Don't let Hash#fetch give you a false sense of security

On the face of it, Hash’s fetch instance method is a pretty handy and fail-safe way of retrieving keys from a hash. It allows one to specify a default value to be returned if a key does not exist.

hash = { :foo => 'bar' }
hash.fetch(:foo, 'default')   # => 'bar'
hash.fetch(:baz, 'default')   # => 'default'

That’s pretty useful behaviour. However, not infrequently I find myself having to work with hashes in which a key may be missing or be explicitly set to nil. (If you have any control over the situation, just don’t create an ambiguous data structure like that. If not, read on.) In cases such as this, fetch is no help: the key is present in the hash, and the method dutifully retrieves its value. In fact, I find that using fetch has tripped me up more than once because I rashly assumed that I was handling the missing value while I was really only handling the missing key.

Consider:

book1 = { :title => "100 Years of Solitude", :author => "marquez" }
book1.fetch(:author, "Author unknown").capitalize   #=> "Marquez"

book2 = { :title => "Autumn of the Patriarch" }
book2.fetch(:author, "Author unknown").capitalize   #=> "Author unknown"

book3 = { :title => "No One Writes to the Colonel", :author => nil }
book3.fetch(:author, "Author unknown").capitalize
# => NoMethodError: undefined method `capitalize' for nil:NilClass

So, as convenient as fetch seems at first, it can still leave the application not only with unsightful nils but with downright unacceptable exceptions.

By contrast, (book3[:author] || "Author unknown").capitalize will work with both missing keys and nil values, and is at least as performant as fetch.

Hash#fetch remains a good tool to use with well formed hashes, particularly because it can take a block that specifies more complex default behaviour, as in

book3.fetch(:author) do
  @books_with_missing_author += 1
  "Author unknown"
end

However, one has to be aware of the method’s limitations, as well as of the nuances of the data structures being parsed.