How should I declare default values for instance variables in Python

Question

Should I give my class members default values like this   class Foo      num   1   or like this   class Foo      def   init   self           self num   1   In this question I discovered that in both cases   bar   Foo   bar num    1   is a well-defined operation   I understand that the first method will give me a class variable while the second one will not  However  if I do not require a class variable  but only need to set a default value for my instance variables  are both methods equally good  Or one of them more  pythonic  than the other   One thing I ve noticed is that in the Django tutorial  they use the second method to declare Models  Personally I think the second method is more elegant  but I d like to know what the  standard  way is

User · Answer

You can also declare class variables as None which will prevent propagation. This is useful when you need a well defined class and want to prevent AttributeErrors. For example:

>>> class TestClass(object):
...     t = None
... 
>>> test = TestClass()
>>> test.t
>>> test2 = TestClass()
>>> test.t = 'test'
>>> test.t
'test'
>>> test2.t
>>>

Also if you need defaults:

>>> class TestClassDefaults(object):
...    t = None
...    def __init__(self, t=None):
...       self.t = t
... 
>>> test = TestClassDefaults()
>>> test.t
>>> test2 = TestClassDefaults([])
>>> test2.t
[]
>>> test.t
>>>

Of course still follow the info in the other answers about using mutable vs immutable types as the default in __init__.

User · Answer

Using class members to give default values works very well just so long as you are careful only to do it with immutable values  If you try to do it with a list or a dict that would be pretty deadly  It also works where the instance attribute is a reference to a class just so long as the default value is None   I ve seen this technique used very successfully in repoze which is a framework that runs on top of Zope  The advantage here is not just that when your class is persisted to the database only the non-default attributes need to be saved  but also when you need to add a new field into the schema all the existing objects see the new field with its default value without any need to actually change the stored data   I find it also works well in more general coding  but it s a style thing  Use whatever you are happiest with

User · Answer

Extending bp s answer  I wanted to show you what he meant by immutable types   First  this is okay    gt  gt  gt  class TestB            def   init   self  attr 1               self attr   attr           gt  gt  gt  a   TestB    gt  gt  gt  b   TestB    gt  gt  gt  a attr   2  gt  gt  gt  a attr 2  gt  gt  gt  b attr 1   However  this only works for immutable  unchangable  types  If the default value was mutable  meaning it can be replaced   this would happen instead    gt  gt  gt  class Test            def   init   self  attr                  self attr   attr           gt  gt  gt  a   Test    gt  gt  gt  b   Test    gt  gt  gt  a attr append 1   gt  gt  gt  a attr  1   gt  gt  gt  b attr  1   gt  gt  gt     Note that both a and b have a shared attribute  This is often unwanted   This is the Pythonic way of defining default values for instance variables  when the type is mutable    gt  gt  gt  class TestC            def   init   self  attr None               if attr is None                  attr                  self attr   attr           gt  gt  gt  a   TestC    gt  gt  gt  b   TestC    gt  gt  gt  a attr append 1   gt  gt  gt  a attr  1   gt  gt  gt  b attr      The reason my first snippet of code works is because  with immutable types  Python creates a new instance of it whenever you want one  If you needed to add 1 to 1  Python makes a new 2 for you  because the old 1 cannot be changed  The reason is mostly for hashing  I believe

User · Answer

With dataclasses  a feature added in Python 3 7  there is now yet another  quite convenient  way to achieve setting default values on class instances  The decorator dataclass will automatically generate a few methods on your class  such as the constructor  As the documentation linked above notes   quot  t he member variables to use in these generated methods are defined using PEP 526 type annotations quot   Considering OP s example  we could implement it like this  from dataclasses import dataclass   dataclass class Foo      num  int   0   When constructing an object of this class s type we could optionally overwrite the value  print  Default val      format Foo       Default val  Foo num 0  print  Custom val      format Foo num 5      Custom val  Foo num 5

User · Answer

Using class members for default values of instance variables is not a good idea  and it s the first time I ve seen this idea mentioned at all  It works in your example  but it may fail in a lot of cases  E g   if the value is mutable  mutating it on an unmodified instance will alter the default    gt  gt  gt  class c          l            gt  gt  gt  x   c    gt  gt  gt  y   c    gt  gt  gt  x l     gt  gt  gt  y l     gt  gt  gt  x l append 10   gt  gt  gt  y l  10   gt  gt  gt  c l  10

User · Answer

The two snippets do different things  so it s not a matter of taste but a matter of what s the right behaviour in your context  Python documentation explains the difference  but here are some examples   Exhibit A  class Foo    def   init   self       self num   1   This binds num to the Foo instances  Change to this field is not propagated to other instances   Thus    gt  gt  gt  foo1   Foo    gt  gt  gt  foo2   Foo    gt  gt  gt  foo1 num   2  gt  gt  gt  foo2 num 1   Exhibit B  class Bar    num   1   This binds num to the Bar class  Changes are propagated    gt  gt  gt  bar1   Bar    gt  gt  gt  bar2   Bar    gt  gt  gt  bar1 num   2  this creates an INSTANCE variable that HIDES the propagation  gt  gt  gt  bar2 num 1  gt  gt  gt  Bar num   3  gt  gt  gt  bar2 num 3  gt  gt  gt  bar1 num 2  gt  gt  gt  bar1   class   num 3   Actual answer     If I do not require a class variable  but only need to set a default value for my instance variables  are both methods equally good  Or one of them more  pythonic  than the other    The code in exhibit B is plain wrong for this  why would you want to bind a class attribute  default value on instance creation  to the single instance   The code in exhibit A is okay   If you want to give defaults for instance variables in your constructor I would however do this   class Foo    def   init   self  num   None       self num   num if num is not None else 1      or even   class Foo    DEFAULT NUM   1   def   init   self  num   None       self num   num if num is not None else DEFAULT NUM      or even   preferrable  but if and only if you are dealing with immutable types    class Foo    def   init   self  num   1       self num   num   This way you can do   foo1   Foo 4  foo2   Foo    use default

[python] How should I declare default values for instance variables in Python?

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