correct way to use super argument passing

Question

So I was following Python s Super Considered Harmful  and went to test out his examples   However  Example 1-3  which is supposed to show the correct way of calling super when handling   init   methods that expect different arguments  flat-out doesn t work   This is what I get       python example1-3 py  MRO    E    C    A    D    B    object   E arg  10 C arg  10 A D arg  10 B Traceback  most recent call last     File  Download example1-3 py   line 27  in  lt module gt      E arg 10    File  Download example1-3 py   line 24  in   init       super E  self    init   arg   args    kwargs    File  Download example1-3 py   line 14  in   init       super C  self    init   arg   args    kwargs    File  Download example1-3 py   line 4  in   init       super A  self    init    args    kwargs    File  Download example1-3 py   line 19  in   init       super D  self    init   arg   args    kwargs    File  Download example1-3 py   line 9  in   init       super B  self    init    args    kwargs  TypeError  object   init     takes no parameters   It seems that object itself violates one of the best practices mentioned in the document  which is that methods which use super must accept  args and   kwargs   Now  obviously Mr  Knight expected his examples to work  so is this something that was changed in recent versions of Python  I checked 2 6 and 2 7  and it fails on both   So what is the correct way to deal with this problem

User · Answer

If you re going to have a lot of inheritence  that s the case here  I suggest you to pass all parameters using   kwargs  and then pop them right after you use them  unless you need them in upper classes    class First object       def   init   self   args    kwargs           self first arg   kwargs pop  first arg           super First  self    init    args    kwargs   class Second First       def   init   self   args    kwargs           self second arg   kwargs pop  second arg           super Second  self    init    args    kwargs   class Third Second       def   init   self   args    kwargs           self third arg   kwargs pop  third arg           super Third  self    init    args    kwargs    This is the simplest way to solve those kind of problems   third   Third first arg 1  second arg 2  third arg 3

User · Answer

As explained in Python s super   considered super  one way is to have class eat the arguments it requires  and pass the rest on  Thus  when the call-chain reaches object  all arguments have been eaten  and object   init   will be called without arguments  as it expects   So your code should look like this   class A object       def   init   self   args    kwargs           print  A          super A  self    init    args    kwargs   class B object       def   init   self   args    kwargs           print  B          super B  self    init    args    kwargs   class C A       def   init   self  arg   args    kwargs           print  C   arg   arg         super C  self    init    args    kwargs   class D B       def   init   self  arg   args    kwargs           print  D    arg   arg         super D  self    init    args    kwargs   class E C D       def   init   self  arg   args    kwargs           print  E    arg   arg         super E  self    init    args    kwargs   print  MRO     x   name   for x in E   mro    E 10  20  30

User · Answer

Sometimes two classes may have some parameter names in common  In that case  you can t pop the key-value pairs off of   kwargs or remove them from  args  Instead  you can define a Base class which unlike object  absorbs ignores arguments   class Base object       def   init   self   args    kwargs   pass  class A Base       def   init   self   args    kwargs           print  A          super A  self    init    args    kwargs   class B Base       def   init   self   args    kwargs           print  B          super B  self    init    args    kwargs   class C A       def   init   self  arg   args    kwargs           print  C   arg   arg         super C  self    init   arg   args    kwargs   class D B       def   init   self  arg   args    kwargs           print  D    arg   arg         super D  self    init   arg   args    kwargs   class E C D       def   init   self  arg   args    kwargs           print  E    arg   arg         super E  self    init   arg   args    kwargs   print  MRO     x   name   for x in E   mro    E 10    yields  MRO    E    C    A    D    B    Base    object   E arg  10 C arg  10 A D arg  10 B   Note that for this to work  Base must be the penultimate class in the MRO

[python] correct way to use super (argument passing)

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