math - What does the [0]*x syntax do in Python?

Question

A flash question, I'm looking at the following code

from __future__ import division
import math
import time

def dft(x, inverse = False, verbose = False) :
    t = time.clock()
    N = len(x)
    inv = -1 if not inverse else 1
    X =[0] * N
    for k in xrange(N) :
        for n in xrange(N) :
            X[k] += x[n] * math.e**(inv * 2j * math.pi * k * n / N)
        if inverse :
            X[k] /= N
    t = time.clock() - t
    if verbose :
        print "Computed","an inverse" if inverse else "a","DFT of size",N,
        print "in",t,"sec."
    return X

and I'm wondering (I do not know python):

• what does the X =[0] * N line do?
• why the double asterisk ** ?

Answer 1

The [0] * x creates a list with x elements. So,

>>> [ 0 ] * 5
[0, 0, 0, 0, 0]
>>> 

Be warned that they all point to the same object. This is cool for immutables like integers but a pain for things like lists.

>>> t = [[]] * 5
>>> t
[[], [], [], [], []]
>>> t[0].append(5)
>>> t
[[5], [5], [5], [5], [5]]
>>> 

The ** operator is used for exponentation.

>>> 5 ** 2 
25