Feature #14313


Support creating KeyError with receiver and key from Ruby

Added by kou (Kouhei Sutou) over 3 years ago. Updated about 3 years ago.

Target version:


KeyError has readers for error details, receiver and key. They are convenient to process KeyError.

We can set receiver and key to KeyError by rb_key_err_new() in C. But we can't set them in Ruby. Because receiver and key use no @ instance variables.

How about adding KeyError#initialize that accepts receiver and key? Because KeyError is useful in pure Ruby libraries. For example, csv library uses KeyError:

def fetch(header, *varargs)
  # ...
  raise KeyError, "key not found: #{header}"
  # ...

I want to use KeyError for Hash like objects such as Arrow::Table in Red Arrow.

The attached patch adds KeyError#initialize that behaves as the following:

# => "KeyError": No change. Keep backward compatibility.

# => "Message": No change. Keep backward compatibility.

# => The Object instance.

p :unknown_key).key
# => :unknown_key

key_error =, key: :unknown_key)
p key_error.receiver
# => The Object instance.
p key_error.key
# => :unknown_key

key_error ="Message", receiver:, key: :unknown_key)
p key_error.message
# => "Message"
p key_error.receiver
# => The Object instance.
p key_error.key
# => :unknown_key


key_error_new.diff (2.91 KB) key_error_new.diff kou (Kouhei Sutou), 01/04/2018 04:58 AM

Related issues

Related to Ruby master - Bug #14670: Objectで定義したmethod_missingでsuperが使えないClosedActions

Updated by nobu (Nobuyoshi Nakada) over 3 years ago

Also NameError?

Updated by Hanmac (Hans Mackowiak) over 3 years ago

should that also extend to the raise function itself like this?

raise KeyError, "key not found: #{header}", key: :header

or is that already done?

Updated by kou (Kouhei Sutou) over 3 years ago

NameError already supports assigning name:

p"message", :x).name # => :x

Ah, did you mention receiver not name?

Updated by kou (Kouhei Sutou) over 3 years ago

Kernel.#raise expects backtrace for the 3rd argument.
If we want to extend the current Kernel.#raise behavior, we should create a new issue instead of discussing in this issue.

Updated by matz (Yukihiro Matsumoto) about 3 years ago

I agree with the original proposal. We are not going to enhance raise behavior (yet).


Actions #6

Updated by kou (Kouhei Sutou) about 3 years ago

  • Status changed from Open to Closed

Applied in changeset trunk|r62049.

KeyError#initialize accepts receiver and key.
[Feature #14313][ruby-core:84626]

Updated by nobu (Nobuyoshi Nakada) about 3 years ago

Shouldn't unspecified attribute raise an exception?

diff --git c/error.c i/error.c
index 990f9f7855..7870e58035 100644
--- c/error.c
+++ i/error.c
@@ -1692,8 +1692,6 @@ key_err_initialize(int argc, VALUE *argv, VALUE self)
     VALUE message;
     VALUE options;
-    VALUE receiver = Qnil;
-    VALUE key = Qnil;

     rb_scan_args(argc, argv, "01:", &message, &options);

@@ -1711,16 +1709,13 @@ key_err_initialize(int argc, VALUE *argv, VALUE self)
    keywords[1] = id_key;
    rb_get_kwargs(options, keywords, 0, 2, values);
    if (values[0] != Qundef) {
-       receiver = values[0];
+       rb_ivar_set(self, id_receiver, values[0]);
    if (values[1] != Qundef) {
-       key = values[1];
+       rb_ivar_set(self, id_key, values[1]);

-    rb_ivar_set(self, id_receiver, receiver);
-    rb_ivar_set(self, id_key, key);
     return self;

diff --git c/test/ruby/test_key_error.rb i/test/ruby/test_key_error.rb
index 852bd32712..fe1d5bb5ab 100644
--- c/test/ruby/test_key_error.rb
+++ i/test/ruby/test_key_error.rb
@@ -15,11 +15,15 @@
     receiver =
     error = receiver)
     assert_equal(receiver, error.receiver)
+    error =
+    assert_raise(ArgumentError) {error.receiver}

   def test_key
     error = :key)
     assert_equal(:key, error.key)
+    error =
+    assert_raise(ArgumentError) {error.key}

   def test_receiver_and_key
Actions #8

Updated by nobu (Nobuyoshi Nakada) about 3 years ago

  • Related to Bug #14670: Objectで定義したmethod_missingでsuperが使えない added

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