Question

I am trying to learn how to “zip” lists. To this end, I have a program, where at a particular point, I do the following:

x1, x2, x3 = stuff.calculations(withdataa)

This gives me three lists, x1, x2, and x3, each of, say, size 20.

Now, I do:

zipall = zip(x1, x2, x3)

However, when I do:

print "len of zipall %s" % len(zipall)

I get 20, which is not what I expected. I expected three. I think I am doing something fundamentally wrong.

Answer 1

When you zip() together three lists containing 20 elements each, the result has twenty elements. Each element is a three-tuple.

See for yourself:

In [1]: a = b = c = range(20)

In [2]: zip(a, b, c)
Out[2]: 
[(0, 0, 0),
 (1, 1, 1),
 ...
 (17, 17, 17),
 (18, 18, 18),
 (19, 19, 19)]

To find out how many elements each tuple contains, you could examine the length of the first element:

In [3]: result = zip(a, b, c)

In [4]: len(result[0])
Out[4]: 3

Of course, this won’t work if the lists were empty to start with.