Aren t Python strings immutable Then why does a b work

Question

My understanding was that Python strings are immutable   I tried the following code   a    Dog  b    eats  c    treats   print a  b  c   Dog eats treats  print a         b         c   Dog eats treats  print a   Dog  a   a         b         c print a   Dog eats treats         Shouldn t Python have prevented the assignment  I am probably missing something   Any idea

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mutable  means that we can change the content of the string   immutable  means that we can t add an extra string

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You can make a numpy array immutable and use the first element   numpyarrayname 0     write once    then   numpyarrayname setflags write False    or  numpyarrayname flags writeable   False

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l    1 2 3  print id l  l append 4  print id l   object l is the same  a    dog  print id a  a    cat  print id a   object a is a new object  previous one is deleted

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Consider    gt  gt  gt  a  asdf   gt  gt  gt  a   repr    lt method-wrapper    repr    of str object at 0x1091aab90 gt   gt  gt  gt  a  asdf   gt  gt  gt  a   repr    lt method-wrapper    repr    of str object at 0x1091aab90 gt   gt  gt  gt  a  qwer   gt  gt  gt  a   repr    lt method-wrapper    repr    of str object at 0x109198490 gt    Notice that the hex memory location did not change when I stored the same value in the variable twice  It did change when I stored a different value  The string is immutable  Not because of zealotry  but because you pay the performance penalty of creating a new object in memory  The variable a is just a label pointing to that memory address  It can be altered to point to anything

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Python strings are immutable  However  a is not a string  it is a variable with a string value  You can t mutate the string  but can change what value of the variable to a new string

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a    dog  address   id a  print id a    a   a    cat  print id a         Address changes  import ctypes ctypes cast address  ctypes py object  value     value at old address is intact

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The string objects themselves are immutable   The variable  a  which points to the string  is mutable   Consider   a    Foo    a now points to  Foo  b   a   b points to the same  Foo  that a points to a   a   a   a points to the new string  FooFoo   but b still points to the old  Foo   print a print b   Outputs     FooFoo   Foo    Observe that b hasn t changed  even though a has

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This image gives the answer  Please read it

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We r just concatenate the two string values  We never change the value of  a   Just now  a  represent another memory block that has  dogdog  value  Because in the backend  one variable never represent two memory blocks at same time  The value of  a  before concatenation was  dog   But after that  a  represent the   dogdog   because now  a  in backend rep  the block that has  dogdog  value  And  dog  is rep  by  b  and  dog  isn t counted as garbage value until  b  represent the  dog     The confusion is we represent the memory blocks that contain data or info   in backend with same variable name

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The statement a   a         b         c can be broken down based upon pointers   a       says give me what a points to  which can t be changed  and add     to my current working set   memory   working set    Dog   a    Dog   b    eats  c    treats      b says give me what b points to  which can t be changed  and add it to current working set   memory   working set    Dog eats  a    Dog   b    eats  c    treats            c says add     to the current set  Then give me what c points to  which can t be changed  and add it to current working set   memory   working set    Dog eats treats  a    Dog   b    eats  c    treats    Finally  a   says set my pointer to point to the resulting set   memory   a    Dog eats treats  b    eats  c    treats     Dog  is reclaimed  because no more pointers connect to it s chunk of memory  We never modified the memory section  Dog  resided in  which is what is meant by immutable  However  we can change which labels  if any  point to that section of memory

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The variable a is pointing at the object  Dog   It s best to think of the variable in Python as a tag  You can move the tag to different objects which is what you did when you changed a    dog  to a    dog eats treats    However  immutability refers to the object  not the tag      If you tried a 1     z  to make  dog  into  dzg   you would get the error   TypeError   str  object does not support item assignment     because strings don t support item assignment  thus they are immutable

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Summarizing   a   3 b   a a   3 2 print b   5   Not immutable   a    OOP  b   a a    p  a print b   OOP   Immutable   a    1 2 3  b   range len a   for i in range len a        b i    a i  1   This is an error in Python 3 because it is immutable  And not an error in Python 2 because clearly it is not immutable

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The built-in function id   returns the identity of an object as an integer  This integer usually corresponds to the object   s location in memory     gt  gt a  dog    gt  gt print id a    139831803293008    gt  gt a a  cat    gt  gt print id a    139831803293120   Initially   a  is stored in 139831803293008 memory location  as the string object is immutable in python if you try to modify and reassign the reference will be removed and will be a pointer to a new memory location 139831803293120

