Bug #12022
closedInconsistent behavior with splatted named arguments
Description
The Bug¶
When an empty hash is splatted (using **) into a call of a method with no parameters, the empty hash is passed in as a positional argument instead of doing nothing. This causes an ArgumentError, which is confusing because when you splat an empty array into a method that doesn't accept any arguments the method is called without raising an error.
Similarly, if you splat a hash into a method that only has positional arguments, the method is called with the hash added as the last argument. This either causes an ArgumentError or unexpected bugs.
Examples¶
(tested in MRI 2.2.2 and 2.3.0)
def without_parameters
# some code
end
def with_parameters(*args)
args
end
def with_one_parameter(arg)
arg
end
empty_hash = {}
filled_hash = { example: "value" }
array = []
without_parameters(*array)
# calls the method without an error because `array' is empty
without_parameters(**empty_hash)
# unexpectedly raises an ArgumentError despite `empty_hash' being empty
with_parameters(**empty_hash)
# unexpectedly returns [{}] instead of []
with_parameters(**filled_hash)
# unexpectedly returns [{ example: "value }] instead of raising an ArgumentError
with_one_parameter(**empty_hash)
with_one_parameter(**filled_hash)
# both unexpectedly do not raise an ArgumentError
Further Information¶
This behavior makes it more difficult to do things like write specialized decorator classes using #method_missing. The following example does not work if the method being called does not have any named parameters. The variable named_args gets passed in as a positional argument, causing ArgumentErrors or unexpected bugs:
class TrivialDecoratorExample
def initialize(value)
@value = value
end
def method_missing(name, *args, **named_args, &block)
@value.send(name, *args, **named_args, &block)
end
end
Instead one has to write something really ugly like:
def method_missing(name, *args, **named_args, &block)
if @value.method(name)
.parameters
.any? { |type, _| [:keyreq, :key].include?(type) }
@value.send(name, *args, **named_args, &block)
elsif named_args.empty?
@value.send(name, *args, &block)
else
raise ArgumentError.new
end
end
Updated by justcolin (Colin Fulton) almost 9 years ago
- ruby -v set to 2.2.2 and 2.3.0
Updated by justcolin (Colin Fulton) over 8 years ago
More details about why errors are not raised in some cases, and wrong errors are raised in other cases:
def example(required_param, **optional_named)
p required_param
end
# Works as expected.
example(42) # => 42
# Should raise an error because no positional argument is being passed in,
# instead it passes in an option hash to required_param.
example(named_arg: 43)
def example2(required_param, second_required_param, **optional_named)
# Some code...
end
# Should say it is missing two arguments, instead says it is missing one:
example2(named: 42)
I understand that all of this is probably related to the option hash syntax leftover from when Ruby didn't have named arguments, but this all leads to very confusing code. Named arguments are much more powerful, easier to deal with, and more "Ruby-like" than the option hash syntax. As such I feel that Ruby should assume that if you use the ** operator in your parameters you never want that method to use an options hash.
In the distance future I even think that option hashes should be deprecated then removed, since named arguments do everything option hashes do and more. What opinions do you all have?
Updated by sawa (Tsuyoshi Sawada) over 8 years ago
This is a duplicate, or is at least related to #11860.
Updated by justcolin (Colin Fulton) over 8 years ago
Sorry, I didn't see that issue. The cause is probably the same, but the difference is that #11860 deals with using the double splat in the argument list of a method call, whereas this issue deals using with the double splat in the parameter list of a method definition. That being said, these two issues can probably be merged.
Thanks for catching that!
Updated by justcolin (Colin Fulton) over 8 years ago
NOTE: I did find a "cleaner" way to do the decorator mentioned in the Further Information section, but it uses Kernel#eval
which never feels like a good idea:
def method_missing *ordered, **named, &block
args = ordered.map.with_index { |_, index| "ordered[#{index}]" } +
named.map { |key, _| "#{key}: named[:#{key}]" }
eval "@value.#{name}(#{args.join(', ')})"
end
Updated by jeremyevans0 (Jeremy Evans) over 5 years ago
- Related to Feature #14183: "Real" keyword argument added
Updated by jeremyevans0 (Jeremy Evans) about 5 years ago
- Status changed from Open to Closed
With the master branch, you now get:
without_parameters(*array)
# => nil
without_parameters(**empty_hash)
# => nil
with_parameters(**empty_hash)
# => []
# This doesn't raise ArgumentError, because the method does not accept keyword
# arguments, so a keyword splat is passed as a positional hash.
with_parameters(**filled_hash)
# => [{:example=>"value"}]
with_one_parameter(**empty_hash)
# ArgumentError (wrong number of arguments (given 0, expected 1))
# This also doesn't raise ArgumentError, for the same reason as with_parameters
with_one_parameter(**filled_hash)
# => {:example=>"value"}
There are no warnings in this code, and the behavior should be the same in Ruby 3.