Q:

Confused about 'respond_to' vs 'respond_to?'

I am learning Rails with railstutorial.org, and I am confused about something:
in this chapter the author tells us to do some testing in the console with the respond_to? method on a User object, and it works ok. But later, when we write the test for the :encrypted_password attribute, he uses respond_to.
Out of curiosity, I tried respond_to in the console, for a User object, and I get an error saying the method doesnt exist. Alas, if I try to write the test using respond_to? instead of respond_to, the test doesnt run. 
Could someone explain me the difference, and why does the test only run with respond_to?

A:

Ruby treats ? and ! as actual characters in a method name. respond_to and respond_to? are different. ? indicates that this should respond with a true or false (by convention; this is not a requirement). Specifically:
respond_to? is a Ruby method for detecting whether the class has a particular method on it. For example,
@user.respond_to?('eat_food')

would return true if the User class has an eat_food method on it.
respond_to is a Rails method for responding to particular request types. For example:
def index
  @people = Person.find(:all)

  respond_to do |format|
    format.html
    format.xml { render :xml => @people.to_xml }
  end
end

However, in the RailsTutorial link you've provided, you're seeing an RSpec method should interacting with RSpec's respond_to method. This wouldn't be available in your console, unless you run rails console test.

A:

respond_to? is a Boolean evaluation. The respond_to is used (normally) for determining the display information.  More information here.  The respond_to? checks to see if a method exists and returns true if it does and false if it doesn't.  

A:

The test uses convenient helpers to be more user friendly.
Ruby is Ruby so using the good old respond_to? would work if you call it this way:
 @user.respond_to?(:encrypted_password).should be_true

There is another respond_to used in controllers but still nothing to do with those you already met.

