如何在访问哈希内部的哈希时处理潜在的错误？

Question

Sometimes while dealing with API responses, I'll end up writing something like:

what_i_need = response["key"]["another key"]["another key 2"]

The problem with that is, it'll throw an error if, say, "another key" is missing. I don't like that. I'd be a lot happier if what_i_need turned up a nil if something along the process broke.

Is there a more elegant solution than:

what_i_need = nil
begin
  what_i_need = response["key"]["another key"]["another key 2"]
rescue Exception => e
end

I also thought about monkey patching NilClass you try to access nil["something"] it would return nil , but I'm not sure if that's the best way to go about it either of if it's possible even.

Answer 1

Use Hash#fetch with default value.

h = {:a => 2}
h.fetch(:b,"not present")
# => "not present"
h.fetch(:a,"not present")
# => 2

Without default value it will throw KeyError .

h = {:a => 2}
h.fetch(:b)
# ~> -:2:in `fetch': key not found: :b (KeyError)

But with nested Hash like your one you can use :

h = {:a => {:b => 3}}
val = h[:a][:b] rescue nil # => 3
val = h[:a][:c] rescue nil # => nil
val = h[:c][:b] rescue nil # => nil

Answer 2

Ruby 2.0有NilClass#to_h 。

what_i_need = response["key"].to_h["another key"].to_h["another key 2"]

Answer 3

Taking some inspiration from Objective-C's key-value coding system, you can do this with a lightweight DSL to walk a series of keys in an arbitrarily-nested data structure:

module KeyValue
  class << self
    def lookup(obj, *path)
      path.inject(obj, &:[]) rescue nil
    end
  end
end

h = { a: { b: { c: 42, d: [ 1, 2 ] } }, e: "test"}

KeyValue.lookup(h, :a, :b, :d, 1) # => 2
KeyValue.lookup(h, :a, :x)        # => nil

Or if you just want the one-liner:

(["key", "another key", "another key 2"].inject(h, &:[]) rescue nil)

Answer 4

To expand on @Priti's answer a bit, you can use a chain of Hash#fetch instead of Hash#[] to return empty hashes till you get to the last in the chain, and then return nil :

what_i_need = response.fetch('key', {})
                      .fetch('another key', {})
                      .fetch('another key 2', nil)

or rely on the KeyError exception being raised (maybe not the best, as exceptions as control flow should be avoided):

what_i_need = begin
                response.fetch('key').fetch('another key').fetch('another key 2')
              rescue KeyError
                nil
              end