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Here is an easy way to accomplish what you want:

sage: A = [[1], [2]]                                                                        
sage: B = ['a', 'b']                                                                        
sage: [[x + [y] for x in A] for y in B]                                                   
[[[1, 'a'], [2, 'a']], [[1, 'b'], [2, 'b']]]

Or if you like:

C = []
for x in A:
    for y in B:
        C.append(x+[y])

I think that the problem with your original code is that you are looping over a list while also modifying that list, which is why working with a copy helps. Note also that code like a.append(1) will change the original list a, and that can be confusing.

for x in A:
     x.append(2)

modifies each list in A, while

for x in A:
    x + [2]

leaves A unchanged.

Here is an easy way to accomplish what you want:

sage: A = [[1], [2]]                                                                        
sage: B = ['a', 'b']                                                                        
sage: [[x + [y] for x in A] for y in B]                                                   
[[[1, 'a'], [2, 'a']], [[1, 'b'], [2, 'b']]]

Or if you like:

C = []
for x in A:
    for y in B:
        C.append(x+[y])

I think that the problem with your original code is that you are looping over a list while also modifying that list, which is why working with a copy helps. Note also that code like a.append(1) will change the original list a, and that can be confusing.

for x in A:
     x.append(2)

modifies each list in A, while

for x in A:
    x + [2]

leaves A unchanged.

from itertools import product
product(A, B, C)

list(product(A, B, C))

Here is an easy way to accomplish what you want:

sage: A = [[1], [2]]                                                                        
sage: B = ['a', 'b']                                                                        
sage: [[x + [y] for x in A] for y in B]                                                   
[[[1, 'a'], [2, 'a']], [[1, 'b'], [2, 'b']]]

Or if you like:

C = []
for x in A:
    for y in B:
        C.append(x+[y])

I think that the problem with your original code is that you are looping over a list while also modifying that list, which is why working with a copy helps. Note also that code like a.append(1) will change the original list a, and that can be confusing.

for x in A:
     x.append(2)

modifies each list in A, while

for x in A:
    x + [2]

leaves A unchanged.

Edit: this looks like a job for itertools: https://docs.python.org/3/library/itertools.html. Create multiple lists A = [1, 2], B = ['a', 'b'], C = ['o', 'p'] and do

from itertools import product
product(A, B, C)

or to get something more explicit:

list(product(A, B, C))

(The product function gives the Cartesian product of the inputs.) If you don't want multiple lists, create a single list C = [[1, 2], ['a', 'b'], ['o', 'p']] and then do product(*C). (Or if you have your two lists A and B as in the comments, you can replace A with A = [x[0] for x in A] and do C = A + B.)