Pythonic way to combine FOR loop and IF statement

Question

I know how to use both for loops and if statements on separate lines  such as    gt  gt  gt  a    2 3 4 5 6 7 8 9 0      xyz    0 12 4 6 242 7 9      for x in xyz          if x in a              print x  0 4 6 7 9   And I know I can use a list comprehension to combine these when the statements are simple  such as   print  x for x in xyz if x in a     But what I can t find is a good example anywhere  to copy and learn from  demonstrating a complex set of commands  not just  print x   that occur following a combination of a for loop and some if statements  Something that I would expect looks like   for x in xyz if x not in a      print x       Is this just not the way python is supposed to work

User · Accepted Answer

You can use generator expressions like this   gen    x for x in xyz if x not in a   for x in gen      print x

User · Answer

a    2 3 4 5 6 7 8 9 0  xyz    0 12 4 6 242 7 9    set a   amp  set xyz    set  0  9  4  6  7

User · Answer

I would probably use   for x in xyz       if x not in a          print x

User · Answer

I personally think this is the prettiest version    a    2 3 4 5 6 7 8 9 0  xyz    0 12 4 6 242 7 9  for x in filter lambda w  w in a  xyz     print x   Edit  if you are very keen on avoiding to use lambda you can use partial function application and use the operator module  that provides functions of most operators    https   docs python org 2 library operator html module-operator  from operator import contains from functools import partial print list filter partial contains  a   xyz

User · Answer

A simple way to find unique common elements of lists a and b   a    1 2 3  b    3 6 2  for both in set a   amp  set b       print both

User · Answer

You can use generators too  if generator expressions become too involved or complex   def gen        for x in xyz          if x in a              yield x  for x in gen        print x

User · Answer

I liked Alex s answer  because a filter is exactly an if applied to a list  so if you want to explore a subset of a list given a condition  this seems to be the most natural way  mylist    1 2 3 4 5  another list    2 3 4   wanted   lambda x x in another list  for x in filter wanted  mylist       print x    this method is useful for the separation of concerns  if the condition function changes  the only code to fiddle with is the function itself  mylist    1 2 3 4 5   wanted   lambda x  x  0 5   gt  10  0 3  for x in filter wanted  mylist       print x    The generator method seems better when you don t want members of the list  but a modification of said members  which seems more fit to a generator  mylist    1 2 3 4 5   wanted   lambda x  x  0 5   gt  10  0 3  generator    x  0 5 for x in mylist if wanted x    for x in generator      print x    Also  filters work with generators  although in this case it isn t efficient  mylist    1 2 3 4 5   wanted   lambda x  x  0 5   gt  10  0 3  generator    x  0 9 for x in mylist   for x in filter wanted  generator       print x    But of course  it would still be nice to write like this   mylist    1 2 3 4 5   wanted   lambda x  x  0 5   gt  10  0 3    for x in filter wanted  mylist   for x in mylist if wanted x       print x

User · Answer

As per The Zen of Python  if you are wondering whether your code is  Pythonic   that s the place to go     Beautiful is better than ugly  Explicit is better than implicit  Simple is better than complex  Flat is better than nested  Readability counts    The Pythonic way of getting the sorted intersection of two sets is    gt  gt  gt  sorted set a  intersection xyz    0  4  6  7  9    Or those elements that are xyz but not in a    gt  gt  gt  sorted set xyz  difference a    12  242    But for a more complicated loop you may want to flatten it by iterating over a well-named generator expression and or calling out to a well-named function  Trying to fit everything on one line is rarely  Pythonic      Update following additional comments on your question and the accepted answer  I m not sure what you are trying to do with enumerate  but if a is a dictionary  you probably want to use the keys  like this    gt  gt  gt  a             2   Turtle Doves           3   French Hens           4   Colly Birds           5   Gold Rings           6   Geese-a-Laying           7   Swans-a-Swimming           8   Maids-a-Milking           9   Ladies Dancing           0   Camel Books          gt  gt  gt   gt  gt  gt  xyz    0  12  4  6  242  7  9   gt  gt  gt   gt  gt  gt  known things   sorted set a iterkeys    intersection xyz    gt  gt  gt  unknown things   sorted set xyz  difference a iterkeys      gt  gt  gt   gt  gt  gt  for thing in known things          print  I know about   a thing      I know about Camel Books I know about Colly Birds I know about Geese-a-Laying I know about Swans-a-Swimming I know about Ladies Dancing  gt  gt  gt  print     but        but     gt  gt  gt   gt  gt  gt  for thing in unknown things          print  I don t know what happened on the  0 th day of Christmas  format thing      I don t know what happened on the 12th day of Christmas I don t know what happened on the 242th day of Christmas

User · Answer

Use intersection or intersection update   intersection    a    2 3 4 5 6 7 8 9 0  xyz    0 12 4 6 242 7 9  ans   sorted set a  intersection set xyz     intersection update   a    2 3 4 5 6 7 8 9 0  xyz    0 12 4 6 242 7 9  b   set a  b intersection update xyz    then b is your answer

User · Answer

The following is a simplification one liner from the accepted answer    a    2 3 4 5 6 7 8 9 0  xyz    0 12 4 6 242 7 9   for x in  x for x in xyz if x not in a       print x   12 242   Notice that the generator was kept inline   This was tested on python2 7 and python3 6   notice the parens in the print

[python] Pythonic way to combine FOR loop and IF statement

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