将列表转换为有序元组字典

Question

I've used the following approach many times to generate a list of tuples from the contents of a dictionary:

dispositions = list(dispositions.items())

In this case, the keys are different types of ways a patient can leave the emergency department, and the values are counts of those types. Now, I wanted to sort this data based on the second item in each of the tuples, so I tried this:

dispositions = list(dispositions.items()).sort(key=lambda x: x[1])

To my surprise, when I ran the code, I found that dispositions had been set to None. I tried breaking it into two parts, as follows:

dispositions = list(dispositions.items())
dispositions.sort(key=lambda x: x[1])

This works. I got my list of sorted tuples, So, I've already solved my problem. but I want to know why the first option didn't work so that I can be a better person (better programmers are better people)? Can anyone help me out here?

Answer 1

list.sort does not return anything. So this:

dispositions = list(dispositions.items()).sort(key=lambda x: x[1])

sets dispositions to None .

sorted on the other hand returns a new list. So you would use:

dispositions = sorted(dispositions.items(), key=lambda x: x[1])

Which will set dispositions to a sorted list of tuples.

Answer 2

The sort function doesn't work like you might think. It does not return a sorted list, it directly modifies the list so it's sorted.

For instance, say you have this code:

a = [0, 2, 1]
a.sort()

a will now be changed [0, 1, 2] . Had I used a = a.sort() rather than just a.sort() , it would have been set to None .

This:

dispositions = list(dispositions.items()).sort(key=lambda x: x[1])

functions the same as this:

dispositions = list(dispositions.items())
dispositions = dispositions.sort(key=lambda x: x[1])

Which sets dispositions to the returned value from sort , which is None .

On the other hand, this:

dispositions = list(dispositions.items())
dispositions.sort(key=lambda x: x[1])

is not setting dispositions to the value of sort , it's simply calling the sort function on dispositions , which causes the list to be sorted as intended.