How to deepcopy a list of user defined lists?

Terry Reedy tjreedy at udel.edu
Thu Jan 8 21:33:59 CET 2009


srinivasan srinivas wrote:
> Hi,
> I have a class which is a subclass of builtin-type list.
> 
> #------------------------------------------------------------------------------
> class clist(list):
>     def __new__(cls, values, ctor):
>         val = []
>         for item in values:
>             item = ctor(item)
>             val.append(item)
>         
>         self = list.__new__(cls, val)
>         self.__values = val
>         self.__ctor = ctor
>         return self

A subclass of list should populate the list in __init__, not __new__, 
usually by calling list.__init__, as lists are mutable and subclasses 
thereof should be too.

class clist(list):
      def __init__(self, values, ctor):
          list.__init__(self, map(ctor, values))
          self.__ctor = ctor

clist1 = clist((1,2,3),str)
clist2 = clist((1,2,3), float)
alist1 = [clist1,clist2]
print(alist1)
#[['1', '2', '3'], [1.0, 2.0, 3.0]]

from copy import deepcopy
alist2 = deepcopy(alist1)
print(alist2)
#[['1', '2', '3'], [1.0, 2.0, 3.0]]

print(alist1[0] is alist2[0])
#False - ie, inner clist was copied

I omitted your __values attribute as redundant with the value of the 
clist itself.  Since clist is not a mixin class, double underscores for 
name-mangling are not needed.

Unless you make use of _ctor in other methods, initializing regular 
lists with 'list(map(ctor,values)) would work as well.

Terry Jan Reedy




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