Why does comparing strings using either or is sometimes produce a different result

Question

I ve got a Python program where two variables are set to the value  public   In a conditional expression I have the comparison var1 is var2 which fails  but if I change it to var1    var2 it returns True   Now if I open my Python interpreter and do the same  is  comparison  it succeeds    gt  gt  gt  s1    public   gt  gt  gt  s2    public   gt  gt  gt  s2 is s1 True   What am I missing here

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is will compare the memory location  It is used for object-level comparison      will compare the variables in the program  It is used for checking at a value level    is checks for address level equivalence     checks for value level equivalence

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is is identity testing     is equality testing  what happens in your code would be emulated in the interpreter like this   gt  gt  gt  a    pub   gt  gt  gt  b      join   p    u    b     gt  gt  gt  a    b True  gt  gt  gt  a is b False  so  no wonder they re not the same  right  In other words  a is b is the equivalent of id a     id b

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Other answers here are correct  is is used for identity comparison  while    is used for equality comparison  Since what you care about is equality  the two strings should contain the same characters   in this case the is operator is simply wrong and you should be using    instead   The reason is works interactively is that  most  string literals are interned by default  From Wikipedia      Interned strings speed up string   comparisons  which are sometimes a   performance bottleneck in applications    such as compilers and dynamic   programming language runtimes  that   rely heavily on hash tables with   string keys  Without interning    checking that two different strings   are equal involves examining every   character of both strings  This is   slow for several reasons  it is   inherently O n  in the length of the   strings  it typically requires reads   from several regions of memory  which   take time  and the reads fills up the   processor cache  meaning there is less   cache available for other needs  With   interned strings  a simple object   identity test suffices after the   original intern operation  this is   typically implemented as a pointer   equality test  normally just a single   machine instruction with no memory   reference at all    So  when you have two string literals  words that are literally typed into your program source code  surrounded by quotation marks  in your program that have the same value  the Python compiler will automatically intern the strings  making them both stored at the same memory location    Note that this doesn t always happen  and the rules for when this happens are quite convoluted  so please don t rely on this behavior in production code    Since in your interactive session both strings are actually stored in the same memory location  they have the same identity  so the is operator works as expected   But if you construct a string by some other method  even if that string contains exactly the same characters   then the string may be equal  but it is not the same string -- that is  it has a different identity  because it is stored in a different place in memory

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is is identity testing and    is equality testing  This means is is a way to check whether two things are the same things  or just equivalent  Say you ve got a simple person object  If it is named  Jack  and is  23  years old  it s equivalent to another 23-year-old Jack  but it s not the same person  class Person object      def   init   self  name  age          self name   name        self age   age     def   eq   self  other          return self name    other name and self age    other age  jack1   Person  Jack   23  jack2   Person  Jack   23   jack1    jack2   True jack1 is jack2   False  They re the same age  but they re not the same instance of person  A string might be equivalent to another  but it s not the same object

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The is keyword is a test for object identity while    is a value comparison   If you use is  the result will be true if and only if the object is the same object  However     will be true any time the values of the object are the same

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This is a side note  but in idiomatic Python  you will often see things like  if x is None        Some clauses  This is safe  because there is guaranteed to be one instance of the Null Object  i e   None

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The    operator tests value equivalence  The is operator tests object identity  and Python tests whether the two are really the same object  i e   live at the same address in memory    gt  gt  gt  a    banana   gt  gt  gt  b    banana   gt  gt  gt  a is b True  In this example  Python only created one string object  and both a and b refers to it  The reason is that Python internally caches and reuses some strings as an optimization  There really is just a string  banana  in memory  shared by a and b  To trigger the normal behavior  you need to use longer strings   gt  gt  gt  a    a longer banana   gt  gt  gt  b    a longer banana   gt  gt  gt  a    b  a is b  True  False   When you create two lists  you get two objects   gt  gt  gt  a    1  2  3   gt  gt  gt  b    1  2  3   gt  gt  gt  a is b False  In this case we would say that the two lists are equivalent  because they have the same elements  but not identical  because they are not the same object  If two objects are identical  they are also equivalent  but if they are equivalent  they are not necessarily identical  If a refers to an object and you assign b   a  then both variables refer to the same object   gt  gt  gt  a    1  2  3   gt  gt  gt  b   a  gt  gt  gt  b is a True

