How to deal with potential errors while accessing a hash inside a hash?

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.

[Don’t rescue from Exception ever!](http://stackoverflow.com/q/10048173/211563). — Andrew Marshall, Jul 07 '13 at 13:49
If you are not using the exception `e`, there is no point of writing `=> e`. — sawa, Jul 07 '13 at 14:14

Arup Rakshit · Accepted Answer · 2013-07-07T12:51:21.740

11

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

edited Jul 07 '13 at 12:51

answered Jul 07 '13 at 12:38

Arup Rakshit

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Wow, had never seen your last example before. I always have `.try` all over the place. – BigRon Jul 24 '18 at 13:33

score 6 · Answer 2 · answered Jul 07 '13 at 12:58

6

Ruby 2.0 has NilClass#to_h.

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

answered Jul 07 '13 at 12:58

sawa

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@naomik Yeah. It is a cool feature. You can do similar things with arrays like `array[0].to_a[1]`. – sawa Jul 08 '13 at 05:12
any reason for the `to_a` and `to_h` names? They don't really seem to fit. I would expect something like `try_a` or `try_h` – Thank you Jul 08 '13 at 05:16
1

They always succeed. `to_a` always turns something into an array. `to_h` always turns something into a hash. – sawa Jul 08 '13 at 05:24

d11wtq · Answer 3 · 2013-07-07T14:25:57.367

2

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)

edited Jul 07 '13 at 14:25

answered Jul 07 '13 at 14:18

d11wtq

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You can write it shorter: `array.inject(h, &:[]) rescue nil`. – sawa Jul 07 '13 at 14:22
@sawa the receiver would be the element in the path in that case, not the hash? – d11wtq Jul 07 '13 at 14:23

score 1 · Answer 4 · edited May 23 '17 at 12:07

1

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

edited May 23 '17 at 12:07

Community

1
1

answered Jul 07 '13 at 14:09

Andrew Marshall

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If you are using `rescue`, there is no particular reason to switch from `Hash#[]` to `Hash#fetch`, which would make your second solution the same as the OPs. – sawa Jul 07 '13 at 14:12
OK. I got you point. You consider rescuing `KeyError` is better than rescuing `Exception`. – sawa Jul 07 '13 at 14:15
2

@sawa Well, just about anything is better than rescuing from Exception, but even if it was just StandardError there’s a whole multitude of other exceptions that could occur which you probably don’t want to rescue from. – Andrew Marshall Jul 07 '13 at 14:17
@AndrewMarshall I do like your `Hash#fetch` implementation. – Arup Rakshit Jul 07 '13 at 15:47
That's pretty cool. Why is begin rescue a bad thing in that scenario? What is a good place to use being rescue? – dsp_099 Jul 07 '13 at 17:01
@dsp_099 You may wish to see the SO question [Why not use exceptions as regular flow of control?](http://stackoverflow.com/questions/729379/why-not-use-exceptions-as-regular-flow-of-control) for some discussion. I think rescuing immediately isn’t so bad, since it’s obvious where the exception was thrown. Then again what’s that point of using an exception that can bubble up when you immediately rescue it? It’s a matter of design and style and semantics more than functional difference. – Andrew Marshall Jul 07 '13 at 17:57

How to deal with potential errors while accessing a hash inside a hash?

4 Answers4