The problem

You're appending the same array a to your list0 4 times. Arrays like a are mutable objects, which means, among other things, that when you assign values to them the underlying object changes. Since the array is present in your list 4 times, those changes (seem to) show up in 4 different places.

Solution

You can fix the code you have with one small change. Append a copy of the array to your list, instead of the array itself:

import numpy as np
a = np.empty((3), int)
list0 = []
for idx in range(4):    
    for i in range(3):
        a[i] = idx*10 + i
    print("idx =",idx,"; a =",a)
    list0.append(a.copy())
print("list0 =",list0)

Output:

idx = 0 ; a = [0 1 2]
idx = 1 ; a = [10 11 12]
idx = 2 ; a = [20 21 22]
idx = 3 ; a = [30 31 32]
list0 = [array([0, 1, 2]), array([10, 11, 12]), array([20, 21, 22]), array([30, 31, 32])]

Optimized solution

Python/Numpy offer many better ways (both in terms of using fewer lines of code and running faster) to initialize arrays. For a bunch of ranges like this, here is a reasonable approach:

list0 = [np.arange(n*10, n*10+3) for n in range(4)]
print(list0)

Output:

[array([0, 1, 2]), array([10, 11, 12]), array([20, 21, 22]), array([30, 31, 32])]

You might also consider just using a single 2D array in place of a list of arrays. One single array is often easier to work with than a heterogenous mix of arrays in a list. Here's how you do that:

arr0 = np.array([np.arange(n*10, n*10+3) for n in range(4)])
print(arr0)

Output:

[[ 0  1  2]
 [10 11 12]
 [20 21 22]
 [30 31 32]]

You can append the elements of one list to another with the "+=" operator. Note that the "+" operator creates a new list.

a = [1, 2, 3]
b = [10, 20]

a = a + b # Create a new list a+b and assign back to a.
print a
# [1, 2, 3, 10, 20]


# Equivalently:
a = [1, 2, 3]
b = [10, 20]

a += b
print a
# [1, 2, 3, 10, 20]

If you want to append the lists and keep them as lists, then try:

result = []
result.append(a)
result.append(b)
print result
# [[1, 2, 3], [10, 20]]

{} represents an empty dictionary, not an array/list. For lists or arrays, you need [].

To initialize an empty list do this:

my_list = []

or

my_list = list()

To add elements to the list, use append

my_list.append(12)

To extend the list to include the elements from another list use extend

my_list.extend([1,2,3,4])
my_list
--> [12,1,2,3,4]

To remove an element from a list use remove

my_list.remove(2)

Dictionaries represent a collection of key/value pairs also known as an associative array or a map.

To initialize an empty dictionary use {} or dict()

Dictionaries have keys and values

my_dict = {'key':'value', 'another_key' : 0}

To extend a dictionary with the contents of another dictionary you may use the update method

my_dict.update({'third_key' : 1})

To remove a value from a dictionary

del my_dict['key']