How to Safely Remove Items from a List While Iterating in Python?

Iterative Removal of Items from a List

Problem

Iterating through a list of tuples in Python, attempting to remove elements based on certain criteria:

for tup in somelist:
    if determine(tup):
         # How to remove 'tup'?

Solution

To remove items while iterating, one approach involves creating a new list containing only the desired elements:

somelist = [x for x in somelist if not determine(x)]

Alternatively, assigning to the slice somelist[:] mutates the existing list:

somelist[:] = [x for x in somelist if not determine(x)]

For direct mutation, the itertools module provides the filterfalse function:

• Python 2:

from itertools import ifilterfalse
somelist[:] = ifilterfalse(determine, somelist)

• Python 3:

from itertools import filterfalse
somelist[:] = filterfalse(determine, somelist)

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