Python, delete JSON element having specific key from a loop

Question

Using Python, I need to delete all objects in JSON array that have specific value of 'name' key. However, I can't do that from a loop.

Imaging I want to delete all items having 'bad' as name in the following input:

{
  'myArray' : [
    {
      'name' : 'good',
      'value' : '1'
    },
    {
      'name' : 'bad',
      'value' : '2'
    }
  ]
}

So I use the following Python test case:

myData = {'myArray': [{'name': 'good', 'value': '1'}, {'name': 'bad', 'value': '2'}]}

for a in myData['myArray']:
  if (a['name'] =='bad'):
    del a

print(json.dumps(myData))

And I see that the myData is not changed.

I assume this is because I try to delete an iterator of a loop, that might be considered as risky action by interpreter, however no runtime error or warning is reported by Python.

What's the recommended approach in this case?

Thanks!

"I assume this is because I try to delete an iterator of a loop, that might be considered as risky action by interpreter, however no runtime error or warning is reported by Python" You are not deleting the iterator. In fact, you don't have access to the iterator. You are simply doing del on a, which is the variable that the result of (implicitely) calling next on the (invisible) iterator is assigned to. So you delete the variable, but then on the next iteration, that variable is simply re-created. You want to mutate your collection, but you probably shouldn't while iterating — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Sep 4, 2018 at 21:43
Use a (nested) dict/list comprehension. Never try to delete elements of an object while iterating over it. — smci
– smci, Commented Sep 4, 2018 at 21:44
juanpa.arrivillaga, thank you, your explanation "that variable is simply re-created" brings the light here. I see one workaround: copy all elements except 'bad' to newly created collection, and then substitute it instead of old one. Not optimal solution but I'll use it if no easier approach is suggested by someone. — Dmytro
– Dmytro, Commented Sep 4, 2018 at 21:52

Dmytro · Accepted Answer · 2018-09-04 22:00:59Z

4

One of approaches from Remove element from list when using enumerate() in python - a loop over a copy of the list referred as [:] - works too:

for a in myData['myArray'][:]:
  if (a['name'] == u'bad'):
     myData['myArray'].remove(a)

Thanks everyone!

answered Sep 4, 2018 at 22:00

Dmytro

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smci · Accepted Answer · 2018-09-04 21:51:29Z

1

Use a (nested) dict/list comprehension. Never try to delete elements of an object while iterating over it

>>> [x for x in myData['myArray'] if x['name'] != 'bad']
[{'name': 'good', 'value': '1'}]

Assign the result to myData['myArray'], or whatever you want.

edited Sep 4, 2018 at 21:51

answered Sep 4, 2018 at 21:46

smci

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PARMESH · Accepted Answer · 2018-09-04 21:46:16Z

0

You can use the list comprehension. Below is the reference.

new_list =  [obj for obj in myData['myArray'] if(obj['name'] != 'bad')] 
print new_list

answered Sep 4, 2018 at 21:46

PARMESH

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Pedro · Accepted Answer · 2018-09-04 21:47:51Z

0

myData = {'myArray': [{'name': 'good', 'value': '1'}, {'name': 'bad', 'value': '2'}]}

myData_clean = [x for x in myData['myArray'] if x['name'] !='bad']
myData_clean = {'myArray':myData_clean}
print(myData_clean)

output:

{'myArray': [{'name': 'good', 'value': '1'}]}

answered Sep 4, 2018 at 21:47

Pedro

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