What is a None value

Question

I have been studying Python  and I read a chapter which describes the None value  but unfortunately this book isn t very clear at some points  I thought that I would find the answer to my question  if I share it there   I want to know what the None value is and what do you use it for   And also  I don t get this part of the book      Assigning a value of None to a variable is one way to reset it to   its original  empty state    What does that mean   The answers were great  although I didn t understand most of answers due to my low knowledge of the computer world  I haven t learned about classes  objects  etc    What does this sentence mean      Assigning a value of None to a variable is one way to reset it   to its original  empty state    Final   Finally I ve got my answer from looking to different answers  I must appreciate all the people who put their times to help me  especially Martijn Pieters and DSM   and I wish that I could choose all answers as the best  but the selection is limited to one  All of the answers were great

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largest none smallest  none  While True             num  raw input   enter a number               if num    done    break            try              inp  int  inp             except                Print Invalid input              if largest is none                  largest inp            elif inp gt largest                  largest  none             print  maximum   largest            if smallest is none                 smallest  none           elif inp lt smallest                  smallest  inp           print  minimum   smallest   print  maximum  minimum  largest  smallest

User · Answer

None is just a value that commonly is used to signify  empty   or  no value here   It is a signal object  it only has meaning because the Python documentation says it has that meaning   There is only one copy of that object in a given Python interpreter session   If you write a function  and that function doesn t use an explicit return statement  None is returned instead  for example  That way  programming with functions is much simplified  a function always returns something  even if it is only that one None object   You can test for it explicitly   if foo is None        foo is set to None  if bar is not None        bar is set to something  other  than None   Another use is to give optional parameters to functions an  empty  default   def spam foo None       if foo is not None            foo was specified  do something clever    The function spam   has a optional argument  if you call spam   without specifying it  the default value None is given to it  making it easy to detect if the function was called with an argument or not   Other languages have similar concepts  SQL has NULL  JavaScript has undefined and null  etc   Note that in Python  variables exist by virtue of being used  You don t need to declare a variable first  so there are no really empty variables in Python  Setting a variable to None is then not the same thing as setting it to a default empty value  None is a value too  albeit one that is often used to signal emptyness  The book you are reading is misleading on that point

User · Answer

I love code examples  as well as fruit   so let me show you  apple    apple  print apple   gt  gt  gt  apple apple   None print apple   gt  gt  gt  None   None means nothing  it has no value   None evaluates to False

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All of these are good answers but I think there s more to explain why None is useful   Imagine you collecting RSVPs to a wedding  You want to record whether each person will attend  If they are attending  you set person attending   True  If they are not attending you set person attending   False  If you have not received any RSVP  then person attending   None  That way you can distinguish between no information - None - and a negative answer

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The book you refer to is clearly trying to greatly simplify the meaning of None  Python variables don t have an initial  empty state      Python variables are bound  only  when they re defined  You can t create a Python variable without giving it a value    gt  gt  gt  print x  Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  NameError  name  x  is not defined  gt  gt  gt  def test x         print x        gt  gt  gt  test   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  test   takes exactly 1 argument  0 given   gt  gt  gt  def test          print x        gt  gt  gt  test   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt    File   lt stdin gt    line 2  in test NameError  global name  x  is not defined   but sometimes you want to make a function mean different things depending on whether a variable is defined or not  You can create an argument with a default value of None    gt  gt  gt  def test x None         if x is None          print  no x here         else          print x        gt  gt  gt  test   no x here  gt  gt  gt  test  x    x    The fact that this value is the special None value is not terribly important in this case  I could ve used any default value    gt  gt  gt  def test x -1         if x    -1          print  no x here         else          print x        gt  gt  gt  test   no x here  gt  gt  gt  test  x    x       but having None around gives us two benefits    We don t have to pick a special value like -1 whose meaning is unclear  and Our function may actually need to handle -1 as a normal input     gt  gt  gt  test -1  no x here   oops   So the book is a little misleading mostly in its use of the word reset      assigning None to a name is a signal to a programmer that that value isn t being used or that the function should behave in some default way  but to reset a value to its original  undefined state you must use the del keyword    gt  gt  gt  x   3  gt  gt  gt  x 3  gt  gt  gt  del x  gt  gt  gt  x Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  NameError  name  x  is not defined

