Is there a difference between and is

Question

My Google-fu has failed me   In Python  are the following two tests for equality equivalent   n   5   Test one  if n    5      print  Yay      Test two  if n is 5      print  Yay     Does this hold true for objects where you would be comparing instances  a list say    Okay  so this kind of answers my question   L      L append 1  if L     1       print  Yay     Holds true  but     if L is  1       print  Yay     Doesn t    So    tests value where is tests to see if they are the same object

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They are completely different   is checks for object identity  while    checks for equality  a notion that depends on the two operands  types    It is only a lucky coincidence that  is  seems to work correctly with small integers  e g  5    4 1    That is because CPython optimizes the storage of integers in the range  -5 to 256  by making them singletons  This behavior is totally implementation-dependent and not guaranteed to be preserved under all manner of minor transformative operations   For example  Python 3 5 also makes short strings singletons  but slicing them disrupts this behavior    gt  gt  gt   foo     bar      foobar  True  gt  gt  gt   foo     bar  is  foobar  True  gt  gt  gt   foo        bar      foobar  True  gt  gt  gt   foo        bar  is  foobar  False

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https   docs python org library stdtypes html comparisons  is tests for identity    tests for equality  Each  small  integer value is mapped to a single value  so every 3 is identical and equal  This is an implementation detail  not part of the language spec though

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Python difference between is and equals         The is operator may seem like the same as the equality operator but   they are not same       The is checks if both the variables point to the same object whereas   the    sign checks if the values for the two variables are the same       So if the is operator returns True then the equality is definitely   True  but the opposite may or may not be True    Here is an example to demonstrate the similarity and the difference    gt  gt  gt  a   b    1 2 3   gt  gt  gt  c    1 2 3   gt  gt  gt  a    b True  gt  gt  gt  a    c True  gt  gt  gt  a is b True  gt  gt  gt  a is c False  gt  gt  gt  a    1 2 3   gt  gt  gt  b    1 2   gt  gt  gt  a    b False  gt  gt  gt  a is b False  gt  gt  gt  del a 2   gt  gt  gt  a    b True  gt  gt  gt  a is b False    Tip  Avoid using is operator for immutable types such as strings and numbers  the result is unpredictable

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In a nutshell  is checks whether two references point to the same object or not    checks whether two objects have the same value or not   a  1 2 3  b a         a and b point to the same object c list a    c points to different object   if a  b      print         output   if a is b      print         output     if a  c      print         output     if a is c      print          no output as c and a point to different object

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As John Feminella said  most of the time you will use    and    because your objective is to  compare values  I d just like to categorise what you would do the rest of the time   There is one and only one instance of NoneType i e  None is a singleton  Consequently foo    None and foo is None mean the same  However the is test is faster and the Pythonic convention is to use foo is None   If you are doing some introspection or mucking about with garbage collection or checking whether your custom-built string interning gadget is working or suchlike  then you probably have a use-case for foo is bar   True and False are also  now  singletons  but there is no use-case for foo    True and no use case for foo is True

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Your answer is correct  The is operator compares the identity of two objects   The    operator compares the values of two objects   An object s identity never changes once it has been created  you may think of it as the object s address in memory   You can control comparison behaviour of object values by defining a   cmp   method or a rich comparison method like   eq

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What s the difference between is and        and is are different comparison  As others already said      compares the values of the objects  is compares the references of the objects   In Python names refer to objects  for example in this case value1 and value2 refer to an int instance storing the value 1000  value1   1000 value2   value1   Because value2 refers to the same object is and    will give True   gt  gt  gt  value1    value2 True  gt  gt  gt  value1 is value2 True  In the following example the names value1 and value2 refer to different int instances  even if both store the same integer   gt  gt  gt  value1   1000  gt  gt  gt  value2   1000   Because the same value  integer  is stored    will be True  that s why it s often called  quot value comparison quot   However is will return False because these are different objects   gt  gt  gt  value1    value2 True  gt  gt  gt  value1 is value2 False  When to use which  Generally is is a much faster comparison  That s why CPython caches  or maybe reuses would be the better term  certain objects like small integers  some strings  etc  But this should be treated as implementation detail that could  even if unlikely  change at any point without warning  You should only use is if you   want to check if two objects are really the same object  not just the same  quot value quot    One example can be if you use a singleton object as constant   want to compare a value to a Python constant  The constants in Python are   None True1 False1 NotImplemented Ellipsis   debug   classes  for example int is int or int is float  there could be additional constants in built-in modules or 3rd party modules  For example np ma masked from the NumPy module     In every other case you should use    to check for equality  Can I customize the behavior  There is some aspect to    that hasn t been mentioned already in the other answers  It s part of Pythons  quot Data model quot   That means its behavior can be customized using the   eq   method  For example  class MyClass object       def   init   self  val           self  value   val      def   eq   self  other           print    eq   method called           try              return self  value    other  value         except AttributeError              raise TypeError  Cannot compare  0  to objects of type  1                                format type self   type other     This is just an artificial example to illustrate that the method is really called   gt  gt  gt  MyClass 10     MyClass 10    eq   method called True  Note that by default  if no other implementation of   eq   can be found in the class or the superclasses    eq   uses is  class AClass object       def   init   self  value           self  value   value   gt  gt  gt  a   AClass 10   gt  gt  gt  b   AClass 10   gt  gt  gt  a    b False  gt  gt  gt  a    a  So it s actually important to implement   eq   if you want  quot more quot  than just reference-comparison for custom classes  On the other hand you cannot customize is checks  It will always compare just if you have the same reference  Will these comparisons always return a boolean  Because   eq   can be re-implemented or overridden  it s not limited to return True or False  It could return anything  but in most cases it should return a boolean    For example with NumPy arrays the    will return an array   gt  gt  gt  import numpy as np  gt  gt  gt  np arange 10     2 array  False  False   True  False  False  False  False  False  False  False   dtype bool   But is checks will always return True or False   1 As Aaron Hall mentioned in the comments  Generally you shouldn t do any is True or is False checks because one normally uses these  quot checks quot  in a context that implicitly converts the condition to a boolean  for example in an if statement   So doing the is True comparison and the implicit boolean cast is doing more work than just doing the boolean cast - and you limit yourself to booleans  which isn t considered pythonic   Like PEP8 mentions   Don t compare boolean values to True or False using     Yes    if greeting  No     if greeting    True  Worse  if greeting is True

