Understanding lambda's sort function in python

Question

I am trying to sort the list of lists in python. I have written the following code:

def sort(intervals):
    if intervals == [] : return []  
    intervals.sort(key = lambda x:x.start)
    return intervals

a = [[1,3],[8,10],[15,18],[2,6]]
print(sort(a))

I am getting the following error:

AttributeError: 'list' object has no attribute 'start'

Please can someone explain lambda function for sort and some details about the above error. Thank you!!

Answer 1

The reason for your error message is that you are sorting based on an attribute that is not for list (start is not an attribute of a list'), so quick fix is, either to use the sort method of list or use built-in method sorted:

1 - Using sort method of list:

intervals.sort(key = lambda l:l[0])

2 - Using built-in method sorted:

intervals = sorted(intervals, key=lambda l:l[0])

Reading more about sorting list in this wiki post, very interesting.

Answer 2

You should use:

intervals.sort(key = lambda x:x[0])

lambda is a fast-way of making functions. For example,

def getFirst(x):
   return x[0]

is equal to:

getFirst = lambda x: x[0]

I guess you should read the official documentation.

PS: Be aware that you are making in place sorting. You can also use sorted(a, key=lambda x:x[0]) which returns another copy of sorted array, if you want otherwise.