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Variables can point to anywhere they want   An error will be thrown if you do the following   a    dog  print a                    dog a 1     g                  ERROR       STRINGS ARE IMMUTABLE

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Something is mutable only when we are able to change the values held in the memory location without changing the memory location itself   The trick is  If you find that the memory location before and after the change are the same  it is mutable   For example  list is mutable  How    gt  gt  a     hello    gt  gt  id a  139767295067632    Now let s modify  1  gt  gt  a 0     hello new   gt  gt  a   hello new   Now that we have changed  a   let s see the location of a  gt  gt  id a  139767295067632 so it is the same as before  So we mutated a  So list is mutable    A string is immutable  How do we prove it    gt  a    hello   gt  a 0   h    Now let s modify it  gt  a 0     n  ----------------------------------------------------------------------   we get     TypeError   str  object does not support item assignment   So we failed mutating the string  It means a string is immutable   In you reassigning  you change the variable to point to a new location itself  Here you have not mutated the string  but mutating the variable itself  The following is what you are doing    gt  gt  a    hello   gt  gt  id a  139767308749440  gt  gt  a   world   gt  gt  id a  139767293625808   id before and after reassignment is different  so it this proves that you are actually not mutating  but pointing the variable to new location  Which is not mutating that string  but mutating that variable

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There is a difference between data and the label it is associated with  For example when you do   a    dog    the data  dog  is created and put under the label a  The label can change but what is in the memory won t  The data  dog  will still exist in memory  until the garbage collector deletes it  after you do   a    cat    In your programm a now  points to   cat  but the string  dog  hasn t changed

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A variable is just a label pointing to an object  The object is immutable  but you can make the label point to a completely different object if you want to

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Adding a bit more to above-mentioned answers   id of a variable changes upon reassignment    gt  gt  gt  a    initial string   gt  gt  gt  id a  139982120425648  gt  gt  gt  a    new string   gt  gt  gt  id a  139982120425776   Which means that we have mutated the variable a to point to a new string  Now there exist two string str  objects    initial string  with id   139982120425648  and   new string  with id   139982120425776  Consider the below code    gt  gt  gt  b    intitial string   gt  gt  gt  id b  139982120425648   Now  b points to the  initial string  and has the same id as a had before reassignment   Thus  the  intial string  has not been mutated

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Consider this addition to your example  a    quot Dog quot   b    quot eats quot   c    quot treats quot   print  a b c    Dog eats treats  d   a    quot   quot    b    quot   quot    c  print  a    Dog  print  d    Dog eats treats  One of the more precise explanations I found in a blog is   In Python   almost  everything is an object  What we commonly refer to as  quot variables quot  in Python are more properly called names  Likewise   quot assignment quot  is really the binding of a name to an object  Each binding has a scope that defines its visibility  usually the block in which the name originates  Eg   some guy    Fred        some guy    George    When we later say some guy    George   the string object containing  Fred  is unaffected  We ve just changed the binding of the name some guy  We haven t  however  changed either the  Fred  or  George  string objects  As far as we re concerned  they may live on indefinitely   Link to blog  https   jeffknupp com blog 2012 11 13 is-python-callbyvalue-or-callbyreference-neither

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gt  gt  gt  a    dogs    gt  gt  gt  a replace  dogs    dogs eat treats     dogs eat treats    gt  gt  gt  print a   dogs   Immutable  isn t it    The variable change part has already been discussed

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First a pointed to the string  Dog    Then you changed the variable a to point at a new string  Dog eats treats    You didn t actually mutate the string  Dog    Strings are immutable  variables can point at whatever they want

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Python string objects are immutable  Example    gt  gt  gt  a    tanim   gt  gt  gt   Address of a is     format id a    Address of a is 64281536   gt  gt  gt  a    ahmed   gt  gt  gt   Address of a is     format id a    Address of a is 64281600    In this example we can see that when we assign different value in a it doesn t modify A new object is created  And it can t be modified  Example      gt  gt  gt  a 0     c      Traceback  most recent call last         File   lt stdin gt    line 1  in  lt module gt        TypeError     str  object does not support item assignment   A error occurs

[python] Aren't Python strings immutable? Then why does a + " " + b work?

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