What does the list() function do in Python?

Question

I know that the list() constructor creates a new list but what exactly are its characteristics?

1. What happens when you call list((1,2,3,4,

Answer 1

aTuple = (123, 'xyz', 'zara', 'abc');
aList = list(aTuple)

print "List elements : ", aList

When we run above program, it produces following result:

List elements :  [123, 'xyz', 'zara', 'abc']

It is another way to create a list in python. How convenient!

Answer 2

list() converts the iterable passed to it to a list. If the itertable is already a list then a shallow copy is returned, i.e only the outermost container is new rest of the objects are still the same.

>>> t = (1,2,3,4,[5,6,7,8],9)
>>> lst = list(t) 
>>> lst[4] is t[4]  #outermost container is now a list() but inner items are still same.
True

>>> lst1 = [[[2,3,4]]]
>>> id(lst1)
140270501696936
>>> lst2 = list(lst1)
>>> id(lst2)
140270478302096
>>> lst1[0] is lst2[0]
True

Answer 3

Yes it is true.

Its very simple. list() takes an iterable object as input and adds its elements to a newly created list. Elements can be anything. It can also be an another list or an iterable object, and it will be added to the new list as it is.

i.e no nested processing will happen.

Answer 4

Your question is vague, but this is the output as follows, it doesn't "replace" the outer braces, it creates a data structure of a list, that can contain any value in a "listed" order (one after the other, after the other, and so on...) in a recursive way, you can add/remove elements to a specified index using append and pop. By the other hand, tuples are static and are not dynamically linked, they are more like an array of any type of element.

WHEN:

list((1,2,3,4,[5,6,7,8],9))

RETURNS:

[1, 2, 3, 4, [5, 6, 7, 8], 9]

WHEN:

list([[[2,3,4]]])

RETURNS:

[[[2, 3, 4]]]

WHEN:

list([[1,2,3],[4,5,6]])

RETURNS:

[[1, 2, 3], [4, 5, 6]]

Answer 5

You said: "From what I can tell, calling the constructor list removes the most outer braces (tuple or list) and replaces them with []. Is this true?"

IMHO, this is not a good way to think about what list() does. True, square brackets [] are used to write a list literal, and are used when you tell a list to represent itself as a string, but ultimately, that's just notation. It's better to think of a Python list as a particular kind of container object with certain properties, eg it's ordered, indexable, iterable, mutable, etc.

Thinking of the list() constructor in terms of it performing a transformation on the kind of brackets of a tuple that you pass it is a bit like saying adding 3 to 6 turns the 6 upside down to make 9. It's true that a '9' glyph looks like a '6' glyph turned upside down, but that's got nothing to do with what happens on the arithmetic level, and it's not even true of all fonts.

Answer 6

Python has a well-established documentation set for every release version, readable at https://docs.python.org/. The documentation for list() states that list() is merely a way of constructing a list object, of which these are the listed ways:

• Using a pair of square brackets to denote the empty list: []
• Using square brackets, separating items with commas: [a], [a, b, c]
• Using a list comprehension: [x for x in iterable]
• Using the type constructor: list() or list(iterable)

The list() function accepts any iterable as its argument, and the return value is a list object.

Further reading: https://docs.python.org/3.4/library/stdtypes.html#typesseq-list