创建列表的多个副本的最快方法

Question

Problem

I am working on a breadth-first search algorithm for my maze solver and it is working so far. I am keeping track of the current stack by copying the previous one and appending the current value to it.

Since copying list takes a great amount of time I would like to create multiple copies of the list within one operation.

What I've tried so far

• Copy the list and assign it to multiple variables.

   l = [1, 2, 3] a = b = c = l[:] # Just creates references and no individual lists 

• Use numpy arrays with copy function (faster than list[:] ).

Question

What is the fastest way to create multiple copies of one list?

Answer 1

Don't use ordinary Python lists for your stacks. Use Lisp-style linked lists, so your stacks share most of their structure with each other and building a new stack with an additional element is constant time:

def empty_stack():
    return ()
def push(stack, item):
    return (item, stack)
def stack_to_list(stack):
    l = []
    while stack:
        item, stack = stack
        l.append(item)
    return l[::-1]

Here, push runs in constant time and produces a new stack, without mutating the old, so you can call push on the old stack repeatedly without copying it.

Answer 2

You can just bulk copy it:

def copyList(thelist, number_of_copies):
    return tuple(thelist[:] for _ in xrange(number_of_copies))

a, b, c = copyList(range(10), 3)

Here you have a live example