Python - Appending to a list within an object within a list of objects

Question

Take the following example:

from pprint import pprint
class ResultsClass:
    def __init__(self, 
    name = None, 
    counts = []): 
        self.name = name
        self.counts = counts

objs = [ResultsClass() for i in range(5)]
i = 0 
while i < 4:
    objs[i].counts.append("count is " + str(i))
    pprint(objs[i].counts)
    i = i+1

The results are:

['count is 0']
['count is 0', 'count is 1']
['count is 0', 'count is 1', 'count is 2']
['count is 0', 'count is 1', 'count is 2', 'count is 3']

I have also tried the following code to no avail:

i = 0
objs = [ResultsClass() for i in range(5)]
for obj in objs:
    obj.counts.append("count is " + str(i))
    pprint(obj.counts)
    i = +1

What I want to do is have each instance of ResultsClass with a different counts list, like so:

['count is 0'] 
['count is 1'] 
['count is 2'] 
['count is 3']

Coming from a PowerShell background, I would have thought Python would iterate through each instance of the object and append to that specific list therein; instead it seems to be appending to the list of objects and all instances of the counts property.

What's the best way of achieving the desired result?

score 0 · Answer 1 · answered Dec 12 '19 at 11:29

0

from pprint import pprint
class ResultsClass:
    def __init__(self, 
    name = None): 
        self.name = name
        self.counts = []

objs = [ResultsClass() for i in range(5)]
i = 0 
while i < 4:
    objs[i].counts.append("count is " + str(i))
    pprint(objs[i].counts)
    i = i+1

answered Dec 12 '19 at 11:29

Jyoti Arora

94
7

What have you done here that is different from the OP? – quamrana Dec 12 '19 at 12:18
Yes In original post, counts is mutable. and i am initialising it in __init__. – Jyoti Arora Dec 13 '19 at 07:55
Thanks, @JyotiArora - that works; but why does initialising it in *init* work when it's not previously initialised within the brackets? – RobTheRobot16 Dec 13 '19 at 12:11
Read this: http://effbot.org/zone/default-values.htm – Jyoti Arora Dec 16 '19 at 06:08

Python - Appending to a list within an object within a list of objects

1 Answers1