Question

Look at the discussion on this thread . I am not able to follow how having a block to a fetch is a better solution.

Answer 1

In the first patch on Rails ticket #4558:

options.fetch(:alt, File.basename(src, '.*').capitalize)

This line executes the basename and capitalize functions and then passes the result into Hash#fetch regardless of if a value for :alt already exists in the options hash.

In the updated patch:

options.fetch(:alt) { File.basename(src, '.*').capitalize }

The same basename/capitalize code is only executed when Hash#fetch needs the default value (i.e. when the :alt key does not exist in the options hash). This means the (possibly expensive) calculation of the default value can be skipped if it’s not needed.

See the documentation on Hash#fetch for more details.