Python array combining

Question

I'm trying to port some Python code to Java. I'm not familiar with Python and have never seen this in any language before:

return [c,] + s

What exactly does this line mean? Specifically the [c,] part. Is it combining two arrays or something? s is an array of integers and c is an integer. The full function is below (from Wikipedia: http://en.wikipedia.org/wiki/Ring_signature )

def sign(self,m,z):
    self.permut(m)
    s,u = [None]*self.n,random.randint(0,self.q)
    c = v = self.E(u) 
    for i in range(z+1,self.n)+range(z):
        s[i] = random.randint(0,self.q)
        v = self.E(v^self.g(s[i],self.k[i].e,self.k[i].n)) 
        if (i+1)%self.n == 0: c = v
    s[z] = self.g(v^u,self.k[z].d,self.k[z].n)
    return [c,] + s

Thanks so much!

Martijn Pieters · Accepted Answer · 2013-03-26T20:29:27.867

5

The comma is redundant. It's just creating a one-element list:

>>> [1,]
[1]
>>> [1] == [1,]
True

The practice comes from creating tuples in Python; a one-element tuple requires a comma:

>>> (1)
1
>>> (1,)
(1,)
>>> (1) == (1,)
False

The [c,] + s statement creates a new list with the value of c as the first element.

edited Mar 26 '13 at 20:29

answered Mar 26 '13 at 20:00

Martijn Pieters

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score 3 · Answer 2 · edited May 23 '17 at 12:11

3

[c,] is exactly the same as [c], i.e. a single-item list.

(See this answer for why this syntax is needed)

edited May 23 '17 at 12:11

Community

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answered Mar 26 '13 at 20:00

bereal

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score 1 · Answer 3 · answered Mar 26 '13 at 20:02

For a list, the extra comma is redundant and can be ignored. The only time it makes a difference if it had been a tuple instead of a list so

[c,] and [c] are the same but,
(c,) and (c) are different. The former being a tuple and later just a parenthesis around an expression

score 0 · Answer 4 · answered Mar 26 '13 at 20:06

0

to answer both your questions, the line concatenates two lists, the first of the two is a one-element list since the comma is just ignored by python

answered Mar 26 '13 at 20:06

furins

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score 0 · Answer 5 · edited May 23 '17 at 10:32

I believe you are correct, it is combining two "arrays" (lists in python). If I'm not mistaken, the trailing comma is unnecessary in this instance.

x = [1,2,3]
y = [1] + x

#is equivalent to

x = [1,2,3]
y = [1,] + x

The reason Python allows the use of trailing commas in lists has to do with another data type called a tuple and ease of use with multi-line list declaration in code.

Why does Python allow a trailing comma in list?

Python array combining

5 Answers5