1
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!!!

Skam
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Jan Zaplatil
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  • See also: [Is there a difference between `==` and `is` in Python?](https://stackoverflow.com/questions/132988/is-there-a-difference-between-and-is-in-python), [String comparison in Python: is vs. ==](https://stackoverflow.com/questions/2988017/string-comparison-in-python-is-vs), [Why does comparing strings in Python using either '==' or 'is' sometimes produce a different result?](https://stackoverflow.com/questions/1504717/why-does-comparing-strings-in-python-using-either-or-is-sometimes-produce) – Brad Solomon Jul 20 '17 at 17:12
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    The odd thing is that if you set `beer = "union"` then the code work as expected (maybe because Python caches/reuses immutable objects?). However, getting the string from `input` produces a different object. – Jared Goguen Jul 20 '17 at 17:13

4 Answers4

3

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).

McGlothlin
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    The `is` operator checks for identity in Python. The `beer` variable and the string `"union"` are different objects in memory. Therefore, `is` will never return true. – Jacobm001 Jul 20 '17 at 17:10
  • @Jacobm001 Actually, `is` will often return `True`, it appears that immutable objects are reused in some situations (albeit not with `input`). – Jared Goguen Jul 20 '17 at 17:16
  • @JaredGoguen: I meant in this situation – Jacobm001 Jul 20 '17 at 17:17
  • @Jacobm001 `is` implies `equality` but the converse is not true. – Skam Jul 20 '17 at 17:29
1

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.

Lev Levitsky
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DeGe
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0

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")
Ramon
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0

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

enter image description here

Root
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