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    <p>One solution I just came up with would be to implement a View of the list via class <code>ListView</code> which takes the following arguments:</p> <ul> <li><code>raw_list</code>: a list of <code>One</code>-objects</li> <li><code>raw2new</code>: a function that converts <code>One</code>-objects to <code>Two</code>-objects</li> <li><code>new2raw</code>: a function that converts <code>Two</code>-objects to <code>One</code>-objects</li> </ul> <p>Here is a the code:</p> <pre><code>class ListView(list): def __init__(self, raw_list, raw2new, new2raw): self._data = raw_list self.converters = {'raw2new': raw2new, 'new2raw': new2raw} def __repr__(self): repr_list = [self.converters['raw2new'](item) for item in self._data] repr_str = "[" for element in repr_list: repr_str += element.__repr__() + ",\n " repr_str = repr_str[:-3] + "]" return repr_str def append(self, item): self._data.append(self.converters['new2raw'](item)) def pop(self, index): self._data.pop(index) def __getitem__(self, index): return self.converters['raw2new'](self._data[index]) def __setitem__(self, key, value): self._data.__setitem__(key, self.converters['new2raw'](value)) def __delitem__(self, key): return self._data.__delitem__(key) def __getslice__(self, i, j): return ListView(self._data.__getslice__(i,j), **self.converters) def __contains__(self, item): return self._data.__contains__(self.converters['new2raw'](item)) def __add__(self, other_list_view): assert self.converters == other_list_view.converters return ListView( self._data + other_list_view._data, **self.converters ) def __len__(self): return len(self._data) def __eq__(self, other): return self._data == other._data def __iter__(self): return iter([self.converters['raw2new'](item) for item in self._data]) </code></pre> <p>Now, <code>DataUI</code> has to look something like this:</p> <pre><code>class DataUI(object): def __init__(self, list_of_ones): for one in list_of_ones: assert type(one) == One self.list_of_ones = list_of_ones self.list_of_twos = ListView( self.list_of_ones, Two.newByDTO, Two.getDTO ) </code></pre> <p>With that, <code>Two</code> needs the following method:</p> <pre><code>def getDTO(self): return self._dto </code></pre> <p>The entire example would now look like the following:</p> <pre><code>import unittest import numpy as np class ListView(list): def __init__(self, raw_list, raw2new, new2raw): self._data = raw_list self.converters = {'raw2new': raw2new, 'new2raw': new2raw} def __repr__(self): repr_list = [self.converters['raw2new'](item) for item in self._data] repr_str = "[" for element in repr_list: repr_str += element.__repr__() + ",\n " repr_str = repr_str[:-3] + "]" return repr_str def append(self, item): self._data.append(self.converters['new2raw'](item)) def pop(self, index): self._data.pop(index) def __getitem__(self, index): return self.converters['raw2new'](self._data[index]) def __setitem__(self, key, value): self._data.__setitem__(key, self.converters['new2raw'](value)) def __delitem__(self, key): return self._data.__delitem__(key) def __getslice__(self, i, j): return ListView(self._data.__getslice__(i,j), **self.converters) def __contains__(self, item): return self._data.__contains__(self.converters['new2raw'](item)) def __add__(self, other_list_view): assert self.converters == other_list_view.converters return ListView( self._data + other_list_view._data, **self.converters ) def __len__(self): return len(self._data) def __iter__(self): return iter([self.converters['raw2new'](item) for item in self._data]) def __eq__(self, other): return self._data == other._data class One(object): """ Data Transfere Object (DTO) """ def __init__(self, name, data): assert type(name) == str assert type(data) == str self.name = name self.data = data def __repr__(self): return "%s(%r, %r)" %(self.__class__.__name__, self.name, self.data) class Two(np.ndarray): _DTO = One def __new__(cls, name, data): dto = cls._DTO(name, data) return cls.newByDTO(dto) @classmethod def newByDTO(cls, dto): obj = np.fromstring(dto.data, dtype="float", sep=',').view(cls) obj.setflags(write=False) # Immutable obj._dto = dto return obj @property def name(self): return self._dto.name def getDTO(self): return self._dto class DataUI(object): def __init__(self, list_of_ones): for one in list_of_ones: assert type(one) == One self.list_of_ones = list_of_ones self.list_of_twos = ListView( self.list_of_ones, Two.newByDTO, Two.getDTO ) class TestListView(unittest.TestCase): def testProperties(self): o1 = One('first object', "1, 3.0, 7, 8,1") o2 = One('second object', "3.7, 8, 10") my_data = DataUI ([o1, o2]) t1 = Two('third object', "4.8, 8.2, 10.3") t2 = Two('forth object', "33, 1.8, 1.0") # append: my_data.list_of_twos.append(t1) # __getitem__: np.testing.assert_array_equal(my_data.list_of_twos[2], t1) # __add__: np.testing.assert_array_equal( (my_data.list_of_twos + my_data.list_of_twos)[5], t1) # __getslice__: np.testing.assert_array_equal( my_data.list_of_twos[1:], my_data.list_of_twos[1:2] + my_data.list_of_twos[2:] ) # __contains__: self.assertEqual(my_data.list_of_twos.__contains__(t1), True) # __setitem__: my_data.list_of_twos.__setitem__(1, t1), np.testing.assert_array_equal(my_data.list_of_twos[1], t1) # __delitem__: l1 = len(my_data.list_of_twos) my_data.list_of_twos.__delitem__(1) l2 = len(my_data.list_of_twos) self.assertEqual(l1 - 1, l2) # __iter__: my_data_2 = DataUI ([]) for two in my_data.list_of_twos: my_data_2.list_of_twos.append(two) if __name__ == '__main__': unittest.main() </code></pre>
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    1. POHow can I change in Python the return/input type of a list that is implemented as an class attribute?
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