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Have a look at Stack Overflow question Python s    is    operator behaves unexpectedly with integers   What it mostly boils down to is that  is  checks to see if they are the same object  not just equal to each other  the numbers below 256 are a special case

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Is there a difference between    and is in Python    Yes  they have a very important difference       check for equality - the semantics are that equivalent objects  that aren t necessarily the same object  will test as equal  As the documentation says      The operators  lt               lt    and    compare the values of two objects    is  check for identity - the semantics are that the object  as held in memory  is the object  Again  the documentation says      The operators is and is not test for object identity  x is y is true   if and only if x and y are the same object  Object identity is   determined using the id   function  x is not y yields the inverse   truth value    Thus  the check for identity is the same as checking for the equality of the IDs of the objects  That is   a is b   is the same as   id a     id b    where id is the builtin function that returns an integer that  is guaranteed to be unique among simultaneously existing objects   see help id   and where a and b are any arbitrary objects   Other Usage Directions  You should use these comparisons for their semantics  Use is to check identity and    to check equality   So in general  we use is to check for identity  This is usually useful when we are checking for an object that should only exist once in memory  referred to as a  singleton  in the documentation   Use cases for is include    None enum values  when using Enums from the enum module  usually modules usually class objects resulting from class definitions usually function objects resulting from function definitions anything else that should only exist once in memory  all singletons  generally  a specific object that you want by identity   Usual use cases for    include    numbers  including integers strings lists sets dictionaries custom mutable objects other builtin immutable objects  in most cases   The general use case  again  for     is the object you want may not be the same object  instead it may be an equivalent one  PEP 8 directions  PEP 8  the official Python style guide for the standard library also mentions two use-cases for is      Comparisons to singletons like None should always be done with is or   is not  never the equality operators       Also  beware of writing if x when you really mean if x is not None --   e g  when testing whether a variable or argument that defaults to None   was set to some other value  The other value might have a type  such   as a container  that could be false in a boolean context    Inferring equality from identity  If is is true  equality can usually be inferred - logically  if an object is itself  then it should test as equivalent to itself    In most cases this logic is true  but it relies on the implementation of the   eq   special method  As the docs say       The default behavior for equality comparison     and     is based on   the identity of the objects  Hence  equality comparison of instances   with the same identity results in equality  and equality comparison of   instances with different identities results in inequality  A   motivation for this default behavior is the desire that all objects   should be reflexive  i e  x is y implies x    y     and in the interests of consistency  recommends      Equality comparison should be reflexive  In other words  identical   objects should compare equal       x is y implies x    y   We can see that this is the default behavior for custom objects    gt  gt  gt  class Object object   pass  gt  gt  gt  obj   Object    gt  gt  gt  obj2   Object    gt  gt  gt  obj    obj  obj is obj  True  True   gt  gt  gt  obj    obj2  obj is obj2  False  False    The contrapositive is also usually true - if somethings test as not equal  you can usually infer that they are not the same object    Since tests for equality can be customized  this inference does not always hold true for all types   An exception  A notable exception is nan - it always tests as not equal to itself    gt  gt  gt  nan   float  nan    gt  gt  gt  nan nan  gt  gt  gt  nan is nan True  gt  gt  gt  nan    nan                   False   Checking for identity can be much a much quicker check than checking for equality  which might require recursively checking members     But it cannot be substituted for equality where you may find more than one object as equivalent   Note that comparing equality of lists and tuples will assume that identity of objects are equal  because this is a fast check   This can create contradictions if the logic is inconsistent - as it is for nan    gt  gt  gt   nan      nan  True  gt  gt  gt   nan       nan   True   A Cautionary Tale   The question is attempting to use is to compare integers  You shouldn t assume that an instance of an integer is the same instance as one obtained by another reference  This story explains why   A commenter had code that relied on the fact that small integers  -5 to 256 inclusive  are singletons in Python  instead of checking for equality      Wow  this can lead to some insidious bugs  I had some code that checked if a is b  which worked as I wanted because a and b are typically small numbers  The bug only happened today  after six months in production  because a and b were finally large enough to not be cached      gwg   It worked in development  It may have passed some unittests    And it worked in production - until the code checked for an integer larger than 256  at which point it failed in production    This is a production failure that could have been caught in code review or possibly with a style-checker   Let me emphasize  do not use is to compare integers

