Referring to the null object in Python

Question

How do I refer to the null object in Python

User · Answer

In Python  to represent the absence of a value  you can use the None value  types NoneType None  for objects and     or len      0  for strings  Therefore   if yourObject is None     if yourObject    None           if yourString           if yourString len      0            Regarding the difference between      and  is   testing for object identity using      should be sufficient  However  since the operation  is  is defined as the object identity operation  it is probably more correct to use it  rather than       Not sure if there is even a speed difference   Anyway  you can have a look at    Python Built-in Constants doc page  Python Truth Value Testing doc page

User · Answer

Null is a special object type like    gt  gt  gt type None   lt class  NoneType  gt    You can check if an object is in class  NoneType     gt  gt  gt variable   None  gt  gt  gt variable is None True   More information is available at Python Docs

User · Answer

It s not called null as in other languages  but None  There is always only one instance of this object  so you can check for equivalence with x is None  identity comparison  instead of x    None  if you want

User · Answer

Use f string for getting this solved  year None year val   null  if year is None else  str year  print f  year val     null

User · Answer

None  Python s null  There s no null in Python  instead there s None  As stated already  the most accurate way to test that something has been given None as a value is to use the is identity operator  which tests that two variables refer to the same object   gt  gt  gt  foo is None True  gt  gt  gt  foo    bar   gt  gt  gt  foo is None False  The basics There is and can only be one None None is the sole instance of the class NoneType and any further attempts at instantiating that class will return the same object  which makes None a singleton  Newcomers to Python often see error messages that mention NoneType and wonder what it is  It s my personal opinion that these messages could simply just mention None by name because  as we ll see shortly  None leaves little room to ambiguity  So if you see some TypeError message that mentions that NoneType can t do this or can t do that  just know that it s simply the one None that was being used in a way that it can t  Also  None is a built-in constant  As soon as you start Python  it s available to use from everywhere  whether in module  class  or function  NoneType by contrast is not  you d need to get a reference to it first by querying None for its class   gt  gt  gt  NoneType NameError  name  NoneType  is not defined  gt  gt  gt  type None  NoneType  You can check None s uniqueness with Python s identity function id    It returns the unique number assigned to an object  each object has one  If the id of two variables is the same  then they point in fact to the same object   gt  gt  gt  NoneType   type None   gt  gt  gt  id None  10748000  gt  gt  gt  my none   NoneType    gt  gt  gt  id my none  10748000  gt  gt  gt  another none   NoneType    gt  gt  gt  id another none  10748000  gt  gt  gt  def function that does nothing    pass  gt  gt  gt  return value   function that does nothing    gt  gt  gt  id return value  10748000  None cannot be overwritten In much older versions of Python  before 2 4  it was possible to reassign None  but not any more  Not even as a class attribute or in the confines of a function    In Python 2 7  gt  gt  gt  class SomeClass object           def my fnc self                   self None    foo  SyntaxError  cannot assign to None  gt  gt  gt  def my fnc            None    foo  SyntaxError  cannot assign to None    In Python 3 5  gt  gt  gt  class SomeClass          def my fnc self                   self None    foo  SyntaxError  invalid syntax  gt  gt  gt  def my fnc            None    foo  SyntaxError  cannot assign to keyword  It s therefore safe to assume that all None references are the same  There isn t any  quot custom quot  None  To test for None use the is operator When writing code you might be tempted to test for Noneness like this  if value  None      pass  Or to test for falsehood like this if not value      pass  You need to understand the implications and why it s often a good idea to be explicit  Case 1  testing if a value is None Why do value is None  rather than value  None    The first is equivalent to  id value   id None   Whereas the expression value  None is in fact applied like this value   eq   None   If the value really is None then you ll get what you expected   gt  gt  gt  nothing   function that does nothing    gt  gt  gt  nothing   eq   None  True  In most common cases the outcome will be the same  but the   eq     method opens a door that voids any guarantee of accuracy  since it can be overridden in a class to provide special behavior  Consider this class   gt  gt  gt  class Empty object           def   eq   self  other               return not other  So you try it on None and it works  gt  gt  gt  empty   Empty    gt  gt  gt  empty  None True  But then it also works on the empty string  gt  gt  gt  empty     True  And yet  gt  gt  gt      None False  gt  gt  gt  empty is None False  Case 2  Using None as a boolean The following two tests if value        Do something  if not value        Do something  are in fact evaluated as if bool value         Do something  if not bool value         Do something  None is a  quot falsey quot   meaning that if cast to a boolean it will return False and if applied the not operator it will return True  Note however that it s not a property unique to None  In addition to False itself  the property is shared by empty lists  tuples  sets  dicts  strings  as well as 0  and all objects from classes that implement the   bool     magic method to return False   gt  gt  gt  bool None  False  gt  gt  gt  not None True   gt  gt  gt  bool     False  gt  gt  gt  not    True   gt  gt  gt  class MyFalsey object           def   bool   self               return False  gt  gt  gt  f   MyFalsey    gt  gt  gt  bool f  False  gt  gt  gt  not f True  So when testing for variables in the following way  be extra aware of what you re including or excluding from the test  def some function value None       if not value          value   init value    In the above  did you mean to call init value   when the value is set specifically to None  or did you mean that a value set to 0  or the empty string  or an empty list should also trigger the initialization  Like