Python - add comments to dictionary as a property

Question

I want to assign comments to the key-value pairs in my dictionary, ideally in a way that I can access them directly. Since I'm new to manipulating classes, I only came this far:

class MyDict(dict):
    def __init__(self, *arg, **kwargs):
        super(MyDict, self).__init__(*arg, **kwargs)

mydict = MyDict()
mydict['first'] = 1
mydict['second'] = 2

Is there a way to add a comment to each of the key-value pairs so I can access them like

for key, value, comment in mydict.iteritems():
    print(key, value, comment)

or

for key, value in mydict.iteritems():
    print(key, value, key.comment)  # or value.comment

Any other way is also fine, as long as the comments are directly accessible (meaning not via indexing of lists or so).

Would have thought the most natural way would be to have a tuple or named tuple as the dictionary value associated with each key. — ChrisOram, Jan 19 '22 at 09:42
in theory you can overload all dictionary functions to get it to work, but it will be much slower than just storing a tuple, because dictionary functions are implemented in C. — Ahmed AEK, Jan 19 '22 at 10:01

Jonathan Weine · Accepted Answer · 2022-01-20T14:39:50.423

1

If you store (value, comment) pairs, for each pair you can use tuple unpacking in the for statement (note the parenthesis). Although, this is kind of contradicting your "non-indexing" request:


mydict[key] = (value, comment)

for key, (value, comment) in mydict.items():
    print(key, value, comment)

Edit: Following specifications in the comments:

You could subclass a dict, that has another dict containing the comments as a member. Every regular dictionary method is left as is, but a specific method is implemented to iterate over the items along with a comment. Comments are added by using the corresponding method add_comment. If you want to include the comments into the initialzation, while keeping full compatibility with regular dictionary construction more work needs to be put into the __init__ definition.

class MyCommentedDict(dict):
  def __init__(self, *args, **kwargs):
    super(MyCommentedDict, self).__init__(*args, **kwargs)
    self._comments = dict()

  def add_comment(self, key, comment):
    self._comments.update({key:comment})
  
  def commented_items(self):
    """ Returns a generator that yields triplets of (key, value, comment).
    For all entries without a provided comment `None` is returned as comment."""
    return ((key, value, self._comments.get(key, None))
             for key, value in self.items())

a_dict = MyCommentedDict({'color': 'blue', 'fruit': 'apple', 'pet': 'dog'})
a_dict.add_comment('color', 'This a is color')

for key, value in a_dict.items():
  print(key, value)

>>>color blue
>>>fruit apple
>>>pet dog

for key, value, comment in a_dict.commented_items():
  print(key, value, comment)

>>>color blue This a is color
>>>fruit apple None
>>>pet dog None

edited Jan 20 '22 at 14:39

answered Jan 19 '22 at 09:45

Jonathan Weine

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3
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This would practically work, true. But is there a way to alter the `dict` class in the described way? Ideally, I (and all others who might use it) don't want to care about indexing when working with the commented dict. – s6hebern Jan 19 '22 at 09:51
1

In that case look into `collections.abc`, you can inherit the base class that supports dictionary operations and implement your spec with those, see https://stackoverflow.com/a/21368848/15981783. – ChrisOram Jan 19 '22 at 10:13
@s6hebern, Is my understanding correct, that you want to have a datacontainer that behaves like a `dict` containing `(key, value)` - pairs in all cases *except* when iterating over the items, in which case you want to return `(key, value, comments)`? Or do you want to be able to have the original return on iteration as well? – Jonathan Weine Jan 19 '22 at 10:51
And additionaly, how to you intend to add the comments to each value? Are these already pairs on initialization? – Jonathan Weine Jan 19 '22 at 10:54
@JonathanWeine, thought about some additional function like `mydict['somekey'].add_comment('this is a comment')`. No need to do it directly on initialization. – s6hebern Jan 19 '22 at 12:09
1

So what should it return for alle keys wthout a comment? `None`? The cleanest option probably is subclassing as ChrisORam suggested especially if you want to stay consistent with all definitions of the `dict` class. An easier option could be to subclass a dict, that has another member dict containing `comments`, which is updated via the `add_comment` method. If that seems sufficient, I'll update the answer – Jonathan Weine Jan 19 '22 at 12:15
Sounds sufficient, yes, although I don't fully understand what you mean (yet). It probably gets clear when I see the code. Thanks already – s6hebern Jan 20 '22 at 06:22
Looks like a very nice solution, but I get `TypeError: dict.get() takes no keyword arguments`. (Python 3.9) – s6hebern Jan 20 '22 at 12:04
1

That shouldn't be a problem. The `default=None`, in `dict.get(...)` can be passed as positional argument then. I'll update the answer – Jonathan Weine Jan 20 '22 at 14:39

Python - add comments to dictionary as a property

1 Answers1