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determines if the values are equal  while is determines if they are the exact same object

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As the other people in this post answer the question in details the difference between    and is for comparing Objects or variables  I would emphasize mainly the comparison between is and    for strings which can give different results and I would urge programmers to carefully use them  For string comparison  make sure to use    instead of is  str    hello  if  str is  hello        print   str is hello   if  str     hello        print   str    hello    Out  str is hello str    hello  But in the below example    and is will get different results  str2    hello sam      if  str2 is  hello sam            print   str2 is hello sam       if  str2     hello sam            print   str2    hello sam    Out  str2    hello sam  Conclusion and Analysis  Use is carefully to compare between strings  Since is for comparing objects and since in Python 3  every variable such as string interpret as an object  let s see what happened in above paragraphs  In python there is id function that shows a unique constant of an object during its lifetime  This id is using in back-end of Python interpreter to compare two objects using is keyword  str    hello  id  hello    gt  140039832615152 id str   gt  140039832615152  But str2    hello sam  id  hello sam    gt  140039832615536 id str2   gt  140039832615792

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Most of them already answered to the point  Just as an additional note  based on my understanding and experimenting but not from a documented source   the statement         if the objects referred to by the variables are equal   from above answers should be read as         if the objects referred to by the variables are equal and objects belonging to the same type class     I arrived at this conclusion based on the below test   list1    1 2 3 4  tuple1    1 2 3 4   print list1  print tuple1  print id list1   print id tuple1    print list1    tuple1  print list1 is tuple1    Here the contents of the list and tuple are same but the type class are different

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is will return True if two variables point to the same object     if the objects referred to by the variables are equal    gt  gt  gt  a    1  2  3   gt  gt  gt  b   a  gt  gt  gt  b is a  True  gt  gt  gt  b    a True    Make a new copy of list  a  via the slice operator     and assign it to variable  b   gt  gt  gt  b   a      gt  gt  gt  b is a False  gt  gt  gt  b    a True   In your case  the second test only works because Python caches small integer objects  which is an implementation detail  For larger integers  this does not work    gt  gt  gt  1000 is 10  3 False  gt  gt  gt  1000    10  3 True   The same holds true for string literals    gt  gt  gt   a  is  a  True  gt  gt  gt   aa  is  a    2 True  gt  gt  gt  x    a   gt  gt  gt   aa  is x   2 False  gt  gt  gt   aa  is intern x 2  True   Please see this question as well

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There is a simple rule of thumb to tell you when to use    or is       is for value equality  Use it when you would like to know if two objects have the same value  is is for reference equality  Use it when you would like to know if two references refer to the same object    In general  when you are comparing something to a simple type  you are usually checking for value equality  so you should use     For example  the intention of your example is probably to check whether x has a value equal to 2       not whether x is literally referring to the same object as 2     Something else to note  because of the way the CPython reference implementation works  you ll get unexpected and inconsistent results if you mistakenly use is to compare for reference equality on integers    gt  gt  gt  a   500  gt  gt  gt  b   500  gt  gt  gt  a    b True  gt  gt  gt  a is b False   That s pretty much what we expected  a and b have the same value  but are distinct entities  But what about this    gt  gt  gt  c   200  gt  gt  gt  d   200  gt  gt  gt  c    d True  gt  gt  gt  c is d True   This is inconsistent with the earlier result  What s going on here  It turns out the reference implementation of Python caches integer objects in the range -5  256 as singleton instances for performance reasons  Here s an example demonstrating this    gt  gt  gt  for i in range 250  260   a   i  print   i   s     i  a is int str i          250  True 251  True 252  True 253  True 254  True 255  True 256  True 257  False 258  False 259  False   This is another obvious reason not to use is  the behavior is left up to implementations when you re erroneously using it for value equality

[python] Is there a difference between "==" and "is"?

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