Python Modifying List Within Function

I am trying to let a function modify a list by passing the list's reference to it. The program below shows when I pass a list into the function, only a local variable is generated.

Solution 1:

You have to return and assign the returned value to list1:

def func(list1):
    list1 = list1[2:]    
    print(list1)   # prints [2, 3]
    return list1

list1=func(list1)

or you have to reference list1 as global inside func().

Solution 2:

please goto http://www.pythontutor.com/visualize.html to visualize your code. It'll give you a more accurate explanation.

Here is a short explanation and two possible solutions for you question:

list1 = list1[2:]

This line creates a new list and assign it to local variable list1 in func namespace, while keeps the global one still.

If you want to change the original list:

1. Please use change in place methods(remove, pop etc) for list which can be found here.

2. Or you can return a new list like this:

   def func(list1):
       list1 = list1[2:]    
       print(list1)   # prints [2, 3]
       return list1
   
   list1 = [0, 1, 2, 3]
   list1 = func(list1)print(list1)       # prints [2, 3]
   

Hope it helps.

Solution 3:

list1 = list[2:] will create a list and make the local variable reference the new list. It does not affect the original list.

Use list slice assignment to get what you want:

deffunc(list1):
    list1[:] = list1[2:]
    print(list1)   # prints [2, 3]

or make the function return the new list, and assign the return value in the calling part:

def func(list1):
    return list[2:]

list1 = [0, 1, 2, 3]
list1 = func(list1)
....

Solution 4:

You've run into "pass by name". The code you show only changes what the "list1" name that is local to func() points to. If you use the "list1" name to change the value of what is being referred to, rather than pointing the "list1" name within func() to something else, then that will be seen by the rest of the program.

def func(list1):
    del list1[0:2]
    # list1 = list1[2:]
    # print(list1)   # prints [2, 3]

list1 = [0, 1, 2, 3]
func(list1)print(list1)

Solution 5:

option 1: don't do it. Side effects are evil :)

option 2: IF YOU MUST, then apply destructive methods on the object that your function receives as a parameter:

def func(list1):
    list1 = list1[2:]
    list1.pop(0)
    list1.pop(0)
    print(list1)   # prints [2, 3]

list1 = [0, 1, 2, 3]
func(list1)print(list1)       # prints [2, 3]