Percent-format strings with named keys don't accept hashes with string keys?

Question

I am using the percent string formatting like so:

'fetching imap (%<host>s/%<user>s port=%<port>s...' % options

This works fine when the options hash has symbol keys { host: 'example.com' }, but raises a KeyError when it has string keys { 'host' => 'example.com' }:

KeyError: key<host> not found

This is problematic because Rails’ HashWithIndifferentAccess uses string keys by default. If using ActiveSupport, one solution is to call symbolize_keys on the hash.

Is there an alternative way to make the String#% method try string keys as well as symbol keys?

Is this a bug in Ruby that deserves a bug report? Or are there reasons for enforcing this behavior, like the edge case where both :host and 'host' keys exist?

Answer 1

It's not a bug that Ruby doesn't treat strings and symbols interchangeably; as you note, you're using Rails' HashWithIndifferentAccess, which exists exactly because Ruby regularly considers strings and symbols unrelated.

This is a similar situation to keyword arguments, which also do not allow string-keyed hashes.

At best, you could try to argue that sprintf should use the ruby-level accessors methods ([] or fetch), which HashWithIndifferentAccess overrides, instead of directly doing the C-level rb_hash_lookup2.

Alternatively, Rails might be able to make this work by partly restoring the default behaviour that existed between c918a3a0691 and a218a35e660.

Answer 2

I am using the percent string formatting … This works fine when the options hash has symbol keys … but raises a KeyError when it has string keys

If using ActiveSupport [from Rails], one solution is to call symbolize_keys on the hash.

For a solution without Rails you can use:

options = options.transform_keys(&:to_sym)

or

options.transform_keys!(&:to_sym)

See the manual. This requires Ruby 2.5.0 or higher, though. For more options, also for earlier Ruby versions, see this related question.