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

Pavol Lisy pavol.lisy at gmail.com
Wed May 4 02:47:00 EDT 2016

2016-05-04 6:03 GMT+02:00, Nick Coghlan <ncoghlan at gmail.com>:

> I was curious as to whether or not it was technically feasible to
> implement this "duplicate keys" check solely for dict displays in
> CPython without impacting other dict use cases, and it turns out it
> should be.


> 1. Report a DeprecationWarning in 3.6
> 2. Report a [RuntimeError? ValueError?] in 3.7+

But what about SyntaxError instead of RuntimeError? Could it be easily
done too?

Probably nobody is interested in my tests with unicode literals which
are "NFKC equivalent" but it help me to understand how things are done
behind the scene.

def f(**kwd):
    for i in kwd:
        print(i, kwd[i])

def tst():
    f(ij=1, ij=2) # this is syntax error

def tst2():
     f(**{1:1}) # this is runtime error

def tst3():
     f(**{'ij':1, 'ij':2}) # this is legal (for me a little unexpected,
maybe bug?)

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