CODEWITHSUNDEEP
python
Setsuna
Setsuna

Reputation: 2141

How to delete from list?

Assuming I have the following list:

array1 = ['A', 'C', 'Desk']

and another array that contains:

array2 = [{'id': 'A', 'name': 'Greg'}, 
{'id': 'Desk', 'name': 'Will'},
{'id': 'E', 'name': 'Craig'},
{'id': 'G', 'name': 'Johnson'}]

What is a good way to remove items from the list? The following does not appear to work

for item in array2:
   if item['id'] in array1:
     array2.remove(item)

Upvotes: 2

Views: 201

Answers (5)

Ashwini Chaudhary
Ashwini Chaudhary

Reputation: 251116

for item in array2:
   if item['id'] in array1:
     array2.remove(item)

A for loop keeps track of the index during the iterations.

Iteration 1 : Index 0

item is assigned {'id': 'A', 'name': 'Greg'} as it is at index 0.

As 'A' as found in array so you removed this entry from the list.

Due to removal of that item all items to it's right actually shift one place to the left.

So, now {'id': 'Desk', 'name': 'Will'} is at index 0.

Iteration 2 : Index value changed to 1

This time item = {'id': 'E', 'name': 'Craig'} as this is at index 1.

So, as you can see {'id': 'Desk', 'name': 'Will'} was skipped (was never assigned to item) and will not be removed as you expected.

A quick solution is to use a shallow copy of the list for iteration:

for item in array2[:]:  #notice the [:]

Upvotes: 2

Jon Clements
Jon Clements

Reputation: 142216

Similar to Paulo's answer, but converting to a set first within the filter:

array2[:] = filter(lambda L, S=set(array1): L['id'] not in S, array2)

Upvotes: 0

Paulo Scardine
Paulo Scardine

Reputation: 77359

You can use filter:

array2 = filter(lambda x: x['id'] not in array1, array2)

Upvotes: 4

sberry
sberry

Reputation: 132098

You could also use a list comprehension for this:

>>> array2 = [{'id': 'A', 'name': 'Greg'},
... {'id': 'Desk', 'name': 'Will'},
... {'id': 'E', 'name': 'Craig'},
... {'id': 'G', 'name': 'Johnson'}]
>>> array1 = ['A', 'C', 'Desk']
>>> filtered = [item for item in array2 if item['id'] not in array1]
>>> filtered
[{'id': 'E', 'name': 'Craig'}, {'id': 'G', 'name': 'Johnson'}]

Upvotes: 7

arshajii
arshajii

Reputation: 129537

You're modifying a list while iterating over it. Instead iterate over a copy of the list

for item in array2[:]:  # [:] creates a copy
    ...

Upvotes: 3

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