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I recently picked up Python and I am alternating between this tutorial and a book called Automate The Boring Stuff With Python.

I find the book quite easy to understand and the exercises are fun.
There is one in particular that I just solved but honestly, I am not satisfied with my answer.

In the question, one is given a grid as follows:

grid = [['.', '.', '.', '.', '.', '.'],

        ['.', '0', '0', '.', '.', '.'],

        ['0', '0', '0', '0', '.', '.'],

        ['0', '0', '0', '0', '0', '.'],

        ['.', '0', '0', '0', '0', '0'],

        ['0', '0', '0', '0', '0', '.'],

        ['0', '0', '0', '0', '.', '.'],

        ['.', '0', '0', '.', '.', '.'],

        ['.', '.', '.', '.', '.', '.']]

Using loops only, one is then required to produce the following output:

..00.00..

.0000000.

.0000000.

..00000..

...000...

....0....

My solution (which works) to this problem is as follows:

for y in range(0,len(grid[0])):

    for x in range(0, len(grid)):

        print(grid[x][y], end='')



    print()

My problem is the grid[0]. I have tried to generalize the code to accommodate scenarios in which the lists within grid are of different values, but failed.

My question, therefore, is how can I retrieve the len of grid[0] in general variable terms?

In other words, how can I retrieve the length of a list within a list?

You don't actually need to retrieve length.

You can do it in one line without knowing dimensions:

print('n'.join(map(lambda x: ''.join(x), zip(*grid))))

Step by step:

1. 
   zip(*grid) will transpose your 2-d array

>>> list(zip(*grid))

[('.', '.', '0', '0', '.', '0', '0', '.', '.'),

 ('.', '0', '0', '0', '0', '0', '0', '0', '.'),

 ('.', '0', '0', '0', '0', '0', '0', '0', '.'),

 ('.', '.', '0', '0', '0', '0', '0', '.', '.'),

 ('.', '.', '.', '0', '0', '0', '.', '.', '.'),

 ('.', '.', '.', '.', '0', '.', '.', '.', '.')]

2. 
   map(lambda x: ''.join(x), zip(*grid)) will join every line

>>> list(map(lambda x: ''.join(x), zip(*grid)))

['..00.00..', '.0000000.', '.0000000.', '..00000..', '...000...', '....0....']

3. And finally 'n'.join joins with separator as new line

>>> print('n'.join(map(lambda x: ''.join(x), zip(*grid))))

..00.00..

.0000000.

.0000000.

..00000..

...000...

....0....

Note: we could also omit 3rd step by printing with sep='n, however in Python3:
Why does map return a map object instead of a list in Python 3? and we would need to convert it to list before printing it. Like so:

print(list(map(lambda x: ''.join(x), zip(*grid))), sep='n')

For example taking the longes of all the content list:

>>> grid = [['.', '.', '.', '.', '.', '.'],

...         ['.', '0', '0', '.', '.', '.'],

...         ['0', '0', '0', '0', '.', '.'],

...         ['0', '0', '0', '0', '0', '.'],

...         ['.', '0', '0', '0', '0', '0'],

...         ['0', '0', '0', '0', '0', '.'],

...         ['0', '0', '0', '0', '.', '.'],

...         ['.', '0', '0', '.', '.', '.'],

...         ['.', '.', '.', '.', '.', '.']]

>>> 

>>> max(map(len, grid))

6

BTW, a pythonic way of solving that (without loops) would look like this list(zip(*grid)):

>>> for l in zip(*grid):

...     print("".join(l))

... 

..00.00..

.0000000.

.0000000.

..00000..

...000...

....0....

If you look at your desired output, you'll observe that final matrix is transpose of your given list.

The transpose of a matrix could be found by using numpy transpose function.

You could use a list comprehension in combination with join method in order to achieve what you want.

import numpy as np

transpose = np.array(grid).transpose()

print('n'.join([''.join(row) for row in transpose]))

Output

..00.00..

.0000000.

.0000000.

..00000..

...000...

....0....

If you really must get the length of a list in a list, just use len(grid[index]):

for i in range(len(grid)):

    for j in range(len(grid[i])):

        print(mylist[i][j], end='')

    print()

However, it is easier to to iterate over the elements of the list than to get the indexes of the list. For example, instead of the above solution, use

for row in mylist:

    for element in row:

         print(element, end='')

    print()

Another way of doing this is to start by transposing the grid, then just loop over the elements of each row

 grid = [['.', '.', '.', '.', '.', '.'],

         ['.', '0', '0', '.', '.', '.'],

         ['0', '0', '0', '0', '.', '.'],

         ['0', '0', '0', '0', '0', '.'],

         ['.', '0', '0', '0', '0', '0'],

         ['0', '0', '0', '0', '0', '.'],

         ['0', '0', '0', '0', '.', '.'],

         ['.', '0', '0', '.', '.', '.'],

         ['.', '.', '.', '.', '.', '.']]

  grid2=zip(*grid)

 # print(grid2)

  for row in grid2:

     for el in row:

         print(el, end='')

     print('n')