Python None comparison should I use is or

Question

My editor warns me when I compare my var    None  but no warning when I use my var is None     I did a test in the Python shell and determined both are valid syntax  but my editor seems to be saying that my var is None is preferred    Is this the case  and if so  why

User · Answer

is is generally preferred when comparing arbitrary objects to singletons like None because it is faster and more predictable  is always compares by object identity  whereas what    will do depends on the exact type of the operands and even on their ordering   This recommendation is supported by PEP 8  which explicitly states that  comparisons to singletons like None should always be done with is or is not  never the equality operators

User · Answer

PEP 8 defines that it is better to use the is operator when comparing singletons

User · Answer

Summary   Use is when you want to check against an object s identity  e g  checking to see if var is None    Use    when you want to check equality  e g  Is var equal to 3     Explanation   You can have custom classes where my var    None will return True  e g   class Negator object       def   eq   self other           return not other  thing   Negator   print thing    None     True print thing is None     False   is checks for object identity   There is only 1 object None  so when you do my var is None  you re checking whether they actually are the same object  not just equivalent objects    In other words     is a check for equivalence  which is defined from object to object  whereas is checks for object identity   lst    1 2 3  lst    lst       This is True since the lists are  equivalent  lst is lst       This is False since they re actually different objects

[python] Python None comparison: should I use "is" or ==?

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