CODEWITHSUNDEEP
pythonfunctionreturn
Iseng
Iseng

Reputation: 17

function return of python

I am reading John Zelle's Python programming 2ed Ch6. And I encounter a concept question: His example code says:

#addinterest3.py 
def addInterest(balances, rate):
    for i in range(len(balances)):
        balances[i] * (1 + rate)


def test():
    amounts = [1000, 2200, 800, 360]
    rate = 0.05
    addInterest(amounts, rate)
    print(amounts)


test()

with upper code the

"assignments into the list caused it to refer to the new values. The old values will actually get cleaned up when Python does garbage collection"

According to this idea I tried a test.py

def add(balance, rate):
    balance = balance * (1 + rate)


def test():
    amount = 1000
    rate = 0.5
    amount = add(amount, rate)
    print(amount)


test()

I think the balance can also be assigned by a new values, but it turns out return "none" in my terminal. Can anyone explain what is the fundamental difference between them? Why list assignment dont need to use return to pass the value but will still be saved, on the other hand, my test.py do required a "return balance" after the function ?

Upvotes: 1

Views: 1004

Answers (3)

Serdalis
Serdalis

Reputation: 10489

The reason you are not getting any value in amount is because your add function is not returning anything.

def add(balance,rate):
    return balance*(1+rate)

Would return a value which would be assigned to amount.

Added to that, the beginning code is wrong. If you we're to try this code in python you would find that it prints out:

[1000, 2200, 800, 360]

it needs to be changed to:

def addInterest(balances,rate):
    for i in range(len(balances)):
        balances[i] = balances[i]*(1+rate)

Upvotes: 1

Roman Fursenko
Roman Fursenko

Reputation: 688

As I may expect, if you pass a list as a function argument it like you pass the pointer on this list. Assignment statements in Python do not copy objects, they create bindings between a target and an object. Therefore if you change the list inside the function you can see these changes. If you pass the variables, the copies of these variables are used inside the function.
As an example you can also try this code:

a = [1, 2]
b = a
a.append(3)
print a, b

Output:

[1, 2, 3] [1, 2, 3]

Ups!

Upvotes: 1

Blender
Blender

Reputation: 298562

Your add function doesn't return anything, so Python automatically has it return None at the end. When you write:

amount = add(amount,rate)

add returns None, so amount is set to None.

Also, not all objects in Python are mutable, so modifying the variable from inside of the function won't do what you expect. Use return instead:

def add(balance,rate):
    return balance * (1 + rate)

Upvotes: 0

Related Questions