The line of code you are asking about is using list comprehension to create a list and assign the data collected in this list to self.cells. It is equivalent to

self.cells = []
for i in xrange(region.cellsPerCol):
    self.cells.append(Cell(self, i))

Explanation:

To best explain how this works, a few simple examples might be instructive in helping you understand the code you have. If you are going to continue working with Python code, you will come across list comprehension again, and you may want to use it yourself.

Note, in the example below, both code segments are equivalent in that they create a list of values stored in list myList.

For instance:

myList = []
for i in range(10):
    myList.append(i)

is equivalent to

myList = [i for i in range(10)]

List comprehensions can be more complex too, so for instance if you had some condition that determined if values should go into a list you could also express this with list comprehension.

This example only collects even numbered values in the list:

myList = []
for i in range(10):
    if i%2 == 0:     # could be written as "if not i%2" more tersely
       myList.append(i)

and the equivalent list comprehension:

myList = [i for i in range(10) if i%2 == 0]

Two final notes:

• You can have "nested" list comrehensions, but they quickly become hard to comprehend :)
• List comprehension will run faster than the equivalent for-loop, and therefore is often a favorite with regular Python programmers who are concerned about efficiency.

Ok, one last example showing that you can also apply functions to the items you are iterating over in the list. This uses float() to convert a list of strings to float values:

data = ['3', '7.4', '8.2']
new_data = [float(n) for n in data]

gives:

new_data
[3.0, 7.4, 8.2]

In Python you can iterate over the list itself:

for item in my_list:
   #do something with item

or to use indices you can use xrange():

for i in xrange(1,len(my_list)):    #as indexes start at zero so you 
                                    #may have to use xrange(len(my_list))
    #do something here my_list[i]

There's another built-in function called enumerate(), which returns both item and index:

for index,item in enumerate(my_list):
    # do something here

examples:

In [117]: my_lis=list('foobar')

In [118]: my_lis
Out[118]: ['f', 'o', 'o', 'b', 'a', 'r']

In [119]: for item in my_lis:
    print item
   .....:     
f
o
o
b
a
r

In [120]: for i in xrange(len(my_lis)):
    print my_lis[i]
   .....:     
f
o
o
b
a
r

In [122]: for index,item in enumerate(my_lis):
    print index,'-->',item
   .....:     
0 --> f
1 --> o
2 --> o
3 --> b
4 --> a
5 --> r

When you do for i in row, i takes the values in row, i.e. '1', then '2', then '3'.

Those are strings, which are not valid as a list index. A list index should be an integer. When doing range(len(row)) you loop on integers up to the size of the list.

By the way, a better approach is:

for elt_id, elt in enumerate(list):
    # do stuff

You were close to it. But you need to append new elements in the inner loop to an empty list, which will be append as element of the outer list. Otherwise you will get (as you can see from your code) a flat list of 100 elements.

newlist = []
for x in range(10):
    innerlist = []
    for y in range(10):
        innerlist.append(y)
    newlist.append(innerlist)

print(newlist)

See the comment below by Błotosmętek for a more concise version of it.