Nested classes scope

Question

I m trying to understand scope in nested classes in Python  Here is my example code   class OuterClass      outer var   1     class InnerClass          inner var   outer var   The creation of class does not complete and I get the error    lt type  exceptions NameError  gt   name  outer var  is not defined   Trying inner var   Outerclass outer var doesn t work  I get    lt type  exceptions NameError  gt   name  OuterClass  is not defined   I am trying to access the static outer var from InnerClass   Is there a way to do this

User · Answer

I think you can simply do   class OuterClass      outer var   1      class InnerClass          pass     InnerClass inner var   outer var   The problem you encountered is due to this      A block is a piece of Python program text that is executed as a unit    The following are blocks  a module  a function body  and a class   definition            A scope defines the visibility of a name within   a block            The scope of names defined in a class block is   limited to the class block  it does not extend to the code blocks of   methods     this includes generator expressions since they are   implemented using a function scope  This means that the following will   fail      class A            a   42           b   list a   i for i in range 10         http   docs python org reference executionmodel html naming-and-binding   The above means  a function body is a code block and a method is a function  then names defined out of the function body  present in a class definition do not extend to the function body   Paraphrasing this for your case  a class definition is a code block  then names defined out of the inner class definition present in an outer class definition do not extend to the inner class definition

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You might be better off if you just don t use nested classes  If you must nest  try this   x   1 class OuterClass      outer var   x     class InnerClass          inner var   x   Or declare both classes before nesting them   class OuterClass      outer var   1  class InnerClass      inner var   OuterClass outer var  OuterClass InnerClass   InnerClass    After this you can del InnerClass if you need to

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All explanations can be found in Python Documentation The Python Tutorial  For your first error  lt type  exceptions NameError  gt   name  outer var  is not defined  The explanation is      There is no shorthand for referencing data attributes  or other methods   from within methods  I find that this actually increases the readability of methods  there is no chance of confusing local variables and instance variables when glancing through a method    quoted from The Python Tutorial 9 4  For your second error  lt type  exceptions NameError  gt   name  OuterClass  is not defined     When a class definition is left normally  via the end   a class object is created    quoted from The Python Tutorial 9 3 1  So when you try inner var   Outerclass outer var  the Quterclass hasn t been created yet  that s why name  OuterClass  is not defined  A more detailed but tedious explanation for your first error      Although classes have access to enclosing functions    scopes  though  they do not act   as enclosing scopes to code nested within the class  Python searches enclosing functions   for referenced names  but never any enclosing classes  That is  a class is a local scope   and has access to enclosing local scopes  but it does not serve as an enclosing local scope   to further nested code     quoted from Learning Python 5th  Mark Lutz

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In Python mutable objects are passed as reference  so you can pass a reference of the outer class to the inner class   class OuterClass      def   init   self           self outer var   1         self inner class   OuterClass InnerClass self          print  Inner variable in OuterClass    d    self inner class inner var       class InnerClass          def   init   self  outer class               self outer class   outer class             self inner var   2             print  Outer variable in InnerClass    d    self outer class outer var

User · Answer

class Outer object       outer var   1      class Inner object            property         def inner var self               return Outer outer var   This isn t quite the same as similar things work in other languages  and uses global lookup instead of scoping the access to outer var   If you change what object the name Outer is bound to  then this code will use that object the next time it is executed    If you instead want all Inner objects to have a reference to an Outer because outer var is really an instance attribute   class Outer object       def   init   self           self outer var   1      def get inner self           return self Inner self             self Inner  is because Inner is a class attribute of this class            Outer Inner  would also work  or move Inner to global scope           and then just use  Inner       class Inner object           def   init   self  outer               self outer   outer           property         def inner var self               return self outer outer var   Note that nesting classes is somewhat uncommon in Python  and doesn t automatically imply any sort of special relationship between the classes  You re better off not nesting   You can still set a class attribute on Outer to Inner  if you want

User · Answer

Easiest solution   class OuterClass      outer var   1     class InnerClass          def   init   self               self inner var   OuterClass outer var   It requires you to be explicit  but doesn t take much effort

[python] Nested classes' scope?

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