Searching a list of objects in Python

Question

Let s assume I m creating a simple class to work similar to a C-style struct  to just hold data elements   I m trying to figure out how to search a list of objects for objects with an attribute equaling a certain value   Below is a trivial example to illustrate what I m trying to do   For instance   class Data      pass  myList       for i in range 20       data   Data       data n   i     data n squared   i   i     myList append data    How would I go about searching the myList list to determine if it contains an element with n    5   I ve been Googling and searching the Python docs  and I think I might be able to do this with a list comprehension  but I m not sure   I might add that I m having to use Python 2 4 3 by the way  so any new gee-whiz 2 6 or 3 x features aren t available to me

User · Answer

Consider using a dictionary   myDict       for i in range 20       myDict i    i   i  print 5 in myDict

User · Answer

You can get a list of all matching elements with a list comprehension    x for x in myList if x n    30     list of all elements with  n  30   If you simply want to determine if the list contains any element that matches and do it  relatively  efficiently  you can do  def contains list  filter       for x in list          if filter x               return True     return False  if contains myList  lambda x  x n    3     True if any element has  n  3       do stuff

User · Answer

x for x in myList if x n    30                  list of all matches  x n squared for x in myList if x n    30        property of matches any x n    30 for x in myList                    if there is any matches  i for i x in enumerate myList  if x n    30     indices of all matches  def first iterable  default None     for item in iterable      return item   return default  first x for x in myList if x n    30             the first match  if any

User · Answer

filter lambda x  x n    5  myList

User · Answer

Another way you could do it is using the next   function  matched obj   next x for x in list if x n    10

User · Answer

You should add a   eq   and a   hash   method to your Data class  it could check if the   dict   attributes are equal  same properties  and then if their values are equal  too   If you did that  you can use  test   Data   test n   5  found   test in myList   The in keyword checks if test is in myList   If you only want to a a n property in Data you could use   class Data object         slots       n       def   init   self  n           self n   n     def   eq   self  other           if not isinstance other  Data               return False         if self n    other n              return False         return True     def   hash   self           return self n      myList     Data 1   Data 2   Data 3        Data 2  in myList      gt  True     Data 5  in myList      gt  False

User · Answer

Simple  Elegant  and Powerful   A generator expression in conjuction with a builtin     python 2 5    any x for x in mylist if x n    10    Uses the Python any   builtin  which is defined as follows      any iterable  - gt     Return True if any element of the iterable is true  Equivalent to    def any iterable       for element in iterable          if element              return True     return False

User · Answer

You can use in to look for an item in a collection  and a list comprehension to extract the field you are interested in  This  works for lists  sets  tuples  and anything that defines   contains   or   getitem       if 5 in  data n for data in myList       print  Found it    See also    Contains Method In operation

User · Answer

Just for completeness  let s not forget the Simplest Thing That Could Possibly Work   for i in list    if i n    5         do something with it      print  YAY  Found one

[python] Searching a list of objects in Python

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