find first sequence item that matches a criterion

Question

What would be the most elegant and efficient way of finding returning the first list item that matches a certain criterion   For example  if I have a list of objects and I would like to get the first object of those with attribute obj val  5  I could of course use list comprehension  but that would incur O n  and if n is large  it s wasteful  I could also use a loop with break once the criterion was met  but I thought there could be a more pythonic elegant solution

User · Accepted Answer

If you don t have any other indexes or sorted information for your objects  then you will have to iterate until such an object is found  next obj for obj in objs if obj val    5   This is however faster than a complete list comprehension  Compare these two   i for i in xrange 100000  if i    1000  0   next i for i in xrange 100000  if i    1000   The first one needs 5 75ms  the second one 58 3  s  100 times faster because the loop 100 times shorter

User · Answer

a  100 200 300 400 500  def search b    try    k a index b    return a k    except ValueError      return  not found  print search 500     it ll return the object if found else it ll return  not found

[python] find first sequence item that matches a criterion

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