如何在列表理解中执行作业？

Question

So while creating test cases for tress in Python 3.6, I was wondering if there is a good way to perform actions such as assignment in the list comprehension. Should I use lambda?

I want to insert

Prev_Node.next = Node(i)
Prev_Node = Node(i)

into

iList = [1,2,3,4,10]
Prev_Node = Node(iList[0])
l1 = [Node(i) <Any assignment action?> for i in iList[1:])]

Thank you for your time in advance,

Answer 1

Use iterators to traverse your list, don't fiddle around with .Next etc to build your own.

If you're looking for list head and tail, then you could do something like:

int_list = [1, 2, 3, 4, 10]
node_list = [Node(i) for i in int_list]
head = node_list[0]  # Get the head Node and leave it in the list.
head, *tail = node_list  # Get the head Node and remove it from the list.

and iterate like

for node in node_list:
    print(node)

Take a look at the various list function on how to further manipulate an existing list.

Answer 2

You can't do assignments within a list comprehension. Trying to do so raises a `SyntaxError, eg:

[x = 0 for _ in range(1)]
# SyntaxError: invalid syntax

I suppose an awful hack to try to achieve this could be to define a function which you call in the list comprehension which does the assignment (using global variables). But this would be much worse than just using a normal for loop instead of a list comprehension. Basically, any time that using a list comprehension doesn't work right away, you're probably better off with a for loop. It'll be easier for you to program and for others to understand.

# hacky; don't do this
def set_x(val):
    global x
    x = val

[set_x(i) for i in range(3)]
x # 2