Question

I don’t have a real reason for doing this, other than to gain understanding, but I’m trying to create a list of lists of lists using list comprehension.

I can create a list of lists just fine:

In[1]: [j for j in [range(3,k) for k in [k for k in range(5,10)]]]
Out[1]: [[3, 4], [3, 4, 5], [3, 4, 5, 6], [3, 4, 5, 6, 7], [3, 4, 5, 6, 7, 8]]

And I can create a list of lists of lists from either the results of that, for example:

In [2]: [range(0,i) for i in [3,4]]
Out[2]: [[0, 1, 2], [0, 1, 2, 3]]

In [3]: [range(0,i) for i in j]
Out[3]: 
[[0, 1, 2],
 [0, 1, 2, 3],
 [0, 1, 2, 3, 4],
 [0, 1, 2, 3, 4, 5],
 [0, 1, 2, 3, 4, 5, 6],
 [0, 1, 2, 3, 4, 5, 6, 7]]

But when I try to combine it into a single statement it goes awry:

In [4]: [range(0,i) for i in [j for j in [range(3,k) for k in [k for k in range(5,10)]]]]
---------------------------------------------------------------------------
TypeError: range() integer end argument expected, got list.

Am I missing some brackets somewhere?

Answer 1

Try the following:

[[range(0, j) for j in range(3, i)] for i in range(5, 10)]

This results in the following list of lists of lists:

>>> pprint.pprint([[range(0, j) for j in range(3, i)] for i in range(5, 10)])
[[[0, 1, 2], [0, 1, 2, 3]],
 [[0, 1, 2], [0, 1, 2, 3], [0, 1, 2, 3, 4]],
 [[0, 1, 2], [0, 1, 2, 3], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4, 5]],
 [[0, 1, 2],
  [0, 1, 2, 3],
  [0, 1, 2, 3, 4],
  [0, 1, 2, 3, 4, 5],
  [0, 1, 2, 3, 4, 5, 6]],
 [[0, 1, 2],
  [0, 1, 2, 3],
  [0, 1, 2, 3, 4],
  [0, 1, 2, 3, 4, 5],
  [0, 1, 2, 3, 4, 5, 6],
  [0, 1, 2, 3, 4, 5, 6, 7]]]

The best way to understand what is happening in a list comprehension is to try to roll it out into normal for loops, lets try that with yours and then mine to see what the difference is:

x = [range(0,i) for i in [j for j in [range(3,k) for k in [k for k in range(5,10)]]]]
# equivalent to
a, b, c, x = [], [], [], []
for k in range(5, 10):
    a.append(k)
for k in a:
    b.append(range(3, k))
for j in b:
    c.append(j)
for i in c:
    x.append(range(0, i))

At the end of this x would be equivalent to your list comprehension, however of course this code will not work because b (and c) will be lists of lists, so i will be a list and range(0, i) will cause an error. Now obviously this is not what you intended to do, since what you would really like to see is those for loops nested instead of one after the other.

Lets look at how mine works:

x = [[range(0, j) for j in range(3, i)] for i in range(5, 10)]
# equivalent to
x = []
for i in range(5, 10):
    a = []
    for j in range(3, i):
        a.append(range(0, j)):
    x.append(a)

Hope this helped to clarify!