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Troubleshooting Question

# deleting contents of one list from another

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Is it possible in python 3 to delete the contents of one list from another list ? and what is the best way to do it ?

eg
a = [1,2,3,4]
b = [1,2,3,4,5,6]
delete a from b to get c = [5,6]

thankyou
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Software Engineer
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Distinguished Expert 2019

Commented:
Try this:
```a = [1,2,3,4]
b = [1,2,3,4,5,6]
for e in a:
b.remove(e)
print(b)```
Principal: IT
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Commented:
This one is on us!
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Commented:
@Brent: This is not a good solution. Fistly, the set does not preserve the order of items. Say, if 1 was at the end of the second list, it would also be removed. Secondly, the set absorbs multiple instances of the same value. Say, if the second list contained multiple 5, then only one would be present after the actions.

Noci's solution is correct if one occurence of the element in the first list is to be removed from the second list. The list.remove() removes only the the first occurence. So, it depends on what is needed.

There is one more possibility that leads to another kind of solution. If the first list is the prefix of the second list, then the second list can be cut using the slicing operator:
``````a = [1,2,3,4]
b = [1,2,3,4,5,6]
if len(a) <= len(b) and b[0:len(a)] == a:
b = b[len(a):]
print(b)
``````
The if just show testing whether a is the prefix of b. The b[len(a):] meas get the part of b from index len(a) to the end.
Senior Developer
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Commented:
hm, as the OP's question is not complete. I would have expected "remove all occurances":

``````elementsToRemove = [1,2,3,4]
sourceList = [1,2,3,4,5,6,4,5]

resultList = list(
filter(
lambda sourceItem : (sourceItem not in elementsToRemove),
sourceList)
)

print(resultList)
``````
gives us

``````[5,6,5]
``````

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