Find first element in a sequence that matches a predicate

Question

I want an idiomatic way to find the first element in a list that matches a predicate   The current code is quite ugly    x for x in seq if predicate x   0    I ve thought about changing it to   from itertools import dropwhile dropwhile lambda x  not predicate x   seq  next     But there must be something more elegant    And it would be nice if it returns a None value rather than raise an exception if no match is found   I know I could just define a function like   def get first predicate  seq       for i in seq          if predicate i   return i     return None   But it is quite tasteless to start filling the code with utility functions like this  and people will probably not notice that they are already there  so they tend to be repeated over time  if there are built ins that already provide the same

User · Accepted Answer

To find first element in a sequence seq that matches a predicate   next x for x in seq if predicate x     Or  itertools ifilter on Python 2    next filter predicate  seq     It raises StopIteration if there is none     To return None if there is no such element   next  x for x in seq if predicate x    None    Or   next filter predicate  seq   None

User · Answer

J F  Sebastian s answer is most elegant but requires python 2 6 as fortran pointed out   For Python version  lt  2 6  here s the best I can come up with   from itertools import repeat ifilter chain chain ifilter predicate seq  repeat None   next     Alternatively if you needed a list later  list handles the StopIteration   or you needed more than just the first but still not all  you can do it with islice    from itertools import islice ifilter list islice ifilter predicate seq  1     UPDATE  Although I am personally using a predefined function called first   that catches a StopIteration and returns None  Here s a possible improvement over the above example  avoid using filter   ifilter   from itertools import islice chain chain  x for x in seq if predicate x   repeat None   next

User · Answer

I don t think there s anything wrong with either solutions you proposed in your question   In my own code  I would implement it like this though    x for x in seq if predicate x   next     The syntax with    creates a generator  which is more efficient than generating all the list at once with

User · Answer

You could use a generator expression with a default value and then next it   next  x for x in seq if predicate x    None    Although for this one-liner you need to be using Python    2 6   This rather popular article further discusses this issue  Cleanest Python find-in-list function

[python] Find first element in a sequence that matches a predicate

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