CODEWITHSUNDEEP
pythonpython-3.xdictionary
0x90
0x90

Reputation: 41002

Why is dict instantiation with repeated keys legit?

Is there a way to avoid such a "less expected" behavior? So if the same key is used more than once something will fire up?

In [3]: a = {"a": 2, "a": 3, "a": 5}

In [4]: a
Out[4]: {'a': 5}

In [5]: type(a)==dict
Out[5]: True

but thankfully this is invalid:

In [6]: dict(a=2,a=4)
  File "<ipython-input-5-733265236ee3>", line 1
    dict(a=2,a=4)
            ^
SyntaxError: keyword argument repeated

Upvotes: 0

Views: 193

Answers (1)

Grismar
Grismar

Reputation: 31354

How about turning the question around: why shouldn't it be valid? You're in charge of the code, it's not like users can make this mistake for you, or provide input you have no control over that would cause it.

And it allows for things like this to work:

{c: n for n, c in enumerate(['a', 'a', 'b'])}

Result:

{'a': 1, 'b': 2}

The example by itself is hardly useful, but you can imagine situations where the simple list is a generator and you may not have control over there being any duplicates. The dict keeping track of the last key generated could be useful.

Also, it's the same for the constructor function:

dict([('a', 0), ('a', 1), ('b', 2)])

As pointed out in the comments by user @sj95126, the reason it doesn't work for the example you gave, is because that's a way of calling a function with duplicate keyword arguments, and that's not allowed - it's not to do with the actual dictionary definition.

User @chepner makes a point in the comments worth repeating as well: an expression like {"a": 2, "a": 3, "a": 5} is a syntactically correct dictionary definition: hashable type keys with values of any type. Disallowing repetition of keys would mean checking the actual values of the keys, not just their types. The keys of a dictionary are data, not identifiers in the language (like function argument names), you check key values at runtime, and you're allowed to add more at runtime (unlike object attributes, for example).

Since {"a": 2, "a": 3, "a": 5} is syntactically correct, having a robust way of interpreting it is vastly preferable to it causing runtime errors - and Python's interpretation is a good example.

Upvotes: 3

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