CODEWITHSUNDEEP
pythonlistfunctionpython-2.7tuples
user2676813
user2676813

Reputation:

Python function return 2 lists

I have a function that creates two lists. I want the function to return the two lists as [], [].

Here is the code. This returns a list that look like this [[], []], close but still not what I want.

def create_xy_lst(start, length, step, up, down):
    new_list = []
    new_list.append(create_x_lst(start, length, step))
    new_list.append(create_y_lst(start, length, up, down))
    return new_list

Then I tried a different approach. This returns ([], []) which is a tuple.

def create_xy_lst(start, length, step, up, down):

    x = (create_x_lst(start, length, step))
    y = (create_y_lst(start, length, up, down))
    return x, y

How can I make the function return [], []?

Python 2.7

Upvotes: 2

Views: 2785

Answers (5)

Nafiul Islam
Nafiul Islam

Reputation: 82470

[], [] is a tuple. () is simple there for syntax, let me demonstrate:

>>> hello = 1,
>>> hello
(1,)
>>> cheese = [1,2,3], [6,7,8]
>>> cheese
([1, 2, 3], [6, 7, 8])

As you can see the , is what makes a tuple.

And you can unpack them like so:

>>> a, b = [1,2], [3,4]
>>> a
>>> [1,2]
>>> b
>>> [3,4]

So, in your function:

x, y = create_xy_lst(start, length, step, up, down)

This is also a neat trick (if you're in Python 3) if you have more than one list in the tuple:

>>> a, *b = [2], [4,5,6], [10]
>>> a
>>> [2]
>>> b
>>> ([4,5,6], [10])

If however, you simple want to return one list, you can do this:

return x + y

Upvotes: 3

Back2Basics
Back2Basics

Reputation: 7806

You are asking about a tuple but you are rejecting the idea that it should be a tuple so here is another idea. You could turn it into a generator.

def create_xy_lst(start, length, step, up, down):

    yield create_x_lst(start, length, step))
    yield create_y_lst(start, length, up, down))

which will only yield 1 list per query.

val = create_xy_lst(a,b,c,d,e)
print(val.next()) #first list
print(val.next()) #second list

or you could loop over it

for val in create_xy_lst(a,b,c,d,e):
    print(val)

Upvotes: 0

thefourtheye
thefourtheye

Reputation: 239463

def returnNumbers():
    return 1, 2

first, second = returnNumbers()
print first, second

It will print

1 2

You can simply return a tuple and unpack the values like shown in the example.

You can even check the type of data returned by the function like this

print type(returnNumbers())

it will print

<type 'tuple'>

Upvotes: 1

astrognocci
astrognocci

Reputation: 1085

Your second try is (mostly) the right way (you just don't need the parenthesis around your function calls).

I'm guessing you're retrieving your results like this:

foo = create_xy_list(...)

try this instead:

foo, bar = create_xy_list(...)

Upvotes: 0

Martijn Pieters
Martijn Pieters

Reputation: 1121714

[], [] is a tuple:

>>> [], []
([], [])

The comma makes the value a tuple, not the parenthesis.

Returning x, y is the correct way to return multiple values.

You can unpack tuples into multiple variables:

x, y = [], []

and this works just fine for function return values too:

x, y = create_xy_lst(...)

Unpacking in an assignment is not limited to just tuples; it works with any sequence:

char1, char2, char3 = 'foo'

Upvotes: 7

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