type of variable through indentation of loops Python

Question

Here is a snippet, in the code through the top- down way, I printed the type of the variable named "podium") this is the code:

def frequence(entranche):
    podium = []
    print("premier podium", type(podium))
    for item in entranche:
        print ("deuxieme podium", type(podium))
        scat = len(entranche)
        for indice in range (len(entranche)):            
            if entranche[indice] == item:
                scat -= 1
            frequence = len(entranche) - scat

        podium = podium.append(frequence)
        print("troisieme podium", type(podium))
        plus_haute_frequence = max(podium)   
    return(plus_haute_frequence)
print(frequence("Je suis né dans le béton Coincé entre deux maisons".split()))

after the code, this is the output:

premier podium <class 'list'>
deuxieme podium <class 'list'>
troisieme podium <class 'NoneType'>

I don't understand why the podium variable loses his type.

Someone said me:

"Now, i think your problem is this: troisieme podium - right? It's because of podium = podium.append(frequence). Just append the new value to your list, re-assigning the variable istn't correct here. Just do podium.append(frequence)".

It's right. but I don't understand why. possibly we consider this as a new variable. And what can I do if I need to make e.g. something with a method that do not alter the named variable until writing variable = variable.method() {in ruby language there is method with "!" and without '!"}

Answer 1

This is because the append() method modifies the list in place. It doesn't return a new list, but modifies the list given as parameter. Its return value is None, and that is why your 'troisieme podium' type is NoneType.

>>> a = [1, 2, 3]
>>> a.append(4)
>>> a
[1, 2, 3, 4]
>>> print(a.append(5))
None
>>> a
[1, 2, 3, 4, 5]

So this line:

podium = podium.append(frequence)

should be simply:

podium.append(frequence)