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)
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'}]
You can use filter:
array2 = filter(lambda x: x['id'] not in array1, array2)
array2 = filter(lambda x: x['id'] not in array1, array2) otherwise: AttributeError: 'dict' object has no attribute 'id'
filter is a generator in Python 3, so you'd need to wrap this in a list call if you needed an actual list. If you are just going to iterate over it once, then it doesn't matter (unless something else mutates array1 or array2 before you do the iteration).filter is a good tool:) Thanks anyway, although 99% of the time I use list comprehensions with if statements because I forget that there is a filter() function for this.. and it turned out, thanks to you, that it is more pythonic. Sweet.You're modifying a list while iterating over it. Instead iterate over a copy of the list
for item in array2[:]: # [:] creates a copy
...
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 [:]
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)
[:] here?