Is there a way to init a list without using square bracket in Python?

Question

For example, is there a function like list_cons so that:

x = list_cons(1, 2, 3, 4)

is equivalent to:

x = [1, 2, 3, 4]

Judging by the `functional-programming` tag and use of the term `cons`, it would seem the asker is trying to get Python to be as much like Lisp as possible. But then, why not just use Lisp? If for some reason an underlying Python implementation is required (to make use of Python libraries or something), then maybe check out the various Lisp-in-Python implementations [here](http://www.biostat.wisc.edu/~annis/creations/PyLisp/) (the linked page has further links to other implementations). — John Y, Jul 03 '12 at 20:55
A Python list is not a cons/linked list, it's an array, so if you think you want this, you've probably mislead yourself badly. — abarnert, Jul 03 '12 at 22:03

score 11 · Accepted Answer · answered Jul 03 '12 at 20:23

11

In [1]: def list_cons(*args):
   ...:     return list(args)
   ...: 

In [2]: list_cons(1,2,3,4)
Out[2]: [1, 2, 3, 4]

answered Jul 03 '12 at 20:23

Lev Levitsky

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Why not just directly use the list method? – Dhara Jul 03 '12 at 20:33
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Not completely sure why the asker asked this question, but this answer clearly matches the sought-after syntax, and is thus almost certainly the best answer for the asker's needs. – John Y Jul 03 '12 at 20:35
@Dhara so he doesnt have to pass it a tuple. list(1,2,3) raises TypeError. list_cons(1,2,3) works – Paul Seeb Jul 03 '12 at 20:35
@Dhara: Because `list()` takes at most 1 argument. – martineau Jul 03 '12 at 20:35
Thanks @JohnY, I was typing the same... except for the "best" part :) – Lev Levitsky Jul 03 '12 at 20:36
I guess my quarrel was with the question, not your answer. It seems fairly pointless to wrap the list call in a function just to avoid typing brackets – Dhara Jul 03 '12 at 20:48

score 6 · Answer 2 · answered Jul 03 '12 at 20:23

6

Use the list constructor and pass it a tuple.

x = list((1,2,3,4))

answered Jul 03 '12 at 20:23

robert

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You could even do: `x = list([1,2,3,4])`. :-) – dubiousjim Jul 13 '12 at 14:10

senderle · Answer 3 · 2012-07-03T21:14:14.867

6

I don't think that would be a particularly useful function. Is typing brackets so hard? Perhaps we could give you a more useful answer if you explained why you want this.

Still, here's a fun thing you can do in Python 3:

>>> (*x,) = 1, 2, 3, 4, 5
>>> x
[1, 2, 3, 4, 5]

You can even omit the parenthesis -- *x, = 1, 2, 3, 4, 5 works too.

edited Jul 03 '12 at 21:14

answered Jul 03 '12 at 20:29

senderle

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Didn't know of that yet, so +1. But I hope I'll never stumble upon this in production code ;-) – Oben Sonne Jul 03 '12 at 20:33
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Why does it make a list and not any other iterable? – Dhara Jul 03 '12 at 20:34
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@Dhara, the short answer is "[because](http://www.python.org/dev/peps/pep-3132/)." Consider [this question](http://stackoverflow.com/questions/6967632/getting-python-sequence-assignments-unpacking-right) for more corner cases. – senderle Jul 03 '12 at 20:37
@Dhara, less flippantly, the PEP I linked to says that using a tuple (and presumably any other type of iterable) would "make further processing of the result harder." (See the [third bullet point](http://www.python.org/dev/peps/pep-3132/#acceptance).) – senderle Jul 03 '12 at 20:42

score 1 · Answer 4 · answered Jul 03 '12 at 20:26

1

works only in python 2.x:

>>> def list_cons(*args):
       return map(None,args)

>>> list_cons(1,2,3,4)
[1, 2, 3, 4]

answered Jul 03 '12 at 20:26

Ashwini Chaudhary

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Is there a way to init a list without using square bracket in Python?

4 Answers4