Bug #16498: Hash#transform_values in 2.7.0 sets new hash's default to old hash's default_proc

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 2.7.0p0 (2019-12-25 revision 647ee6f091) [x86_64-linux]
• Backport: 2.5: DONTNEED, 2.6: DONTNEED, 2.7: DONE

[ptsengineer (Peter Tsengineer)]

The following is unexpected for me.

Hash.new { }.transform_values { }.default
# => #<Proc:0x000055eecda01510 -e:1>
Hash.new { }.transform_values { }[:any]
# => #<Proc:0x0000563a12e35510 -e:1>
Hash.new { :default }.transform_values { }[:any][]
# => :default

I did not think a default_proc would turn into a default. The previous behaviour in 2.6 was that the default would be nil, so this is new in 2.7. But I didn't see this mentioned in the 2.7 NEWS. May I learn of the proper use for this and/or whether it is intentional?

If you wonder when we might see something like this, I had found it with code similar in idea to the following:

weights = Hash.new { |h, k| h[k] = [] }
weights[:apple] << 1
weights[:apple] << 2

prices = weights.transform_values { :arbitrary }

puts (prices[:mango] || [])[0] || "We're all out of mangoes today"

in `block in <main>': undefined method `[]=' for 0:Integer (NoMethodError)

That was quite a confusing error message. It took a while to realise why []= was called on 0.

We also cannot fix this by using dig (puts prices.dig(:mango, 0) || "We're all out of mangoes today") because then, we would get:

in `dig': Proc does not have #dig method (TypeError)

[ptsengineer (Peter Tsengineer)]

• Description updated (diff)

[ptsengineer (Peter Tsengineer)]

• Subject changed from Hash#transform_value in 2.7.0 sets new hash's default to old hash's default_proc to Hash#transform_values in 2.7.0 sets new hash's default to old hash's default_proc

[ptsengineer (Peter Tsengineer)]

• Description updated (diff)

[sawa (Tsuyoshi Sawada)]

• Description updated (diff)

[nobu (Nobuyoshi Nakada)]

• Status changed from Open to Closed

Applied in changeset git|1b4d406e3a04032b6d01e92b6d184a16945c6ac3.

---

Hash#transform_values should return a plain new Hash

[Bug #16498]

[vo.x (Vit Ondruch)]

• Backport changed from 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN to 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: REQUIRED

This does break ActiveSupport 5.2 test suite.

[nobu (Nobuyoshi Nakada)]

• Related to Bug #16914: Hash.new with `default_proc` returns this `default_proc` instead of calling it sometimes added

[nobu (Nobuyoshi Nakada)]

• Related to deleted (Bug #16914: Hash.new with `default_proc` returns this `default_proc` instead of calling it sometimes)

[nobu (Nobuyoshi Nakada)]

• Has duplicate Bug #16914: Hash.new with `default_proc` returns this `default_proc` instead of calling it sometimes added

[nobu (Nobuyoshi Nakada)]

• Backport changed from 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: REQUIRED to 2.5: DONTNEED, 2.6: DONTNEED, 2.7: REQUIRED

[nagachika (Tomoyuki Chikanaga)]

• Backport changed from 2.5: DONTNEED, 2.6: DONTNEED, 2.7: REQUIRED to 2.5: DONTNEED, 2.6: DONTNEED, 2.7: DONE

ruby_2_7 05c4c88767b54808cd1459a1d6da11179c422cfa merged revision(s) 1b4d406e3a04032b6d01e92b6d184a16945c6ac3.