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Martijn s answer explains what None is in Python  and correctly states that the book is misleading   Since Python programmers as a rule would never say     Assigning a value of None to a variable is one way to reset it to   its original  empty state    it s hard to explain what Briggs means in a way which makes sense and explains why no one here seems happy with it   One analogy which may help   In Python  variable names are like stickers put on objects   Every sticker has a unique name written on it  and it can only be on one object at a time  but you could put more than one sticker on the same object  if you wanted to   When you write  F    fork    you put the sticker  F  on a string object  fork    If you then write  F   None   you move the sticker to the None object   What Briggs is asking you to imagine is that you didn t write the sticker  F   there was already an F sticker on the None  and all you did was move it  from None to  fork    So when you type F   None  you re  reset ting  it to its original  empty state   if we decided to treat None as meaning empty state   I can see what he s getting at  but that s a bad way to look at it   If you start Python and type print F   you see   gt  gt  gt  print F  Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  NameError  name  F  is not defined   and that NameError means Python doesn t recognize the name F  because there is no such sticker   If Briggs were right and F   None resets F to its original state  then it should be there now  and we should see   gt  gt  gt  print F  None   like we do after we type F   None and put the sticker on None     So that s all that s going on   In reality  Python comes with some stickers already attached to objects  built-in names   but others you have to write yourself with lines like F    fork  and A   2 and c17   3 14  and then you can stick them on other objects later  like F   10 or F   None  it s all the same    Briggs is pretending that all possible stickers you might want to write were already stuck to the None object

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Other answers have already explained meaning of None beautifully  However  I would still like to throw more light on this using an example   Example   def extendList val  list          list append val      return list  list1   extendList 10  list2   extendList 123     list3   extendList  a    print  list1    s    list1 print  list2    s    list2 print  list3    s    list3   Now try to guess output of above list  Well  the answer is surprisingly as below   list1    10   a   list2    123  list3    10   a       But Why   Many will mistakenly expect list1 to be equal to  10  and list3 to be equal to   a    thinking that the list argument will be set to its default value of    each time extendList is called   However  what actually happens is that the new default list is created only once when the function is defined  and that same list is then used subsequently whenever extendList is invoked without a list argument being specified  This is because expressions in default arguments are calculated when the function is defined  not when it   s called   list1 and list3 are therefore operating on the same default list  whereas list2 is operating on a separate list that it created  by passing its own empty list as the value for the list parameter       None  the savior   Modify example above to produce desired behavior   def extendList val  list None       if list is None         list          list append val      return list  list1   extendList 10  list2   extendList 123     list3   extendList  a    print  list1    s    list1 print  list2    s    list2 print  list3    s    list3   With this revised implementation  the output would be   list1    10  list2    123  list3     a     Note - Example credit to toptal com

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This is what the Python documentation has got to say about None      The sole value of types NoneType  None is frequently used to represent   the absence of a value  as when default arguments are not passed to a   function       Changed in version 2 4  Assignments to None are illegal and raise a   SyntaxError       Note The names None and debug cannot be reassigned  assignments to   them  even as an attribute name  raise SyntaxError   so they can be   considered    true    constants     Let s confirm the type of None first  print type None  print None   class     Output   lt type  NoneType  gt   lt type  NoneType  gt     Basically  NoneType is a data type just like int  float  etc  You can check out the list of default types available in Python in 8 15  types     Names for built-in types    And  None is an instance of NoneType class  So we might want to create instances of None ourselves  Let s try that  print types IntType   print types NoneType     Output  0 TypeError  cannot create  NoneType  instances    So clearly  cannot create NoneType instances  We don t have to worry about the uniqueness of the value None    Let s check how we have implemented None internally   print dir None    Output      class        delattr        doc        format        getattribute          hash        init        new        reduce        reduce ex         repr        setattr        sizeof        str        subclasshook        Except   setattr    all others are read-only attributes  So  there is no way we can alter the attributes of None    Let s try and add new attributes to None  setattr types NoneType   somefield    somevalue   setattr None   somefield    somevalue   None somefield    somevalue    Output  TypeError  can t set attributes of built-in extension type  NoneType  AttributeError   NoneType  object has no attribute  somefield  AttributeError   NoneType  object has no attribute  somefield     The above seen statements produce these error messages  respectively  It means that  we cannot create attributes dynamically on a None instance    Let us check what happens when we assign something None  As per the documentation  it should throw a SyntaxError  It means  if we assign something to None  the program will not be executed at all   None   1   Output  SyntaxError  cannot assign to None    We have established that   None is an instance of NoneType None cannot have new attributes Existing attributes of None cannot be changed  We cannot create other instances of NoneType We cannot even change the reference to None by assigning values to it    So  as mentioned in the documentation  None can really be considered as a true constant   Happy knowing None

User · Answer

None is a singleton object  meaning there is only one None   used in many places in the language and library to represent the absence of some other value    For example  if d is a dictionary  d get k  will return d k  if it exists  but None if d has no key k   Read this info from a great blog  http   python-history blogspot in

[python] What is a None value?

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