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@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-re... [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
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