Posted By: Anonymous
I’m iterating over a list of tuples in Python, and am attempting to remove them if they meet certain criteria.
for tup in somelist: if determine(tup): code_to_remove_tup
What should I use in place of
code_to_remove_tup? I can’t figure out how to remove the item in this fashion.
You can use a list comprehension to create a new list containing only the elements you don’t want to remove:
somelist = [x for x in somelist if not determine(x)]
Or, by assigning to the slice
somelist[:], you can mutate the existing list to contain only the items you want:
somelist[:] = [x for x in somelist if not determine(x)]
This approach could be useful if there are other references to
somelist that need to reflect the changes.
Instead of a comprehension, you could also use
itertools. In Python 2:
from itertools import ifilterfalse somelist[:] = ifilterfalse(determine, somelist)
Or in Python 3:
from itertools import filterfalse somelist[:] = filterfalse(determine, somelist)
For the sake of clarity and for those who find the use of the
[:] notation hackish or fuzzy, here’s a more explicit alternative. Theoretically, it should perform the same with regards to space and time than the one-liners above.
temp =  while somelist: x = somelist.pop() if not determine(x): temp.append(x) while temp: somelist.append(templist.pop())
It also works in other languages that may not have the replace items ability of Python lists, with minimal modifications. For instance, not all languages cast empty lists to a
False as Python does. You can substitute
while somelist: for something more explicit like
while len(somelist) > 0:.