Initializing a list and appending in one line

Question

How can I initialize a list, if it is not already initialized and append to it in one line. For example,

function example(a=None):
   a = a or []
   a.append(1)
   return another_function(a)

How can I combine those two statements in the function into one?

I am looking for something like this, but that does not work:

function example(a):
   a = (a or []).append(1)
   return another_function(a)

EDIT: I don't reference a elsewhere, it is just being passed from function to function. Practically just value is important, so it is OK if it is another object with the right value. I also added a default value of None.

How would it ever not be initialized? What other value would it have instead? — Morgan Thrapp, Oct 07 '16 at 19:43
this parameter a is being passed between several such functions and any one of them might initialize. — mehmet, Oct 07 '16 at 19:45
As @MorganThrapp says, it's not clear what else it could be. You would have to send something to that function. It's either a list and will work, or it isn't and won't. — TigerhawkT3, Oct 07 '16 at 19:45
And is the list supposed to be used only within the function (you rebind the name to a local reference, with the same object or a different one)? Or is it supposed to mutate the original list? Or return a new one? — TigerhawkT3, Oct 07 '16 at 19:51
You should generally check for `None` explicitly, to differentiate between `None` and another falsey object like an empty list. If you have a list reference that you'd like to preserve, passing a reference to an empty list will be replaced with a new empty list object. Your use case isn't clear. — TigerhawkT3, Oct 07 '16 at 19:53
I used `a` within scope of several functions and don't reference it elsewhere. Therefore, it is like passing by value. I edited the question to clarify. — mehmet, Oct 07 '16 at 19:57
The reason your attempt isn't working is that `a.append(x)` mutates the list object in place and returns `None`. That is a pattern you should assume in Python. Read [Alex Martelli's answer here](http://stackoverflow.com/questions/1682567/why-does-pythons-list-append-evaluate-to-false). Also, although `x or y` works, it perhaps more common in modern Python to use the conditional expression `x if x else y` — juanpa.arrivillaga, Oct 07 '16 at 20:04
@user2357112 you can call it OCD (obsessive compulsive disorder). I could do it with object properties in javascript and found it very elegant.. That is why. — mehmet, Oct 07 '16 at 20:18

Cory Kramer · Accepted Answer · 2016-10-07T19:47:40.250

3

def example(a):
    return a + [1] if a else [1]

>>> example([1,2,3])
[1, 2, 3, 1]
>>> example([])
[1]

edited Oct 07 '16 at 19:47

answered Oct 07 '16 at 19:43

Cory Kramer

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5

This doesn't exactly _append_ to a list. I creates a new list that has the last item added. There's a slight difference in some cases... – mgilson Oct 07 '16 at 19:44
2

OP actually said that the same list object could be initialized by another function, implying that the reference should be preserved. I don't think this is the answer – salezica Oct 07 '16 at 19:49
@slezica I said "parameter a is being passed between serveral such functions and any one of them might initialize". I did not say "same object". Until it is initialized I don't care. – mehmet Oct 07 '16 at 20:00
On another note, what a vibrant python community is here! If I knew I would be much more deliberate when asking the question. I would ask about say 'django' and wait days to get an answer. – mehmet Oct 07 '16 at 20:01

Initializing a list and appending in one line

1 Answers1