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.

Answer 1

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