Using Append - Python

Question

What is the need to use the "Append" function in my code? They say it can be used to add another element to my Python list, but wouldn’t it be simpler to add this other element in a traditional way? Append only serves for this "addition"?

Answer 1

Add an element to a list dynamically and there are several reasons why this is possible:

• Do not know previously the value to be added;
• Make sure your list has room for the new value;

And possibly other reasons.

Do not know previously the value to be added

Let’s say I need to ask my user for a list of 5 integer values. How can I create this list if I don’t know what values the user will enter? So I need to create my list and insert the values dynamically:

numeros = []

for _ in range(5):
    numeros.append(int(input('Digite um número: '))

print(numeros)

With this, no matter what values the user enters, the program will add in the list in the same way.

>>> Digite um número: 1
>>> Digite um número: 2
>>> Digite um número: 3
>>> Digite um número: 4
>>> Digite um número: 5
[1, 2, 3, 4, 5]

That means I’ll always use the append? No. Many problems can be solved in other ways. It’s up to you, as a developer, to evaluate which one is ideal. If you want a list with 5 user read values, could do:

numeros = [int(input('Digite um número: ')) for _ in range(5)]

The result is exactly the same as the previous one, without using the append. If I just want the even values, as quoted in the other answer, you could do:

pares = [numero for numero in range(20) if numero % 2 == 0]

Same result without using the append.

Even, it is often more feasible for you to do without the append, for he spends memory for nothing.

Make sure your list has room for the new value

If my list has 5 elements, it will occupy in memory the equivalent of 5 times the memory that is demanded by that type of object. If I need to add a sixth element to the list, what assures me that there is memory available? The method append will ensure this for you as it will perform what we call dynamic memory allocation. Whenever you insert a new element it will be responsible for allocating memory so that it is possible to save the new value.

This dynamic allocation of append, even, seeks to work in an optimized way. How to make the memory allocation for each new value can be costly for application, instead of allocating only a new space will be allocated several, for if added more values already exist available memory. This is interesting if your list undergoes many modifications, but is bad in opposite cases as it can be allocated much more memory than was actually needed.

I commented on this behavior in:

• Why do these two ways of initializing the same list in Python generate structures of different sizes?

For this reason it is common for you to read that it is best for you to use the list-comprehension (above solutions without the append) than the solutions that use the append. But this is not always valid; there are problems that it is better to use the append - otherwise he wouldn’t even exist.

Answer 2

It is recommended to use the append when you use a loop to write this data in a list, example: In this example I want only the even numbers from 0 to 20

lista_par = []
for i in range(20):
   if i%2 == 0:
      lista_par.append(i)
print(lista_par)   

Output = [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]