[ruby-core:67415] [ruby-trunk - Bug #10713] Assigning default value for a Hash as an empty Array creating unpredictable results

Issue #10713 has been updated by Don Burks.


Martin D端rst wrote:
> Hiroshi SHIBATA wrote:
> > It's expected behavior
> 
> Hiroshi, can you tell us why it's expected behavior? It looks quite surprising.

I would agree that this is surprising behaviour. It would appear that in this case, the append operator is not re-assigning the value, the way it does any other time it is used. And it would appear to be not doing this specifically in the case where the default value for the Hash is specified as an empty array. I would like to understand why this behaviour is the way it is.

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Bug #10713: Assigning default value for a Hash as an empty Array creating unpredictable results
https://bugs.ruby-lang.org/issues/10713#change-50847

* Author: Arvinder Singh
* Status: Rejected
* Priority: Normal
* Assignee: 
* Category: core
* Target version: current: 2.2.0
* ruby -v: ruby 2.2.0p0 (2014-12-25 revision 49005) [x86_64-darwin14]
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN
----------------------------------------
Creating a Hash with a default value works fine, unless the default value is an empty Array.

E.g. the following returns an empty Hash...

~~~
irb(main):001:0> letters = Hash.new([])
=> {}
irb(main):002:0> letters[:a] << 1
=> [1]
irb(main):003:0> letters[:a] << 2
=> [1, 2]
irb(main):004:0> letters[:a]
=> [1, 2]
irb(main):005:0> letters
=> {}
~~~

whereas the following code explicitly defining hash keys works.

~~~
irb(main):001:0> letters = {a: [], b: []}
=> {:a=>[], :b=>[]}
irb(main):002:0> letters[:a] << 1
=> [1]
irb(main):003:0> letters[:a] << 2
=> [1, 2]
irb(main):004:0> letters[:a]
=> [1, 2]
irb(main):005:0> letters
=> {:a=>[1, 2], :b=>[]}
~~~

Is this an unpredictable(bug) or an expected behavior(feature). I tend to lean towards the former, but I might be wrong.



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