprehensions are generally preferred to map  thus  I d expect generator expressions to be preferred to imap    gt  gt  gt  l    1   foo   4   bar    gt  gt  gt      join str bit  for bit in l   1 foo 4 bar

[python] How would you make a comma-separated string from a list of strings?

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