python: Can't make list.sort to work

Question

def sort_line(line1,line2):

    for x1,y1,x2,y2 in line1:
        mn1 = min(x1,x2)
    for x1,y1,x2,y2 in line2:
        mn2 = min(x1,x2)
    return mn1 < mn2 

lines.sort(sort_line)

Here lines has 4 int in every row. From what I understood from python wiki is that I need to pass a compare function as argument in the sort() function. But I get the following error,

<ipython-input-107-b0e2f3c756cf> in draw_lines(img, lines, color, thickness)
     69     """
     70 
---> 71     lines.sort(sort_line)
     72 
     73     for line in lines:

TypeError: an integer is required (got type function)

I tried with sorted() but couldn't make it work either.

Python and anaconda version

Python 3.5.2 |Anaconda 4.2.0 (64-bit)| (default, Jul 5 2016, 11:41:13) [MSC v.1900 64 bit (AMD64)] on win32

I'm running the code in jupyter notebook.

My implementation I think is same as the following question but mine doesn't work.

Custom Python list sorting

After following Martijn's answer I modified my code to this

lineList = lines.tolist()  //lines is a numpy array so I converted it to list
print ('lineList: ',lineList)
lineList.sort(key=lambda l: min(l[0], l[2]))
lines = np.array(lineList)

Gives me the following error

<ipython-input-115-06412e8f5aba> in <lambda>(l)
     72     lineList = lines.tolist()
     73     print ('lineList: ',lineList)
---> 74     lineList.sort(key=lambda l: min(l[0], l[1]))
     75     lines = np.array(lineList)
     76 

IndexError: list index out of range

Answer 1

You are making several mistakes. You didn't read the documentation close enough; the cmp function option is gone in Python 3. You also implemented your cmp function incorrectly, and last but not least, you don't need to use a cmp function at all, you can use a key function to extract the minimum of the two exact values in the lists you are sorting.

You need to pay close attention to the section you appear to be reading, The Old Way Using the cmp Parameter:

In Py3.0, the cmp parameter was removed entirely (as part of a larger effort to simplify and unify the language, eliminating the conflict between rich comparisons and the __cmp__ methods).

The error message is a little confusing here, but list.sort() takes no positional arguments. From the list.sort() documentation:

sort(*, key=None, reverse=None)

The * indicates no positional arguments are accepted, and there is no cmp option.

Going back to the Sorting Howto:

When porting code from Python 2.x to 3.x, the situation can arise when you have the user supplying a comparison function and you need to convert that to a key function. The following wrapper makes that easy to do:

def cmp_to_key(mycmp):

[...]

In Python 2.7, the cmp_to_key() tool was added to the functools module.

and again from the list.sort() documentation:

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

You also incorrectly implemented your cmp function; there is no need to iterate, and you have to return -1, 0 or 1 to indicate relative order (so you would have to use cmp(min(list1), min(list2)) if there still was a built-in cmp() function).

However, you can sort your data without a (slow) cmp function. All you need to do is to extract the min() of the two values of each list, in a sort key:

lines.sort(key=lambda l: min(l[0], l[2]))

Answer 2

According to the documentation:

sort() accepts two arguments that can only be passed by keyword (keyword-only arguments): key and reverse.

The cmp parameter only exists in Python 2, it is generally considered as a deprecated usage.