Thomas Johnson Thomas Johnson - 2 months ago 4
Python Question

Remove the first N items that match a condition in a Python list

If I have a function

matchCondition(x)
, how can I remove the first
n
items in a Python list that match that condition?

One solution is to iterate over each item, mark it for deletion (e.g., by setting it to
None
), and then filter the list with a comprehension. This requires iterating over the list twice and mutates the data. Is there a more idiomatic or efficient way to do this?

n = 3

def condition(x):
return x < 5

data = [1, 10, 2, 9, 3, 8, 4, 7]
out = do_remove(data, n, condition)
print(out) # [10, 9, 8, 4, 7] (1, 2, and 3 are removed, 4 remains)

Answer

One way using itertools.filterfalse and itertools.count:

from itertools import count, filterfalse

data = [1, 10, 2, 9, 3, 8, 4, 7]
output = filterfalse(lambda L, c=count(): L < 5 and next(c) < 3, data)

Then list(output), gives you:

[10, 9, 8, 4, 7]
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