Is it possible to use else in a list comprehension

Question

Here is the code I was trying to turn into a list comprehension   table      for index in xrange 256       if index in ords to keep          table    chr index      else          table    replace with   Is there a way to add the else statement to this comprehension   table      join chr index  for index in xrange 15  if index in ords to keep

User · Answer

Great answers  but just wanted to mention a gotcha that  pass  keyword will not work in the if else part of the list-comprehension  as posted in the examples mentioned above     works list1    10  20  30  40  50  newlist2    x if x  gt  30 else x  2 for x in list1   print newlist2  type newlist2     but this WONT work list1    10  20  30  40  50  newlist2    x if x  gt  30 else pass for x in list1   print newlist2  type newlist2     This is tried and tested on python 3 4  Error is as below   newlist2    x if x  gt  30 else pass for x in list1                                       SyntaxError  invalid syntax   So  try to avoid pass-es in list comprehensions

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Also  would I be right in concluding that a list comprehension is the most efficient way to do this    Maybe  List comprehensions are not inherently computationally efficient  It is still running in linear time   From my personal experience  I have significantly reduced computation time when dealing with large data sets by replacing list comprehensions  specifically nested ones  with for-loop list-appending type structures you have above  In this application I doubt you will notice a difference

User · Answer

Yes  else can be used in Python inside a list comprehension  with a Conditional Expression   ternary operator      gt  gt  gt     A  if b   e  else  c   for b in  comprehension     c    c    c    c    c    A    c    A    c    c    c    c    c     Here  the parentheses      are just to emphasize the conditional expression  they are not necessarily required  Operator precedence     Additionaly  several expressions can be nested  resulting in more elses and harder to read code    gt  gt  gt    A  if b   e  else  d  if True else  x  for b in  comprehension     d    d    d    d    d    A    d    A    d    d    d    d    d    gt  gt  gt    On a related note  a comprehension can also contain its own if condition s  at the end    gt  gt  gt    A  if b   e  else  c  for b in  comprehension  if False      gt  gt  gt    A  if b   e  else  c  for b in  comprehension  if  comprehension  index b  2    c    c    A    A    c    c     Conditions  Yes  multiple ifs are possible  and actually multiple fors  too    gt  gt  gt   i for i in range 3  for   in range 3    0  0  0  1  1  1  2  2  2   gt  gt  gt   i for i in range 3  if i for   in range 3  if   if True if True   1  1  2  2     The single underscore   is a valid variable name  identifier  in Python  used here just to show it s not actually used  It has a special meaning in interactive mode   Using this for an additional conditional expression is possible  but of  no real use    gt  gt  gt   i for i in range 3    0  1  2   gt  gt  gt   i for i in range 3  if i   1  2   gt  gt  gt   i for i in range 3  if  True if i else False    1  2    Comprehensions can also be nested to create  multi-dimensional  lists   arrays      gt  gt  gt    i for j in range i   for i in range 3         1    2  2       Last but not least  a comprehension is not limited to creating a list  i e  else and if can also be used the same way in a set comprehension    gt  gt  gt   i for i in  set comprehension     o    p    m    n    c    r    i    t    h    e    s          and a dictionary comprehension    gt  gt  gt   k v for k v in    key   value      dict   comprehension       key    value    dict    comprehension     The same syntax is also used for Generator Expressions    gt  gt  gt  for g in   a  if b else  c  for b in  generator            print g  end         aaaaaaaaa gt  gt  gt    which can be used to create a tuple  there is no tuple comprehension     Further reading    The Python Tutorial on Data Structures   List Comprehensions Sets  Dictionaries

User · Answer

To use the else in list comprehensions in python programming you can try out the below snippet  This would resolve your problem  the snippet is tested on python 2 7 and python 3 5    obj     Even  if i 2  0 else  Odd  for i in range 10

User · Answer

The syntax a if b else c is a ternary operator in Python that evaluates to a if the condition b is true - otherwise  it evaluates to c  It can be used in comprehension statements    gt  gt  gt   a if a else 2 for a in  0 1 0 3    2  1  2  3    So for your example   table      join chr index  if index in ords to keep else replace with                 for index in xrange 15

User · Answer

If you want an else you don t want to filter the list comprehension  you want it to iterate over every value  You can use true-value if cond else false-value as the statement instead  and remove the filter from the end   table      join chr index  if index in ords to keep else replace with for index in xrange 15

[python] Is it possible to use 'else' in a list comprehension?

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