Sorting a list containing objects of a class in python

Question

I have the following code, where I use a class stat to keep my data. Objects of type stat are inserted in a list. However, when I try to call the method printStats, I get the error, AttributeError: stat instance has no attribute 'printStats'. Secondly I want to know how can I sort the list containing objects of stat. My sorting should be based on the blocks field of stat.

fi = open( 'txt/stats.txt', 'r' )
fo = open( 'compile/stats.txt', 'w' )
list = []

class stat():
    def __init__(self, fname, blocks, backEdges):
        self.fname = fname
        self.blocks = blocks
        self.backEdges = backEdges
    def printStats(self):
        print self.fname + str(self.blocks) + str(self.backEdges)

while True:
    line_str = fi.readline()
    if line_str == '':
        break

    str = line_str.split()
    list.append( stat(str[0], int(str[1]), int(str[2])) )

for item in list:
    item.printStats() # <-- problem calling this

Answer 1

As far as the sorting is concerned, you definitely can use the key function:

import operator
lst.sort(key=lambda x: x.blocks)
lst.sort(key=operator.attrgetter('blocks') ) #alternative without lambda.

However, if you want to be able to compare stats objects in a non-sorting context, you can override __eq__,__gt__,__lt__ (and to make your life easier, you can use the functools.total_ordering class decorator to define most of the comparisons for you):

import functools
@functools.total_ordering
class stats(object): #inherit from object.  It's a good idea
    def __init__(self, fname, blocks, backEdges):
        self.fname = fname
        self.blocks = blocks
        self.backEdges = backEdges
    def printStats(self):
        print self.fname + str(self.blocks) + str(self.backEdges)
    def __eq__(self,other):
         return self.blocks == other.blocks
    def __lt__(self,other):
         return self.blocks < other.blocks

with stats defined this way, sorting should again be as simple as:

lst.sort()  #or if you want a new list:  new_lst = sorted(lst)

Answer 2

list.sort(key= lambda x:x.blocks)

example:

>>> a=stat('foo',20,30)
>>> a.printStats()
foo2030
>>> b=stat('foo',15,25)
>>> c=stat('foo',22,23)
>>> lis=[a,b,c]
>>> lis.sort(key= lambda x:x.blocks)
>>> '  '.join(str(x.blocks) for x in lis)   #sorted
'15  20  22'

Answer 3

The function printStats is not actually part of your stat class because it uses tabbed indentation while the rest of the class uses space indentation. Try print dir(stat) and you'll see that printStats is absent. To fix this, change the tabbing style so that it is consistent throughout the class.

You should also look at this line:

str = line_str.split()

You are overwriting the built-in type str with your own list. As a result, you can no longer use str to convert things into strings. When you do successfully call printStats, it will give you a TypeError. Change the name of your str variable to something else.