Why does ruby define variables even if it never executes the variable assignment code?

Question

Given the following code:

a = true # let's assign `a` a value

# and let's test if calling `b`, an unassigned variable, throws an error
begin
  puts "The value of b is: #{b.inspect}"
rescue NameError => e
  puts "Caught an error: #{e}"
end

a || b = true # the assignment should never be executed because `a` is `true`

puts "The value of b is: #{b.inspect}" # will calling `b` still raise an error?

We get the following result:

Caught an error: undefined local variable or method `b' for main:Object
The value of b is: nil

Even though we expected calling b to raise an error the second time, we see that b is now, in fact, nil.

Why is that? Why does b get assigned nil? Since the || never reached the assignment, I would expect b to remain undefined. How can it be defined, but not assigned a value?

Answer 1

Some of the docs explain how variables are created; the explanation as I understand it is that's just how the parser works:

The local variable is created when the parser encounters the assignment, not when the assignment occurs:

a = 0 if false # does not assign to a
p local_variables # prints [:a]
p a # prints nil

You can see other examples of this:

b = true if false # b is nil
"test" || c = true # c is nil

And others it doesn't get assigned:

puts d if false # d generates a NameError