I am testing a reverse function. My code is as follow:
def deep_reverse(L):
new_list = []
for element in L:
new_list += [element[::-1]]
L = new_list[::-1]
L = [[1, 2], [3, 4], [5, 6, 7]]
deep_reverse(L)
print(L)
I expect my output L can be like [[7, 6, 5], [4, 3], [2, 1]], but it only works inside my function.
I wasn't able to change it in the global variable. How can I make it happen? Thank you
L is just a local variable, and assigning a new object to it won't change the original list; all it does is point the name L in the function to another object. See Ned Batchelder's excellent explanation about Python names.
You could replace all elements in the list L references by assigning to the [:] identity slice instead:
L[:] = new_list[::-1]
However, mutable sequences like lists have a list.reverse() method, so you can just reverse the list in-place with that:
L.reverse()
Do the same for each sublist:
def deep_reverse(L):
""" assumes L is a list of lists whose elements are ints
Mutates L such that it reverses its elements and also
reverses the order of the int elements in every element of L.
It does not return anything.
"""
for element in L:
element.reverse()
L.reverse()
