Confusion Using defaultdicts with assignments and without assignments

Question

I'm trying to under behaviour of using defalultdict with and without assignments in python

data is as follows :

data = {'APPLaunch_ftrace': [63.3, 24.5, 8.4, 2.3, 0.9, 0.3, 0.2, 0.0], '1080pEncode_ftrace': [22.9, 27.6, 21.3, 16.1, 12.1, 0.0, 0.0, 0.0]}

Please help me understand difference of the following :

d = {}
for k,v in data.iteritems():
        for i in range(0,8):
            d['cores{0}'.format(i)] = d.setdefault('cores{0}'.format(i),0) + v[i] * 2

print d
{'cores2': 59.400000000000006, 'cores3': 36.800000000000004, 'cores0': 172.39999999999998, 'cores1': 104.2, 'cores6': 0.4, 'cores7': 0.0, 'cores4': 26.0, 'cores5': 0.6}

and following :

d = {}
for k,v in data.iteritems():
        for i in range(0,8):
            d.setdefault('cores{0}'.format(i), 0) + v[i] *2

126.6
49.0
16.8
4.6
1.8
0.6
0.4
0.0
45.8
55.2
42.6
32.2
24.2
0.0
0.0
0.0
>>> print d
{'cores2': 0, 'cores3': 0, 'cores0': 0, 'cores1': 0, 'cores6': 0, 'cores7': 0, 'cores4': 0, 'cores5': 0}

Answer 1

In first code in line:

d['cores{0}'.format(i)] = d.setdefault('cores{0}'.format(i),0) + v[i] * 2

you real do three jobs

1. in part d.setdefault('cores{0}'.format(i),0) you make dict keys with names and with values 0.

2. next you sum this entry's value (0) with v[i]*2

3. and last you assign this summation value to d dict's entry with key=='cores{0}'.format(i)

this process repeats 8 times for each entry of data.

But

In second code you have:

d.setdefault('cores{0}'.format(i), 0) + v[i] *2

it is similar to your first code but you dont have assignation part ( part 3 ). you just do part 1 and 2 but not part 3.

so you really do:

1. in part d.setdefault('cores{0}'.format(i),0) you make dict keys with names and with values 0.

2. next you sum this entry's value (0) with v[i]*2

this summation occures but its value doesn't save on d dict , it is just show on console output and nothing else.