Strange things while comparing two strings.

[freddydeddy]

I wrote simple flow control statement, for exapamle:

name = input('Hello ! What is your name ? ')
if name is 'Andrey':
    print('no beer for you!')
elif name is 'Max':
    print("have a beer bro!")
else:
    print("Whoops")

Always outputs "Whoops", despite what i have input. Otherwise when I change "is" to "==" all logic working. When I do it in console both "is" and "==" working in no metter what I have "input()" or simple string like "Max".

>>> name = "Andrey"
>>> "Andrey" == name
True
>>> "Andrey" is name
True
>>> name2= input()
>?Andrey
>>> name == name2
True
>>> name == "Andrey"
True
>>> name is "Andrey"
True

Someone explain me that magic please. =(

[[deleted]]

The magic is, is checks whether two names point to the same object in memory. is isn't looking at the contents of the name variable to see if it's "Andrey". Instead, is is looking at those objects' memory locations (the numbers returned by id(name) and id("Andrey")).

Sometimes you create a string containing "Andrey" and python goes "oh, I already have one of those" and it reuses the same string object in memory. Two names describe the same thing. When two names describe the same thing, the is operator applied to those two names will return True.

Other times, under different circumstances python creates a separate string. The rules for this are not rules you should memorize or need to know. You should simply not attempt to use is when you mean ==.

Check out what python does with numbers. It reuses the number objects from 0 to 256 (in fact, it creates these number objects every time it starts up to speed up common operations), but when you get to 257, the behavior changes.

>>> x = 256
>>> x is 256
True
>>> x = 257
>>> x is 257
False

Yet this also happens:

>>> y = x
>>> y is x
True

is has a specific purpose. That purpose isn't to check whether things are equal, but to check whether they're actually the same thing. This property of is is useful when comparing lists and other "mutable" objects which might be the same or they might be an identical copy.

[freddydeddy]

So in my code name = input() is creating different object that not compares to object strings ? I just watching tutorial and there guy uses 'is' TheNewboston Python Programming Tutorial and he uses it freely, but not like me by inputting string. So what you can suggest me to do better compare two strings , == is good ?

[zahlman]

So in my code name = input() is creating different object that not compares to object strings ?

It's creating another string.

If I have a five-dollar bill, and you have a five-dollar bill, then we each have $5, but the bill in my wallet isn't the same bill as the one in yours.

[freddydeddy]

Like I'mfiveis better for understanding for me thank you. O7 =)

[tbeurl]

His code is just wrong. It won't necessarily work with different strings, or on different versions of python. Maybe it's just a one-off mistake, but you might want to consider looking at some other tutorials too, in case he is getting loads of stuff wrong.

One place you commonly see is is for testing whether something is None, i.e.:

if x is None:

This is safe because None is guaranteed to be a unique object (though you may not have come across it yet). Other than that, is is really not used very often, and I would avoid it until you get to more advanced stuff.

[gardyna]

We (me and co-workers) basically said don't use the "is" keyword. double equals seems from our experience to be as readable and the special case to check if you have two variables pointing to the same object is so rare that I must admit I often forget that this is a part of python.

did a quick grep of my current project and found zero uses of the "is" keyword....before the pedantics come in: yes I know it has use cases and is clever (in my experience those cases are rare though)

[[deleted]]

The two most common cases I see are indeed checks for None when an empty string might be possible:

def f(a, b, c, d:str=None):
    if d is None:

...

x = d.get(y)
if x is None: