How to iterate over `dict` in class like if just referring to `dict`?

Question

In a defined object some values are kept in a dict, and I want to iterate over the contents in that dict as when referring to a plain dict, thus with directly access using [], and loop using e.g. .items(). Code structure is:

class Klass: def __init__(self, values): self.values = values self.more = None def __getitem__(self, name): return self.values[name] def __iter__(self): pass # TBD[How to make this ?] d = {'alfa': 1, 'bravo': 2, 'charlie': 3} k = Klass(d) for key in k: print(key) # Expected to print keys from self.values for (key, value) in k.items(): print(key, value) # Expected to print key and value from self.values for key in k.keys(): print(key) # Expected to print key from self.values for value in k.values(): print(value) # Expected to print value from self.values

How to write the __iter__ and, other required methods, so this kind of access is possible through an instance of Klass?

The intention with OOP is to hide specific implementation, so direct reference to instance attribute .values should be avoided.. — EquipDev
– EquipDev, Commented Mar 13, 2015 at 13:21
@EquipDev: within the implementation you are still going to have to rely on your own state. Torxed was proposing a implementation for __iter__, making it a generator function. Not needed here, not with iter(), however. — Martijn Pieters
– Martijn Pieters, Commented Mar 13, 2015 at 13:23
@MartijnPieters: Now I see; yes that adheres to OOP principles. Thanks for pointing that out. — EquipDev
– EquipDev, Commented Mar 13, 2015 at 13:28

Martijn Pieters · Accepted Answer · 2015-03-13 13:27:53Z

You'll have to implement the .keys(), .values() and .items() methods yourself; together with __iter__ they can all delegate the actual work to the self.values() dictionary:

class Klass: def __init__(self, values): self._values = values self.more = None def __getitem__(self, name): return self._values[name] def __iter__(self): return iter(self._values) def keys(self): return self._values.keys() def items(self): return self._values.items() def values(self): return self._values.values()

I renamed the attribute to avoid masking the .values() method.

The easiest way to delegate __iter__ to iteration over the dictionary (by key) is to use the iter() function to get an iterator for the dictionary object.

To be explicit: __iter__ plays no role in how .keys(), .values() and .items() are handled; the latter are just more methods.

Thanks for the help, and for renaming values to _values; it was a bad choice of name when having to implement the values method.
There's no need to implement .keys() etc. Just subclass from collections.abc.Mapping.
@Kevin: yup, that's the next step. The fact that collections.abc.Mapping then implements those methods doesn't diminishes the fact that those methods need to be implemented.

Pyglouthon · Accepted Answer · 2015-03-13 13:50:57Z

 class Klass(dict): def __init__(self, *arg, **kw): super(Klass, self).__init__(*arg, **kw) self.choosebettername = super(Klass, self).keys() def __iter__(self): return iter(self.choosebettername) def keys(self): return self.choosebettername def itervalues(self): return (self[key] for key in self) d = {'alfa': 1, 'bravo': 2, 'charlie': 3} k = Klass(d) for key in k: print(key) # Expected to print keys from self.values for (key, value) in k.items(): print(key, value) # Expected to print key and value from self.values for key in k.keys(): print(key) # Expected to print key from self.values print(k.values()) for value in k.values(): print(value) # Expected to print value from self.values

Maicon Mauricio · Accepted Answer · 2023-09-14 11:48:53Z

Contrary to the dict behavior, one might simply let the default iteration method be the items' dictionary view:

class Klass: def __init__(self, values): self._values = values def __getitem__(self, name): return self._values[name] def __iter__(self): iter(self._values.items()) klass = Klass({'alfa': 1, 'bravo': 2, 'charlie': 3}) for key, value in klass: print(key, value)

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