Difference for string comparison in Python is vs

Question

I noticed a Python script I was writing was acting squirrelly, and traced it to an infinite loop, where the loop condition was while line is not ''. Running through it in the debugger, it turned out that line was in fact ''. When I changed it to !='' rather than is not '', it worked fine.

Also, is it generally considered better to just use '==' by default, even when comparing int or Boolean values? I've always liked to use 'is' because I find it more aesthetically pleasing and pythonic (which is how I fell into this trap...), but I wonder if it's intended to just be reserved for when you care about finding two objects with the same id.

Answer 1

I would like to show a little example on how is and ==are involved in immutable types. Try that:
a = 19998989890
b = 19998989889 +1
>>> a is b
False
>>> a == b
True
is compares two objects in memory, == compares their values. For example, you can see that small integers are cached by Python:
c = 1
b = 1
>>> b is c
True
You should use == when comparing values and iswhen comparing identities. (Also, from an English point of view, "equals" is different from "is".

Answer 2

For all built-in Python objects (like strings, lists, dicts, functions, etc.), if x is y, then x==y is also True.

Not always. NaN is a counterexample. But usually, identity (is) implies equality (==). The converse is not true: Two distinct objects can have the same value.

Also, is it generally considered better to just use '==' by default, even when comparing int or Boolean values?

You use == when comparing values and is when comparing identities.

When comparing ints (or immutable types in general), you pretty much always want the former. There's an optimization that allows small integers to be compared with is, but don't rely on it.

For boolean values, you shouldn't be doing comparisons at all. Instead of:

if x == True: # do something

write:

if x: # do something

For comparing against None, is None is preferred over == None.

I've always liked to use 'is' because I find it more aesthetically pleasing and pythonic (which is how I fell into this trap...), but I wonder if it's intended to just be reserved for when you care about finding two objects with the same id.

Yes, that's exactly what it's for.

Answer 3

In simple terms, the == expression is used to check whether the value of the objects are equal. The is expression is used to check whether the objects are pointing to the same objects/values. Let's try to understand this with an example.

>>> a = [1, 2, 3]
>>> b = a

>>> a
[1, 2, 3]
>>> b
[1, 2, 3]

>>> a == b
True

>>> a is b
True

Well, this is as expected because "a is equal to b" and "a is pointing to the same data as b."

>>> c = list(a)

>>> c
[1, 2, 3]

>>> a == c
True

>>> a is c
False

Now, you can see how it's different. Obviously "a is equal to c" because their values are same but "a is not pointing to the same data as c". a is pointing to [1,2,3] whereas c is pointing to list(a)

Answer 4

Difference between == and is operator in Python. The == operator compares the values of both the operands and checks for value equality. Whereas is operator checks whether both the operands refer to the same object or not. ... Hence list1 and list2 refer to different objects.
Hope this helps.

Answer 5

The is expression is identity testing, == is equality testing. what happens in your code would be emulated in the interpreter like this:
>>> a = 'pub' 
>>> b = ''.join(['p', 'u', 'b']) 
>>> a == b 
True 
>>> a is b 
False
so, no wonder they're not the same, right?
In other words: is is the id(a) == id(b)

Answer 6

If we use "==" means both variables have same value. But if we use "is" it means both variables refer to same object.
If two variables belong to immutable data type. Then "==" and "is" works as same.
But if two variables belong to mutable data type then the result for "==" and "is" will be different. Because they will refer to different objects.
For example:
a = 10, b = 10, c = [1,2], d = [1, 2]

a == b
True
a is b
True
c == d
True
c is d
False

Answer 7

‘Is’ use to check address of object , and == you can use to comparison of object value

Answer 8

'is' checks is it is the same object. meaning.. if you change a, you also change b and vice versa. '==' will check for equivalence. if you change a it won't necessarily change b or vice versa.