Why, after assigning a list as a value for a dictionary, performing actions on the list change the value inside the dictionary? How to avoid this?

Question

I need help with the following code:

dict={0:[1],2:[3],4:[5]} 
a=[7,9] 
print(dict) 
dict[6]=a 
print(dict) 
a.pop() 
print(dict) 

Output:

{0: [1], 2: [3], 4: [5]} 
{0: [1], 2: [3], 4: [5], 6: [7, 9]} 
{0: [1], 2: [3], 4: [5], 6: [7]}