I didn't find 2 surprising either, but I'm a little surprised you never see it. If you want to treat the args to a function as immutable, what can you do besides copy, modify, and return a new object?
You can directly produce a modified copy, rather than using a mutating operation to implement the modifications.
It should be noted that "return a modified copy" algorithms can be much more efficient than "mutate the existing data" ones. For example, consider the case of removing multiple elements from a list, specified by a predicate. The version of this code that treats the input as immutable, producing a modified copy, can perform a single pass:
def without(source, predicate):
return [e for e in source if not predicate(e)]
def remove_which(source, predicate):
i = 0
while i < len(source):
if predicate(source[i]):
# Each deletion requires O(n) elements to shift position.
del source[i]
else:
# The index increment must be conditional,
# since removing an element shifts the next one
# and that shifted element must also be considered.
i += 1Or if you do care about order, you can emulate the C++ "erase-remove" idiom, by keeping track of separate "read" and "write" positions in the source, iterating until "read" reaches the end, and only incrementing "write" for elements that are kept; and then doing a single `del` of a slice at the end. But this, too, is complex to write, and very much the sort of thing one chooses Python in order to avoid. And you do all that work, in essence, just to emulate what the list comprehension does but in-place.
It will also probably be significantly slower than just copying the vector.