CODEWITHSUNDEEP
pythonpython-3.x
vinwin
vinwin

Reputation: 49

changing an immutable object in python, methods?

As I know integers are immutable, how would I go about printing 4 instead of 2?

a = 2
def add(num):
   num = 4
print(a)

Upvotes: 2

Views: 2368

Answers (2)

Rick
Rick

Reputation: 45251

In Python, variables are just baskets that hold values. If you want a to hold a different value, just put it in. Simple as that. 

a = 2 # value
a = 3 # new value

Although the integer 2 is immutable, a is just an object reference to that integer. a itself - the object reference- can be changed to reference any object at any time.

As you have discovered, you can't simply pass a variable to a function and change its value: 

def f(a):
    a = 4
f(a)
print(a) # still 3

Why still 3? Because in Python objects are passed by * object reference*. This is neither the same as by value, nor is it the same as by reference, in other languages.

The a inside of the function is a different object reference than outside of the function. It's a different namespace. If you want to change the function so that its a is the other, final a, you have to explicitly specify that this is what you want. 

def f():
    global a # this makes any following references to a refer to the global namespace
    a = 4
f()
print (a) # 4

I highly recommend this link for further reading.

Upvotes: 2

doratheexplorer0911
doratheexplorer0911

Reputation: 220

Seems like this is what you're trying to achieve:

a = 2
def add():
    global a
    a = 4

add()
print(a)

The add() function will change a's value to 4 and the output would be 4.

Upvotes: 0

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