I said  be mindful  As it s often the case  in Python explicit is better than implicit  None in practice None used as a signal value None has a special status in Python  It s a favorite baseline value because many algorithms treat it as an exceptional value  In such scenarios it can be used as a flag to signal that a condition requires some special handling  such as the setting of a default value   You can assign None to the keyword arguments of a function and then explicitly test for it  def my function value  param None       if param is None            Do something outrageous   You can return it as the default when trying to get to an object s attribute and then explicitly test for it before doing something special  value   getattr some obj   some attribute   None  if value is None        do something spectacular   By default a dictionary s get   method returns None when trying to access a non-existing key   gt  gt  gt  some dict       gt  gt  gt  value   some dict get  foo    gt  gt  gt  value is None True  If you were to try to access it by using the subscript notation a KeyError would be raised  gt  gt  gt  value   some dict  foo   KeyError   foo   Likewise if you attempt to pop a non-existing item  gt  gt  gt  value   some dict pop  foo   KeyError   foo   which you can suppress with a default value that is usually set to None value   some dict pop  foo   None  if value is None        Booom   None used as both a flag and valid value The above described uses of None apply when it is not considered a valid value  but more like a signal to do something special  There are situations however where it sometimes matters to know where None came from because even though it s used as a signal it could also be part of the data  When you query an object for its attribute with getattr some obj   attribute name   None  getting back None doesn t tell you if the attribute you were trying to access was set to None or if it was altogether absent from the object  The same situation when accessing a key from a dictionary  like some dict get  some key    you don t know if some dict  some key   is missing or if it s just set to None  If you need that information  the usual way to handle this is to directly attempt accessing the attribute or key from within a try except construct  try        Equivalent to getattr   without specifying a default       value   getattr some obj   some attribute       value   some obj some attribute       Now you handle  None  the data here     if value is None            Do something here because the attribute was set to None except AttributeError        We re now handling the exceptional situation from here        We could assign None as a default value if required      value   None       In addition  since we now know that some obj doesn t have the       attribute  some attribute  we could do something about that      log something some obj   Similarly with dict  try      value   some dict  some key       if value is None            Do something here because  some key  is set to None except KeyError        Set a default     value   None       And do something because  some key  was missing       from the dict      log something some dict   The above two examples show how to handle object and dictionary cases  What about functions  The same thing  but we use the double asterisks keyword argument to that end  def my function   kwargs       try          value   kwargs  some key           if value is None                Do something because  some key  is explicitly               set to None     except KeyError            We assign the default         value   None           And since it s not coming from the caller          log something  did not receive  quot some key quot     None used only as a valid value If you find that your code is littered with the above try except pattern simply to differentiate between None flags and None data  then just use another test value  There s a pattern where a value that falls outside the set of valid values is inserted as part of the data in a data structure and is used to control and test special conditions  e g  boundaries  state  etc    Such a value is called a sentinel and it can be used the way None is used as a signal  It s trivial to create a sentinel in Python  undefined   object    The undefined object above is unique and doesn t do much of anything that might be of interest to a program  it s thus an excellent replacement for None as a flag  Some caveats apply  more about that after the code  With function def my function value  param1 undefined  param2 undefined       if param1 is undefined            We know nothing was passed to it  not even None         log something  param1 was missing           param1   None       if param2 is undefined            We got nothing here either         log something  param2 was missing           param2   None  With dict value   some dict get  some key   undefined  if value is None      log something  quot  some key  was set to None quot    if value is undefined        We know that the dict didn t have  some key      log something  quot  some key  was not set at all quot       value   None  With an object value   getattr obj   some attribute   undefined  if value is None      log something  quot  obj some attribute  was set to None quot   if value is undefined        We know that there s no obj some attribute     log something  quot no  some attribute  set on obj quot       value   None  As I mentioned earlier  custom sentinels come with some caveats  First  they re not keywords like None  so Python doesn t protect them  You can overwrite your undefined above at any time  anywhere in the module it s defined  so be careful how you expose and use them  Next  the instance returned by object   is not a singleton  If you make that call 10 times you get 10 different objects  Finally  usage of a sentinel is highly idiosyncratic  A sentinel is specific to the library it s used in and as such its scope should generally be limited to the library s internals  It shouldn t  quot leak quot  out  External code should only become aware of it  if their purpose is to extend or supplement the library s API

User · Answer

In Python  the  null  object is the singleton None   The best way to check things for  Noneness  is to use the identity operator  is   if foo is None

User · Answer

Per Truth value testing   None  directly tests as FALSE  so the simplest expression will suffice   if not foo

[python] Referring to the null object in Python

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