How to use is and == in Python

I've only been in Python for about a year now, but I've noticed that I couldn't clearly distinguish between the comparison operators ʻisand==`. Described as a commandment to yourself.

1. Organize

ʻIsis Object Identity ==` is Object Equality

Is. As the name implies, Object Identity determines whether they are the same object.

On the other hand, for ==, it is the same implementation as the __eq__ method. For example, it is determined whether or not the character strings match even between different objects.

a = 'hoge'

print(a.__eq__('hoge'))  # True
  1. Usage
a = None

if a == None:
    print('Not good')

On the other hand, if it complies with PEP8, the following will be alerted. E711 comparison to None should be 'if cond is Nond:'

This says that for singletons like None, identity in Object Identity should be compared. Therefore, it is desirable to write as follows.

a = None

if a is None:

However, if it is not necessary to clearly indicate that it is None, ↓ is Pythonic. (3. I wrote it in a digression)

a = None

if not a:

Also, for example, if ʻisis used for character string comparison as shown below, it cannot be evaluated properly. If you want to make a comparison such as strings match, you need to use==`. (Also, in the case of string comparison, you should also be careful about unicode or str)

a = 'hoghoge'

if a is 'hogehoge':
    print('This is not called!')
    print('This is called!')

3. Digression

if a:

The if clause is also often used in Python.

this is,

if a is not None:

Unlike a,

It means that it is different from any of.

If it is better to make an explicit comparison with these,

ʻIf a is not : `

If not,

if a:


4. Summary

Make sure you understand the usage and write correct, Pythonic code.

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