CODEWITHSUNDEEP
pythonpython-3.x
Jan Zaplatil
Jan Zaplatil

Reputation: 43

Difference between == and is operator in Python

print("enter your age")
age = int(input())
if age < 21:
    print("no beer")
if age > 21:
    print("what beer do you like?")
    beer = input()
if beer is "union":
    print("this is water")

After typing union to answer nothing happens. Why? Thanks in advance!!!

Upvotes: 1

Views: 2209

Answers (4)

Root
Root

Reputation: 97

I think this picture will illustrate clearly about the difference between "is" and "==".

enter image description here

Upvotes: 0

McGlothlin
McGlothlin

Reputation: 2099

There are a few things wrong here:

You check for age < 21 and age > 21, but what happens if I'm exactly 21 years old? You'll want to change the > to >= in order to catch this case (assuming you're in America where the drinking age is 21 and over).

Another thing I noticed. What happens if I'm not over 21?

Traceback (most recent call last):
  File "beer.py", line 8, in <module>
    if beer is "union":
NameError: name 'beer' is not defined

Oops. You've defined beer, but this condition is never reached if you're not over 21. Therefore when you reference beer in your last if statement, the interpreter has no idea what you're talking about. You'll want to move the last if inside the second one: 

if age > 21:
    print("what beer do you like?")
    beer = input()
    if beer is "union":
        print("this is water")

Lastly, it's not a wise idea to use is as a substitute for the == operator, because they are not equivalent. In this case, beer is "union" evaluates to False, whereas beer == "union" does not, but there are some cases where the two statements are functionally equivalent. For example:

$ python -i
>>> x = 3
>>> if x is 3:
...     print('x is 3!')
...
x is 3!

Therefore, your final code would look like this:

print("enter your age")
age = int(input())
if age < 21:
    print("no beer")
if age >= 21:
    print("what beer do you like?")
    beer = input()
    if beer == "union":
        print("this is water")

Edit: see this answer for why they are not equivalent. is checks that they are the same object, whereas == checks that the values are equivalent (which is what you want).

Upvotes: 3

Ramon
Ramon

Reputation: 1299

There is an obvious bug in your code, the test if beer is "union", which should be if beer == "union", is not under the age > 21 check.

That is, if age < 21 the beer check will still happen, and beer will be undefined, raising an exception.

NameError: name 'beer' is not defined

This is a bit more sane:

if age < 21:
    print("no beer")
if age > 21:
    print("what beer do you like?")
    beer = input()
    if beer is "union":
        print("this is water")

Upvotes: 0

DeGe
DeGe

Reputation: 110

From the documentation for the "is" operator:

The operators is and is not test for object identity: x is y is true if and only if x and y are the same object.

Just change your 2nd last line to

if beer == "union":

The code would work.

Upvotes: 1

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