Using an attribute of the current class instance as a default value for method s parameter

Question

Possible Duplicate  default value of parameter as result of instance method   While it is possible to set default values to function parameters in python  def my function param one  default            It seems not to be possible to access the current instance  self   class MyClass           def my function self  param one self one of the vars                My question s    Is this true that I cannot access the current instance to set the default parameter in functions  If it is not possble  what are the reasons and is it imaginable that this will be possible in future versions of python

User · Answer

It s written as   def my function self  param one None     Or custom sentinel if None is vaild     if param one is None          param one   self one of the vars   And I think it s safe to say that will never happen in Python due to the nature that self doesn t really exist until the function starts     you can t reference it  in its own definition - like everything else   For example  you can t do d     x   3   y   d  x     5

User · Answer

There is much more to it than you think  Consider the defaults to be static   constant reference pointing to one object  and stored somewhere in the definition  evaluated at method definition time  as part of the class  not the instance  As they are constant  they cannot depend on self   Here is an example  It is counterintuitive  but actually makes perfect sense   def add item  s          s append item      print len s   add 1        1 add 1        2 add 1        1 add 1        1 add 1        3   This will print 1 2 1 1 3   Because it works the same way as  default s    def add item  s default s       s append item    Obviously  if you modify default s  it retains these modifications   There are various workarounds  including  def add item  s None       if not s  s          s append item    or you could do this   def add self  item  s None       if not s  s   self makeDefaultS       s append item    Then the method makeDefaultS will have access to self   Another variation   import types def add item  s lambda self          if isinstance s  types FunctionType   s   s  example       s append item    here the default value of s is a factory function   You can combine all these techniques   class Foo      import types     def add self  item  s Foo defaultFactory           if isinstance s  types FunctionType   s   s self          s append item       def defaultFactory self               Can be overridden in a subclass  too             return

User · Answer

There are multiple false assumptions you re making here - First  function belong to a class and not to an instance  meaning the actual function involved is the same for any two instances of a class  Second  default parameters are evaluated at compile time and are constant  as in  a constant object reference - if the parameter is a mutable object you can change it   Thus you cannot access self in a default parameter and will never be able to

User · Answer

Default value for parameters are evaluated at  compilation   once  So obviously you can t access self  The classic example is list as default parameter  If you add elements into it  the default value for the parameter changes   The workaround is to use another default parameter  typically None  and then check and update the variable

[python] Using an attribute of the current class instance as a default value for method's parameter

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