New submission from Luigi Semenzato:
This was already discussed and rejected at https://bugs.python.org/issue16385.
I am reopening because I am not convinced that the discussion presented all
arguments properly.
The original problem description:
lives_in = { 'lion': ['Africa', 'America],
'parrot': ['Europe'],
#... 100+ more rows here
'lion': ['Europe'],
#... 100+ more rows here
}
The above constructor overwrites the 'lion' entry silently, often causing
unexpected behavior.
These are the arguments presented in favor of the rejection, followed by my
rebuttals.
1. "An error is out of the question for compatibility reasons". No real
rebuttal here, except I wonder if exceptions are ever made and under what
circumstances. I should point out that a warning may also create
incompatibilities.
2. "There are ways to rewrite this as a loop on a list". Yes of course, but
the entire point of the dictionary literal is to offer a concise and convenient
notation for entering a dictionary as program text.
3. "Being able to re-write keys is fundamental to Python dicts and why they can
be used for Python's mutable namespaces". This is fine but it applies to the
data structure, not the literal constructor.
4. "A code generator could depend on being able to write duplicate keys without
having to go back and erase previous output". Yes, but it seems wrong to
facilitate automated code generation at the expense of human code writing. For
hand-written code, I claim that in most (all?) cases it would be preferable to
have the compiler detect key duplications. It is easier for an automated code
generator to check for duplications than it is for a human.
5. "There is already pylint". True, but it forces an additional step, and
seems like a cop-out for not wanting to do the "right thing" in the language.
For context, someone ran into this problem in my team at Google. We fixed it
using pylint, but I really don't see the point of having these constructor
semantics. From the discussions I have seen, it seems just an oversight in the
implementation/specification of dictionary literals (search keywords: dict
constructor, dict literal). I'd be happy to hear stronger reasoning in favor
of the status quo.
----------
components: Interpreter Core
messages: 264657
nosy: Luigi Semenzato
priority: normal
severity: normal
status: open
title: dictionary literal should not allow duplicate keys
type: behavior
versions: Python 3.4
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