Find object in list that has attribute equal to some value that meets any condition

Question

I ve got list of objects  I want to find one  first or whatever  object in this list that has attribute  or method result - whatever  equal to value  What s is the best way to find it  Here s test case    class Test        def   init   self  value             self value   value              import random    value   5      test list    Test random randint 0 100   for x in range 1000                            that I would do in Pascal  I don t believe it s anywhere near  Pythonic    for x in test list        if x value    value            print  quot i found it  quot            break     I think using generators and reduce   won t make any difference because it still would be iterating through list  ps   Equation to value is just an example  Of course we want to get element which meets any condition

User · Answer

I just ran into a similar problem and devised a small optimization for the case where no object in the list meets the requirement.(for my use-case this resulted in major performance improvement):

Along with the list test_list, I keep an additional set test_value_set which consists of values of the list that I need to filter on. So here the else part of agf's solution becomes very-fast.

User · Answer

You could also implement rich comparison via   eq   method for your Test class and use in operator  Not sure if this is the best stand-alone way  but in case if you need to compare Test instances based on value somewhere else  this could be useful   class Test      def   init   self  value           self value   value      def   eq   self  other              To implement  in  operator              Comparing with int  assuming  value  is int          if isinstance other  int               return self value    other           Comparing with another Test object         elif isinstance other  Test               return self value    other value  import random  value   5  test list    Test random randint 0 100   for x in range 1000    if value in test list      print  i found it

User · Answer

You could do something like this  dict          id   1      name    Doom Hammer              id   2       name    Rings ov Saturn        for x in dict    if x  id      2      print x  name      Thats what i use to find the objects in a long array of objects

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For below code  xGen is an anonomous generator expression  yFilt is a filter object  Note that for xGen the additional None parameter is returned rather than throwing StopIteration when the list is exhausted   arr    10 0    11 1    12 2    13 2    14 3    value   2 xGen    x for x in arr if x 1     value  yFilt   filter lambda x  x 1     value  arr  print type xGen   print type yFilt    for i in range 1 4       print  xGen  pass   i   result   next xGen None       print  yFilt  pass   i   result   next yFilt     Output    lt class  generator  gt   lt class  filter  gt  xGen  pass  1  result   12  2  yFilt  pass  1  result   12  2  xGen  pass  2  result   13  2  yFilt  pass  2  result   13  2  xGen  pass  3  result  None Traceback  most recent call last     File  test py   line 12  in  lt module gt      print  yFilt  pass   i   result   next yFilt   StopIteration

User · Answer

Old question but I use this quite frequently  for version 3 8   It s a bit of syntactic salt  but it has the advantage over the top answer in that you could retrieve a list of results  if there are multiple  by simply removing the  0  and it still defaults to None if nothing is found  For any other condition  simply change the x value  value to what ever you re looking for    0  if      x for x in test list if x value  value   else None

User · Answer

next  x for x in test list if x value    value   None   This gets the first item from the list that matches the condition  and returns None if no item matches  It s my preferred single-expression form  However  for x in test list      if x value    value          print  quot i found it  quot           break  The naive loop-break version  is perfectly Pythonic -- it s concise  clear  and efficient  To make it match the behavior of the one-liner  for x in test list      if x value    value          print  quot i found it  quot           break else      x   None  This will assign None to x if you don t break out of the loop

User · Answer

Since it has not been mentioned just for completion  The good ol  filter to filter your to be filtered elements   Functional programming ftw            Set Up         class X       def   init   self  val           self val   val  elem   5  my unfiltered list    X 1   X 2   X 3   X 4   X 5   X 5   X 6            Set Up              Filter one liner     filter lambda x  condition x   some list  my filter iter   filter lambda x  x val    elem  my unfiltered list      Returns a flippin  iterator at least in Python 3 5 and that s what I m on  print next my filter iter  val  print next my filter iter  val  print next my filter iter  val        1  2  3  4  5  5  6  Will Return        5   5   Traceback  most recent call last       File  C  Users mousavin workspace Scripts test py   line 22  in  lt module gt        print next my filter iter  value    StopIteration     You can do that None stuff or whatever at this point  if you don t like exceptions    I know that generally in python list comprehensions are preferred or at least that is what I read  but I don t see the issue to be honest  Of course Python is not an FP language  but Map   Reduce   Filter are perfectly readable and are the most standard of standard use cases in functional programming    So there you go  Know thy functional programming    filter condition list  It won t get any easier than this   next filter lambda x  x val    value   my unfiltered list     Optionally  next      None  or some other default value to prevent Exceptions

User · Answer

A simple example  We have the following array  li      id  1  name   ronaldo     id  2  name   messi      Now  we want to find the object in the array that has id equal to 1   Use method next with  list comprehension   next x for x in li if x  id      1      Use list comprehension and return first item    x for x in li if x  id      1   0     Custom Function   def find arr   id       for x in arr          if x  id      id              return x find li   1    Output all the above methods is   id   1   name    ronaldo

[python] Find object in list that has attribute equal to some value (that meets any condition)

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