Python Find in list

Question

I have come across this   item   someSortOfSelection   if item in myList      doMySpecialFunction item    but sometimes it does not work with all my items  as if they weren t recognized in the list  when it s a list of string    Is this the most  pythonic  way of finding an item in a list  if x in l

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Definition and Usage  the count   method returns the number of elements with the specified value   Syntax  list count value    example   fruits     apple    banana    cherry    x   fruits count  cherry     Question s example   item   someSortOfSelection    if myList count item   gt   1        doMySpecialFunction item

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If you want to find one element or None use default in next  it won t raise StopIteration if the item was not found in the list   first or default   next  x for x in lst if       None

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Finding the first occurrence  There s a recipe for that in itertools   def first true iterable  default False  pred None          Returns the first true value in the iterable       If no true value is found  returns  default       If  pred  is not None  returns the first item     for which pred item  is true                 first true  a b c   x  -- gt  a or b or c or x       first true  a b   x  f  -- gt  a if f a  else b if f b  else x     return next filter pred  iterable   default    For example  the following code finds the first odd number in a list    gt  gt  gt  first true  2 3 4 5   None  lambda x  x 2  1  3

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Instead of using list index x  which returns the index of x if it is found in list or returns a  ValueError message if x is not found  you could use list count x  which returns the number of occurrences of x in list  validation that x is indeed in the list  or it returns 0 otherwise  in the absence of x   The cool thing about count   is that it doesn t break your code or require you to throw an exception for when x is not found

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lstr  1  2  3   lstr map str lstr   r re compile    3  1     results list filter r match lstr    print results

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Check there are no additional unwanted whites space in the items of the list of strings  That s a reason that can be interfering explaining the items cannot be found

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If you are going to check if value exist in the collectible once then using  in  operator is fine  However  if you are going to check for more than once then I recommend using bisect module  Keep in mind that using bisect module data must be sorted  So you sort data once and then you can use bisect  Using bisect module on my machine is about 12 times faster than using  in  operator   Here is an example of code using Python 3 8 and above syntax   import bisect from timeit import timeit  def bisect search container  value       return          index    bisect bisect left container  value    lt  len container         and container index     value        data   list range 1000     value to search true value   666 false value   66666    times to test ttt   1000  print f  bisect search data  true value     bisect search data  false value       t1   timeit lambda  true value in data  number ttt  t2   timeit lambda  bisect search data  true value   number ttt   print  Performance    f  t1   4f    t2   4f   diffs  t1 t2   2f      Output   bisect search data  true value  True bisect search data  false value  False Performance  t1 0 0220  t2 0 0019  diffs t1 t2 11 71

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You may want to use one of two possible searches while working with list of strings    if list element is equal to an item   example  is in   one   example   two      if item in your list  some function on true     ex    in   one   ex   two      True   ex 1  in   one   ex   two      False if list element is like an item   ex  is in   one  example   two   or  example 1  is in   one   example   two      matches    el for el in your list if item in el   or  matches    el for el in your list if el in item   then just check len matches  or read them if needed

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While the answer from Niklas B  is pretty comprehensive  when we want to find an item in a list it is sometimes useful to get its index   next  i for i  x in enumerate lst  if  condition on x     default value

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Another alternative  you can check if an item is in a list with if item in list   but this is order O n   If you are dealing with big lists of items and all you need to know is whether something is a member of your list  you can convert the list to a set first and take advantage of constant time set lookup   my set   set my list  if item in my set     much faster on average than using a list       do something   Not going to be the correct solution in every case  but for some cases this might give you better performance   Note that creating the set with set my list  is also O n   so if you only need to do this once then it isn t any faster to do it this way  If you need to repeatedly check membership though  then this will be O 1  for every lookup after that initial set creation

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As for your first question  that code is perfectly fine and should work if item equals one of the elements inside myList  Maybe you try to find a string that does not exactly match one of the items or maybe you are using a float value which suffers from inaccuracy   As for your second question  There s actually several possible ways if  finding  things in lists   Checking if something is inside  This is the use case you describe  Checking whether something is inside a list or not  As you know  you can use the in operator for that   3 in  1  2  3      gt  True   Filtering a collection  That is  finding all elements in a sequence that meet a certain condition  You can use list comprehension or generator expressions for that   matches    x for x in lst if fulfills some condition x   matches    x for x in lst if x  gt  6    The latter will return a generator which you can imagine as a sort of lazy list that will only be built as soon as you iterate through it  By the way  the first one is exactly equivalent to  matches   filter fulfills some condition  lst    in Python 2  Here you can see higher-order functions at work  In Python 3  filter doesn t return a list  but a generator-like object   Finding the first occurrence  If you only want the first thing that matches a condition  but you don t know what it is yet   it s fine to use a for loop  possibly using the else clause as well  which is not really well-known   You can also use  next x for x in lst if        which will return the first match or raise a StopIteration if none is found  Alternatively  you can use  next  x for x in lst if        default value     Finding the location of an item  For lists  there s also the index method that can sometimes be useful if you want to know where a certain element is in the list    1 2 3  index 2      gt  1  1 2 3  index 4      gt  ValueError   However  note that if you have duplicates   index always returns the lowest index          1 2 3 2  index 2      gt  1   If there are duplicates and you want all the indexes then you can use enumerate   instead    i for i x in enumerate  1 2 3 2   if x  2      gt   1  3

[python] Python: Find in list

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