Mailman 3 Change the __repr__ of the `MappingView`s - Python-ideas

Dec. 31, 2017

      === This proposition is purely aesthetic ===

At this time, the __repr__ of the mapping views is showing the whole mapping:
...
...
...
from collections.abc import ValuesView, KeysView, ItemsView
d = {3: 'three', 4: 'four'}
KeysView(d)
KeysView({3: 'three', 4: 'four'})
ValuesView(d)
ValuesView({3: 'three', 4: 'four'})
ItemsView(d)
ItemsView({3: 'three', 4: 'four'})
Witch is not consistent with dict_keys, dict_values, dict_items:
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...
...
d.keys()
dict_keys([3, 4])
d.values()
dict_values(['three', 'four'])
d.items()
dict_items([(3, 'three'), (4, 'four')])
We could easily change that, since all the views are iterables on what they are designed for, in MappingView:

def __repr__(self):
    viewname = self.__class__.__name__
    elements = ', '.join(map(repr, self))
    return f'{viewname}([elements])

And now:
...
...
...
KeysView(d)
KeysView([3, 4])
ValuesView(d)
ValuesView(['three', 'four'])
ItemsView(d)
ItemsView([(3, 'three'), (4, 'four')])
It's not breaking any test (it seems that there isn't any for this), but it have a real drawback: it's breaking the convention about instantiation by copy/pasting:
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...
...
ValuesView(['three', 'four'])
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/IPython/core/formatters.py", line 684, in __call__
    return repr(obj)
  File "/usr/lib/python3.6/_collections_abc.py", line 706, in __repr__
    elements = ', '.join(map(repr, self))
  File "/usr/lib/python3.6/_collections_abc.py", line 764, in __iter__
    yield self._mapping[key]
TypeError: list indices must be integers or slices, not str
It's because __init__ in MappingView treat the passed argument -- wich is stored in self._mapping -- as the whole mapping, not just keys, values or items... And all the other methods (__contains__ and __iter__) in the subclasses are using this _mapping attribute to work.

So what is to prioritize?

Change the repr of the `MappingView`s

Yahya Abou 'Imran

Guido van Rossum

Guido van Rossum

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