如何使用 python 中的 function 连续扩展列表？

Question

as the title says I have the problem that I can't do a function, where the list is expanding continuously to some value. I need this for a bigger program I'm writing.

Here are two examples that don't work. first one:

from random import *
import time

A = []

def list_expander(A):
    A = A + [randint(-5,5)]
    print (A)

while True:
    list_expander(A)
    time.sleep(2)

and second one:

from random import *
import time


def list_expander():
    A = []
    A = A + [randint(-5,5)]
    print (A)

while True:
    list_expander()
    time.sleep(2)

thank you for your help!

Answer 1

from random import *
import time
def list_expander(A):
    A.append(randint(-5,5))
    print (A)
    return A
A=[]
while True:
    A=list_expander(A)
    time.sleep(2)

Answer 2

To modify an immutable item (for example a list ), you can use its mutating methods ( .append() in this case). So in your first example if you replace A = A + [randint(-5,5)] with A.append(randint(-5, 5)) , you should get what you want. Your first example does not work because the function creates a "new" A every time you call it, it does not "change" the outside list A . The second one obviously won't work either because of the same reason, and also the fact that it reinitialises A with an empty list every time it's called ( A = [] ).

All in all, I would rewrite your code as:

from random import randint
from time import sleep

A = []

def list_expander(A):
    A.append(randint(-5,5))
    print(A) # are you sure you need this?

while True:
    list_expander(A)
    time.sleep(2)

You can read How do I pass a variable by reference? to better understand why your first example doesn't change the list A .

Answer 3

As far as I understand you want to keep appending to the list, so you will have to return it so that you can append to it (extend) again next iteration.

from random import *
import time

def list_expander(A):
    A.append(randint(-5,5))
    print (A)

A = []
while True:
    list_expander(A)
    time.sleep(2)
    x+=1

This code will print

[1]
[1, -5]
[1, -5, 4]
[1, -5, 4, 5]
[1, -5, 4, 5, 2]

Another approach you can take is to have the list as a global variable but keep in mind it's bard practice.