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,[5,6,7,8],9))?

2. What happens when you call list([[[2,3,4]]])?

3. What happens when you call list([[1,2,3],[4,5,6]])?

From what I can tell, calling the constructor list removes the most outer braces (tuple or list) and replaces them with []. Is this true? What other nuances does list() have?

Answer 1

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 2

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