Python operation vs is not

Question

In a comment on this question  I saw a statement that recommended using  result is not None   vs  result    None   I was wondering what the difference is  and why one might be recommended over the other

User · Answer

is    True     1 is 1 True      1       1   True      1   is  1   False     a    1       b   a     a is b True   Some objects are singletons  and thus is with them is equivalent to      Most are not

User · Answer

First  let me go over a few terms  If you just want your question answered  scroll down to  Answering your question    Definitions  Object identity  When you create an object  you can assign it to a variable  You can then also assign it to another variable  And another    gt  gt  gt  button   Button    gt  gt  gt  cancel   button  gt  gt  gt  close   button  gt  gt  gt  dismiss   button  gt  gt  gt  print cancel is close  True   In this case  cancel  close  and dismiss all refer to the same object in memory  You only created one Button object  and all three variables refer to this one object  We say that cancel  close  and dismiss all refer to identical objects  that is  they refer to one single object   Object equality  When you compare two objects  you usually don t care that it refers to the exact same object in memory  With object equality  you can define your own rules for how two objects compare  When you write if a    b   you are essentially saying if a   eq   b    This lets you define a   eq   method on a so that you can use your own comparison logic   Rationale for equality comparisons  Rationale  Two objects have the exact same data  but are not identical   They are not the same object in memory   Example  Strings   gt  gt  gt  greeting    It s a beautiful day in the neighbourhood    gt  gt  gt  a   unicode greeting   gt  gt  gt  b   unicode greeting   gt  gt  gt  a is b False  gt  gt  gt  a    b True   Note  I use unicode strings here because Python is smart enough to reuse regular strings without creating new ones in memory   Here  I have two unicode strings  a and b  They have the exact same content  but they are not the same object in memory  However  when we compare them  we want them to compare equal  What s happening here is that the unicode object has implemented the   eq   method   class unicode object                  def   eq   self  other           if len self     len other               return False          for i  j in zip self  other               if i    j                  return False          return True   Note    eq   on unicode is definitely implemented more efficiently than this   Rationale  Two objects have different data  but are considered the same object if some key data is the same  Example  Most types of model data   gt  gt  gt  import datetime  gt  gt  gt  a   Monitor    gt  gt  gt  a make    Dell   gt  gt  gt  a model    E770s   gt  gt  gt  a owner    Bob Jones   gt  gt  gt  a warranty expiration   datetime date 2030  12  31   gt  gt  gt  b   Monitor    gt  gt  gt  b make    Dell   gt  gt  gt  b model    E770s   gt  gt  gt  b owner    Sam Johnson   gt  gt  gt  b warranty expiration   datetime date 2005  8  22   gt  gt  gt  a is b False  gt  gt  gt  a    b True   Here  I have two Dell monitors  a and b  They have the same make and model  However  they neither have the same data nor are the same object in memory  However  when we compare them  we want them to compare equal  What s happening here is that the Monitor object implemented the   eq   method   class Monitor object                  def   eq   self  other           return self make    other make and self model    other model   Answering your question  When comparing to None  always use is not  None is a singleton in Python - there is only ever one instance of it in memory   By comparing identity  this can be performed very quickly  Python checks whether the object you re referring to has the same memory address as the global None object - a very  very fast comparison of two numbers   By comparing equality  Python has to look up whether your object has an   eq   method  If it does not  it examines each superclass looking for an   eq   method  If it finds one  Python calls it  This is especially bad if the   eq   method is slow and doesn t immediately return when it notices that the other object is None   Did you not implement   eq    Then Python will probably find the   eq   method on object and use that instead - which just checks for object identity anyway   When comparing most other things in Python  you will be using

User · Answer

is an equality test  It checks whether the right hand side and the left hand side are equal objects  according to their   eq   or   cmp   methods    is is an identity test  It checks whether the right hand side and the left hand side are the very same object  No methodcalls are done  objects can t influence the is operation   You use is  and is not  for singletons  like None  where you don t care about objects that might want to pretend to be None or where you want to protect against objects breaking when being compared against None

User · Answer

None is a singleton  therefore identity comparison will always work  whereas an object can fake the equality comparison via    eq

User · Answer

Consider the following   class Bad object       def   eq   self  other           return True  c   Bad   c is None   False  equivalent to id c     id None  c    None   True  equivalent to c   eq   None

[python] Python != operation vs "is not"

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