[Python-ideas] dictionary constructor should not allow duplicate keys

[Michael Selik]

On Wed, May 4, 2016 at 12:04 AM Nick Coghlan <ncoghlan at gmail.com> wrote:

> As such, the technical change being requested here (at least for
> CPython), is that BUILD_MAP be updated to do a PyDict_Contains()
> check prior to each key insertion, and:
> 1. Report a DeprecationWarning in 3.6
> 2. Report a [RuntimeError? ValueError?] in 3.7+
>

To be clear, this implementation would potentially raise an error in the
random example -- ``{random(): 0, random(): 1}`` -- correct? If so, then
code smell or not, I think that's too much breakage.

We could restrict the PyDict_Contains() call to only enforcing uniqueness
of string or numeric literals, but then we'd still have some oddball
situations:

def a():
    return 'a'

{a(): 0, 'a': 1}
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