CODEWITHSUNDEEP
pythonpython-3.xdictionary
Sasuke Uchiha
Sasuke Uchiha

Reputation: 900

how to create a new dictionary in for loop?

In python 3.5.1

og_dict = {'name':'user1', 'salary':'1k'}
og_dict_list =[]
for i in range(2,5):
    og_dict['salary'] = str(i)+'k'
    og_dict_list.append(og_dict)
for og_dict_obj in og_dict_list:
    print(og_dict_obj)

Above code produces following output :

{'name': 'user1', 'salary': '4k'}
{'name': 'user1', 'salary': '4k'}
{'name': 'user1', 'salary': '4k'}

But I am expecting below output :

{'name': 'user1', 'salary': '2k'}
{'name': 'user1', 'salary': '3k'}
{'name': 'user1', 'salary': '4k'}

Apparantely python modifies the existing dictionary only and does not create a new one. What can I do to solve this issue?

Is there exist a way to create a new dictionary object from existing one? Is there any other collection which is more suitable for this?

Upvotes: 3

Views: 16183

Answers (2)

Henin RK
Henin RK

Reputation: 298

og_dict_list =[]
for i in range(2,5):
    og_dict = {'name':'user1', 'salary':str(i) +'k'}
    og_dict_list.append(og_dict)

for og_dict_obj in og_dict_list:
    print(og_dict_obj)`enter code here

There is always a better way to do this. You can also use the update() method in Python to add new items to dictionaries.

Upvotes: 0

Bakuriu
Bakuriu

Reputation: 101919

As has been pointed out, it doesn't matter what you do, when in the loop you have:

for ob_dict_obj in ob_dict_list:
    print(ob_dict)    # wrong!

it always print the value for ob_dict and doesn't matter what values you put in ob_dict_list.

This said the problem is that you are modifying the only dictionary. You want to create a new one for each iteration:

og_dict = {'name':'user1', 'salary':'1k'}
og_dict_list =[]
for i in range(2,5):
    new = og_dict.copy()
    new['salary'] = str(i)+'k'
    og_dict_list.append(new)
for og_dict_obj in og_dict_list:
    print(og_dict_obj)

There is a number of ways to copy a dictionary, like just doing dict(of_dict) instead of calling .copy(). However, AFAIK, there is no single method that allows you to copy & update a dictionary at the same time.

However there isn't much point in having the og_dict outside the loop at this point:

og_dict_list =[]
for i in range(2,5):
    og_dict = {'name':'user1', 'salary':str(i) +'k'}
    og_dict_list.append(og_dict)
for og_dict_obj in og_dict_list:
    print(og_dict_obj)

To answer your broader question: the kind of behaviour you are looking for is typical of functional languages which have persistent data structures. However in imperative languages having an operation that copies & updates would be a simple way to shoot yourself on the foot by creating tons of unwanted copies.

All built-in cointainers act the same as dict: update methods simply modify the current object and you have to explicitly copy the object to avoid the side effects.

Upvotes: 8

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