Why python does not replace the new value of a list by iteration?

Question

I am wondering why my for loop is not updating the old_hosts values and why is updating the workers in the following code? I am NOT assigning any new value to the workers variable!!!

workers = [['w1', 1],['w2',2]]
old_hosts = [['w1',-1],['w1',-11]]

for old_host in old_hosts:
    old_host = [*(worker for worker in workers if worker[0]==old_host[0])][0]
    old_host[1]= 3
    print('old_host: ' + str(old_host))
print('workers: ' + str(workers))
print('old_hosts: ' + str(old_hosts))

output:

old_host: ['w1', 1]
old_host: ['w1', 1]
workers: [['w1', 3], ['w2', 2]]
old_hosts: [['w1', -1], ['w1', -11]]

Please don't edit your question to a different topic after valid answers have been posted. You can always post a new question. — Michael Szczesny, Sep 05 '21 at 11:07
To answer your new question: `old_host` is a reference to the current worker from `workers` list in your loop. When you're changing an element in `old_host` you are changing the original list. — Michael Szczesny, Sep 05 '21 at 11:14
thank you. how can I detach them from each other, so if old_host gets a copy of the value from workers? — Mohammad Sadegh Aslanpour, Sep 06 '21 at 03:05

I'mahdi · Answer 1 · 2021-09-05T10:38:01.030

3

because in for-loop you create a local variable and assign your array to a local variable and you don't change your old_hots at all.

For changing your array you need index and assign what you want to this index. for this purpose I use enumerate like below:

workers = [['w1', 1],['w2',2]]
old_hosts = [['w1',-1],['w1',-11]]

for idx , old_host in enumerate(old_hosts):
    old_hosts[idx] = [*(worker for worker in workers if worker[0]==old_host[0])][0]
print('workers: ' + str(workers))
print('old_hosts: ' + str(old_hosts))

Output:

workers: [['w1', 1], ['w2', 2]]
old_hosts: [['w1', 1], ['w1', 1]]

edited Sep 05 '21 at 10:38

answered Sep 05 '21 at 10:31

I'mahdi

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thank you, just I edited the question and am asking why the workers value is changed? – Mohammad Sadegh Aslanpour Sep 05 '21 at 10:59
@MohammadSadeghAslanpour welcome. this question is closing by other. maybe this is better ask another question. if this is correct please read this link : https://meta.stackexchange.com/questions/5234/how-does-accepting-an-answer-work – I'mahdi Sep 05 '21 at 11:01

Michael Szczesny · Answer 2 · 2021-09-05T10:47:48.623

Your code was nearly correct. You have to assign to the list element you want to change not the variable. Your solution will break if old_hosts contains a worker id that is not in workers. I removed the redundant generator expression.

workers = [['w1', 1],['w2',2]]
old_hosts = [['w1',-1],['w1',-11]]

for old_host in old_hosts:
    old_host[1] = [worker[1] for worker in workers if worker[0]==old_host[0]][0]
    print('old_host: ' + str(old_host))
print('workers: ' + str(workers))
print('old_hosts: ' + str(old_hosts))

Output

old_host: ['w1', 1]
old_host: ['w1', 1]
workers: [['w1', 1], ['w2', 2]]
old_hosts: [['w1', 1], ['w1', 1]]

score 1 · Answer 3 · edited Sep 05 '21 at 10:52

1

print('old_hosts: ' + str(old_hosts))

check this last line, change to

print('old_hosts: ' + str(old_host))

edited Sep 05 '21 at 10:52

Michael Szczesny

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answered Sep 05 '21 at 10:45

Yasmin

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Please format your code in your answer to make it easier to read. – Michael Szczesny Sep 05 '21 at 10:54

Why python does not replace the new value of a list by iteration?

3 Answers3