list comprehension in python with if and else

Question

after i test,

len_stat = [len(x) if len(x) > len_stat[i] else len_stat[i] for i, x in enumerate(a_list)]

is same with

for i, x in enumerate(a_list):
    if len(x) > len_stat[i]:
        len_stat[i] = len(x)

of course, but,

len_stat = [len(x) if len(x) > len_stat[a_list.index(x)] else len_stat[a_list.index(x)] for x in a_list]

is different with

for x in a_list:
    if len(x) > len_stat[a_list.index(x)]:
        len_stat[a_list.index(x)] = len(x)

later, i know <list>.index is a bad method used here.

but why they are different in last two examples?

---

my fault! here is my test code,

a_list = ['Bacteroides fragilis YCH46', 'YCH46', '20571', '-', 'PRJNA13067', 'FCB group', 'Bacteroidetes/Chlorobi group', 'GCA_000009925.1 ', '5.31099', '43.2378', 'chromosome:NC_006347.1/AP006841.1; plasmid pBFY46:NC_006297.1/AP006842.1', '-', '2', '4717', '4625', '2004/09/17', '2016/08/03', 'Complete Genome', 'ftp://ftp.ncbi.nlm.nih.gov/genomes/all/GCF_000009925.1_ASM992v1', 'ftp://ftp.ncbi.nlm.nih.gov/genomes/all/GCA_000009925.1_ASM992v1']

len_stat_1 = [0 for x in a_list]
len_stat_2 = [0 for x in a_list]
len_stat_3 = [0 for x in a_list]
len_stat_4 = [0 for x in a_list]

len_stat_1 = [len(x) if len(x) > len_stat_1[i] else len_stat_1[i] for i, x in enumerate(a_list)]

for i, x in enumerate(a_list):
    if len(x) > len_stat_2[i]:
        len_stat_2[i] = len(x)

len_stat_3 = [len(x) if len(x) > len_stat_3[a_list.index(x)] else len_stat_3[a_list.index(x)] for x in a_list]

for x in a_list:
    if len(x) > len_stat_4[a_list.index(x)]:
        len_stat_4[a_list.index(x)] = len(x)

print len_stat_1
print len_stat_2
print len_stat_3
print len_stat_4

output:

[26, 5, 5, 1, 10, 9, 28, 16, 7, 7, 72, 1, 1, 4, 4, 10, 10, 15, 63, 63]
[26, 5, 5, 1, 10, 9, 28, 16, 7, 7, 72, 1, 1, 4, 4, 10, 10, 15, 63, 63]
[26, 5, 5, 1, 10, 9, 28, 16, 7, 7, 72, 1, 1, 4, 4, 10, 10, 15, 63, 63]
[26, 5, 5, 1, 10, 9, 28, 16, 7, 7, 72, 0, 1, 4, 4, 10, 10, 15, 63, 63]

as you can see, the last two is different!

it really confused me.

Answer 1

This list comprehension:

len_stat = [len(x) if len(x) > len_stat[a_list.index(x)] else len_stat[a_list.index(x)] for x in a_list]

Is equivalent to:

temp = []
for x in a_list:
    temp.append(len(x) if len(x) > len_stat[a_list.index(x)] else len_stat[a_list.index(x)])
len_stat = temp

Which is equivalent to:

temp = []
for x in a_list:
    if len(x) > len_stat[a_list.index(x)]:
        val = len(x)
    else:
        val = len_stat[a_list.index(x)]
    temp.append(val)
len_stat = temp

Equivalent, of course, except for the temp list. The comprehension approach will replace len_stat with a new list, that will always be the same length as len_stat that will have either len(x) for every x in len_stat or will have len_stat[a_list.index(x)] . The for-loop you wrote will mutate len_stat based on the conditional, and it's hard to say exactly what will happen without knowing the content of your lists, but it could potentially change values of len_stat all over the place.

To be clear

I do not recommend this approach, but I am trying to illustrate how the comprehension is different than the for-loop.