I refer to the solution given for How to sort (list/tuple) of lists/tuples?.
sorted_by_second = sorted(data, key=lambda tup: tup[1])
or
data.sort(key=lambda tup: tup[1]) # sorts in place
Where is tup passed to the lambda function in either sort function?
tup is the argument to lambda.
lambda tup: tup[1]
Is equivalent to:
def anonymous_func(tup):
return tup[1]
By sort itself. The key parameter is a callback function which sort calls on each element of the sequence it is sorting. It lets you write something like the following in a more concise manner:
new_data = [(tup[1], tup) for tup in data]
tmp = sorted(data)
sorted_by_second = [orig for _, orig in tmp]
In the lambda function lambda tup: tup[1], tup refers to the element of list data, and tup[1] refers to the second element of the item.
When you pass this to sorted function, you tell it to use the second element of the item in the list as a comparison key, so the result you get is the sorted list on the second element
From the docs
key specifies a function of one argument that is used to extract a comparison key from each element in iterable (for example, key=str.lower). The default value is None (compare the elements directly).
Hence the behaviour is as follows
In [54]: data = [[4,5,6], [1,2,3], [7,8,9]]
In [55]: sorted_by_second = sorted(data, key=lambda tup: tup[1])
In [56]: sorted_by_second
Out[56]: [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
Note that you can also use operator.itemgetter to do the same thing, which automatically picks the second element from the tuple
In [60]: import operator
In [64]: data = [[4,5,6], [1,2,3], [7,8,9]]
In [65]: sorted_by_second = sorted(data, key=operator.itemgetter(1))
In [66]: sorted_by_second
Out[66]: [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
Here key is a transform function applied to each element of the array to provide a new value, which will then be used to sort the array. Essentially this makes it such that an element should be sorted before an element based if key(a) < key(b) rather than a < b.
The list.sort and sorted functions will call key for each element individually.
