How do I convert this list sort function from Python 2 to Python 3

Question

When I try to port it it errors out asking for key2

Python 2:

def SortItems(self,sorter=cmp):
    items = list(self.itemDataMap.keys())
    items.sort(sorter)
    self.itemIndexMap = items
    self.Refresh()

Python 3:

try:
    cmp
except NameError:
    def cmp(x, y):
        if x < y:
            return -1
        elif x > y:
           return 1
        else:
            return 0

def SortItems(self,sorter=cmp):
    items = list(self.itemDataMap.keys())
    items.sort(key=sorter)
    self.itemIndexMap = items
    self.Refresh()

Getting the error:

items.sort(key=sorter)
TypeError: __ColumnSorter() missing 1 required positional argument: 'key2'

It looks like lambda function needs second argument Any idea how to make it work?

Also tried functools.cmp_to_key:

def SortItems(self):
    import locale
    items = list(self.itemDataMap.keys())
    items= sorted(items, key=cmp_to_key(locale.strcoll)) 
    self.itemIndexMap = items   
    self.Refresh()

Getting error:

    items= sorted(items, key=cmp_to_key(locale.strcoll))
TypeError: strcoll() argument 1 must be str, not int

Probably because I'm sorting integers not strings

How do I make it work for int?

Daniel Roseman · Accepted Answer · 2017-03-16 14:20:32Z

1

cmp and key are fundamentally different. However there is a conversion function you can use: functools.cmp_to_key().

answered Mar 16, 2017 at 14:20

Daniel Roseman

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2 Comments

olekb Over a year ago

Tried it, gives different error. TypeError: strcoll() argument 1 must be str, not int. I'm sorting integers. Any idea how to use it for integers?

Daniel Roseman Over a year ago

What? Why are you using the strcoll function to compare integers? I don't understand what you are trying to do.

parsethis · Accepted Answer · 2017-03-16 15:00:07Z

From the docs Python3 list.sort():

sort() accepts two arguments that can only be passed by keyword (keyword-only arguments)

key specifies a function of one argument that is used to extract a comparison key from each list element (for example, key=str.lower).

That is, the key callable only takes a single argument in py3. So in this case doing

items.sort(int), or equivalently items.sort(lambda x: x)

will sort a list of int in ascending order.

In general cmp should return the comparison property of the each element of the list.

def cmp(x):
   # code to compute comparison property or value of x
   # eg. return x % 5

Additionally, you can convert the python2 cmp function:

The functools.cmp_to_key() utility is available to convert a 2.x style cmp function to a key function.

https://docs.python.org/3/library/stdtypes.html#list.sort

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