CODEWITHSUNDEEP
pythonlistpython-3.x
Ihmahr
Ihmahr

Reputation: 1120

Python list.append as an argument

Why does the following code give 'None'? How can I resolve this?

def f1(list1):
    f2(list1.append(2))

def f2(list1):
    print(list1)

f1([1])

What also doesn't work:

def f1(list1):
    arg1 = list1.append(2) 
    f2(arg1)

Upvotes: 3

Views: 208

Answers (2)

Tim Pietzcker
Tim Pietzcker

Reputation: 336228

It depends on what you want to do. If you want list1 to have changed after a call to f1, use

def f1(list1):
    list1.append(2)
    f2(list1)

See what happens:

>>> l = [1]
>>> f1(l)       # Modifies l in-place!
[1, 2]
>>> l
[1, 2]

If you don't want list1 to be changed:

def f1(list1):
    f2(list1 + [2])

Now see this:

>>> l = [1]
>>> f1(l)       # Leaves l alone!
[1, 2]
>>> l
[1]

Upvotes: 6

unutbu
unutbu

Reputation: 879849

In general, Python methods that mutate an object (such as list.append, list.extend, or list.sort) return None.

If you wish to print out the new list:

def f1(list1):    
    list1.append(2)
    f2(list1)

Upvotes: 7

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