[Python-ideas] dict literal allows duplicate keys

[Inada Naoki]

https://docs.python.org/3/reference/expressions.html#dictionary-displays

> If a comma-separated sequence of key/datum pairs is given, they are evaluated from left to right to define the entries of the dictionary: each key object is used as a key into the dictionary to store the corresponding datum. This means that you can specify the same key multiple times in the key/datum list, and the final dictionary’s value for that key will be the last one given.

On Thu, Mar 7, 2019 at 2:09 AM Jonathan Fine <jfine2358 at gmail.com> wrote:
>
> SUMMARY: The outcome of a search for: python dict literal duplicate
> keys. No conclusions (so far).
>
> BACKGROUND
> In the thread  "PEP: Dict addition and subtraction" I wrote
>
> >     >>>  {'a': 0, 'a': 1}
> >     {'a': 1}
>
> > I wonder, is this behaviour of {'a': 0, 'a': 1} documented (or tested)
> > anywhere? I didn't find it in these URLs:
> > https://docs.python.org/3/library/stdtypes.html#mapping-types-dict
> > https://docs.python.org/3/tutorial/datastructures.html#dictionaries
>
> LINKS
> I've since found some relevant URLs.
>
> [1] https://stackoverflow.com/questions/34539772/is-a-dict-literal-containing-repeated-keys-well-defined
> [2] https://help.semmle.com/wiki/display/PYTHON/Duplicate+key+in+dict+literal
> [3] https://bugs.python.org/issue26910
> [4] https://bugs.python.org/issue16385
> [5] https://realpython.com/python-dicts/
>
> ANALYSIS
> [1] gives a reference to [6], which correctly states the behaviour of
> {'a':0, 'a':1}, although without giving an example. (Aside: Sometimes
> one example is worth 50 or more words.)
>
> [2] is from Semmle, who provide an automated code review tool, called
> LGTM. The page [2] appears to be part of the documentation for LGTM.
> This page provides a useful link to [7].
>
> [3] is a re-opening of [4]. It was rapidly closed by David Murray, who
> recommended reopening the discussion on python-ideas.
> [4] was raised by Albert Ferras, based on his real-world experience.
> In particular, a configuration file that contains a long dict literal.
> This was closed by Benjamin Peterson, who said that raising an error
> was "out of the question for compatibility isssues". Given few use
> case and little support on python-ideas,Terry Ready supported the
> closure. Raymond Hettinger supported the closure.
>
> [5] is from RealPython, who provide online tutorials. This page
> contains the statement "a given key can appear in a dictionary only
> once. Duplicate keys are not allowed." Note that
>     {'a': 0, 'a': 1}
> can reasonably be thought of as a dictionary with duplicate keys.
>
> NOTE
> As I recall SGML (this shows my age) allows multiple entity declarations, as in
>     <!ENTITY key "original">
>     <!ENTITY key "updated">
>
> And as I recall, in SGML the first value "original" is the one that is
> in effect. This is what happens with the LaTeX command
> \providecommand.
>
> FURTHER LINKS
> [6] https://docs.python.org/3/reference/expressions.html#dictionary-displays
> [7] https://cwe.mitre.org/data/definitions/561.html # CWE-561: Dead Code
>
> --
> Jonathan
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-- 
Inada Naoki  <songofacandy at gmail.com>