variable reference and manipulation from object to function in python

Question

I have wrote following code in python

class check(object):
 def __init__(self):
 self.a = [1,2,3,4]
 self.b = 5
 appending(self.a, self.b)
 print "a", self.a
 print "b", self.b
def appending(a,b):
 a.append(5)
 b +=1

If now I run check() function, I got output as following:

a [1,2,3,4,5] [1,2,3,4,5]
b 5 5

So, I have following question

• Why it is that list(a) is got updated but not int(b)?

• It is related that I am modifying a but i am creating new object when I add 1 in b, in short, it is difference due to immutable or mutable data types.

• I have define self.a and self.b in object, i have define a,b in function, then why I can write print a and print b in object, get same output as self.a and self.b

• Håken Lid: I think print "a", a should be print "a", self.a, right?
• Kush Patel: Thanks , you are right
• Håken Lid: And the simple answer to your question is "yes". It's because the integer is immutable. self.b still points to the integer 5. When you do b += 1 you create a new integer and assign it to the identifier b, but only inside the scope of the function appending.

Answer 1

self.b is a name for an integer 5. The integer is an immutable object.

When you appear to mutate b you actually create a new object 6 and assign b as a name for it. This in no way affects the object 5 which self.b is a name for.

Answer 2

Since a is a list, it's passed by reference to the method and any changed done to it within the method, will be done directly to the a.

Variable b is an integer, therefore it's passed by value and a copy of the variable is created to be used within the method. Any change will be visible only within the body of the method, but not to the "outside world".

• Håken Lid: This is incorrect. In python everything is passed by assignment. Here's an explanation: stackoverflow.com/a/986145/1977847
• Kush Patel: What is difference between passed by assignment and passed by reference or passed by value ? I am little bit confused about this
• Marko Jevtić: reading this article should explain the difference: robertheaton.com/2014/02/09/…
• Kush Patel: Thanks you for guidance