Oct 31, 2008 · For example, if the list is an attribute of an object, self.l1 += l2 and self.l1.extend(l2) have identical performance on my Python 3.6 install, simply because real operation is more like self.l1 = self.l1.__iadd__(l2), which means it must perform a moderately expensive STORE_ATTR that self.l1.extend(l2) doesn't have to.

╰─ time ./list_plus.py ./list_plus.py 36.03s user 6.39s system 99% cpu 42.558 total ╰─ time ./list_extend.py ./list_extend.py 0.03s user 0.01s system 92% cpu 0.040 total The first script also uses over 200MB of memory, while the second one doesn't use any more memory than a 'naked' python3 process.

0.520906925201 #append 0.602569103241 #extend-list 0.357008934021 #extend-tuple And the same ordering of the results my linux box (Ubuntu, x86-64 core i7): 0.307395935059 #append 0.319436073303 #extend-list 0.238317012787 #extend-tuple

May 2, 2015 · The function extend is an in-place function i.e. it will make the changes to the original list itself. From the docs. Extend the list by appending all the items in the given list; equivalent to a[len(a):] = L. Since it returns None, you should not re-assign it back to the list variable. You can do

Nov 20, 2013 · list.extend() extends a list in place.Python standard library methods that alter objects in-place always return None (the default); your list comprehension executed a.extend() twice and thus the resulting list consists of two None return values.

Feb 16, 2010 · You would've re-bound the first element to a new list containing the element "1", and you would've got your expected result. Containers in python store references, and it's possible in most sequence containers to reference the same item multiple times. A list can actually reference itself as an element, though the usefulness of this is limited.

Sep 12, 2011 · from matplotlib.cbook import flatten a_list.extend(flatten(fun(item) for item in a_list)) should work but I do not want my code to depend on matplotlib. for item in a_list: a_list.extend(fun(item)) would be nice enough for my taste but seems to cause an infinite loop.

Dec 3, 2016 · A list of lists named xss can be flattened using a nested list comprehension:. flat_list = [ x for xs in xss for x in xs ]

Apr 26, 2016 · If you look at the docstring for extend, you will see that it can be extended by any iterable: """ L.extend(iterable) -> None -- extend list by appending elements from the iterable """ This means you are not only limited to list s or tuple s, but also string s, set s, and other iterables:

Mar 18, 2013 · Ruby has something like this where if you actually want to change the original list you'd do something like: li.extend!([5,6,7]) otherwise it would just give you the result without mutating the original. Does this same thing exist in Python?