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The basic concept we have to be clear while approaching this question is to understand the difference between is and       quot is quot  is will compare the memory location  if id a   id b   then a is b returns true else it returns false   so  we can say that is is used for comparing memory locations  Whereas      is used for equality testing which means that it just compares only the resultant values  The below shown code may acts as an example to the above given theory   code  In the case of string literals strings without getting assigned to variables   the memory address will be same as shown in the picture  so  id a   id b   remaining this is self-explanatory

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Actually  the is operator checks for identity and    operator checks for equality  From the language reference   Types affect almost all aspects of object behavior  Even the importance of object identity is affected in some sense  for immutable types  operations that compute new values may actually return a reference to any existing object with the same type and value  while for mutable objects this is not allowed  E g   after a   1  b   1  a and b may or may not refer to the same object with the value one  depending on the implementation  but after c       d       c and d are guaranteed to refer to two different  unique  newly created empty lists   Note that c   d      assigns the same object to both c and d    So from the above statement we can infer that the strings  which are immutable types  may fail when checked with  quot is quot  and may succeed when checked with  quot is quot   The same applies for int and tuple which are also immutable types

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From my limited experience with python  is is used to compare two objects to see if they are the same object as opposed to two different objects with the same value      is used to determine if the values are identical    Here is a good example    gt  gt  gt  s1   u public   gt  gt  gt  s2    public   gt  gt  gt  s1 is s2 False  gt  gt  gt  s1    s2 True   s1 is a unicode string  and s2 is a normal string   They are not the same type  but are the same value

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One last thing to note  you may use the sys intern function to ensure that you re getting a reference to the same string    gt  gt  gt  from sys import intern  gt  gt  gt  a   intern  a    gt  gt  gt  a2   intern  a    gt  gt  gt  a is a2 True   As pointed out above  you should not be using is to determine equality of strings   But this may be helpful to know if you have some kind of weird requirement to use is   Note that the intern function used to be a builtin on Python 2 but was moved to the sys module in Python 3

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If you re not sure what you re doing  use the       If you have a little more knowledge about it you can use  is  for known objects like  None    Otherwise you ll end up wondering why things doesn t work and why this happens    gt  gt  gt  a   1  gt  gt  gt  b   1  gt  gt  gt  b is a True  gt  gt  gt  a   6000  gt  gt  gt  b   6000  gt  gt  gt  b is a False   I m not even sure if some things are guaranteed to stay the same between different python versions implementations

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I think it has to do with the fact that  when the  is  comparison evaluates to false  two distinct objects are used  If it evaluates to true  that means internally it s using the same exact object and not creating a new one  possibly because you created them within a fraction of 2 or so seconds and because there isn t a large time gap in between it s optimized and uses the same object   This is why you should be using the equality operator     not is  to compare the value of a string object    gt  gt  gt  s    one   gt  gt  gt  s2    two   gt  gt  gt  s is s2 False  gt  gt  gt  s2   s2 replace  two    one    gt  gt  gt  s2  one   gt  gt  gt  s2 is s False  gt  gt  gt     In this example  I made s2  which was a different string object previously equal to  one  but it is not the same object as s  because the interpreter did not use the same object as I did not initially assign it to  one   if I had it would have made them the same object

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is is identity testing and    is equality testing  see the Python documentation   In most cases  if a is b  then a    b  But there are exceptions  for example   gt  gt  gt  nan   float  nan    gt  gt  gt  nan is nan True  gt  gt  gt  nan    nan False  So  you can only use is for identity tests  never equality tests

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I believe that this is known as  interned  strings  Python does this  so does Java  and so do C and C   when compiling in optimized modes   If you use two identical strings  instead of wasting memory by creating two string objects  all interned strings with the same contents point to the same memory   This results in the Python  is  operator returning True because two strings with the same contents are pointing at the same string object  This will also happen in Java and in C   This is only useful for memory savings though  You cannot rely on it to test for string equality  because the various interpreters and compilers and JIT engines cannot always do it

[python] Why does comparing strings using either '==' or 'is' sometimes produce a different result?

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