optional local variables in rails partial templates: how do I get out of the (defined? foo) mess?

I've been a bad kid and used the following syntax in my partial templates to set default values for local variables if a value wasn't explicitly defined in the :locals hash when rendering the partial --

<% foo = default_value unless (defined? foo) %>

This seemed to work fine until recently, when (for no reason I could discern) non-passed variables started behaving as if they had been defined to nil (rather than undefined).

As has been pointed by various helpful people on SO, http://api.rubyonrails.org/classes/ActionView/Base.html says not to use

defined? foo

and instead to use

local_assigns.has_key? :foo

I'm trying to amend my ways, but that means changing a lot of templates.

Can/should I just charge ahead and make this change in all the templates? Is there any trickiness I need to watch for? How diligently do I need to test each one?

I do this:

<% some_local = default_value if local_assigns[:some_local].nil? %>

Since local_assigns is a hash, you could also use fetch with the optional default_value.

local_assigns.fetch :foo, default_value

This will return default_value if foo wasn't set.

WARNING:

Be careful with local_assigns.fetch :foo, default_value when default_value is a method, as it will be called anyway in order to pass its result to fetch.

If your default_value is a method, you can wrap it in a block: local_assigns.fetch(:foo) { default_value } to prevent its call when it's not needed.

How about

<% foo ||= default_value %>

This says "use foo if it is not nil or true. Otherwise assign default_value to foo"

I think this should be repeated here (from http://api.rubyonrails.org/classes/ActionView/Base.html):

If you need to find out whether a certain local variable has been assigned a value in a particular render call, you need to use the following pattern:

<% if local_assigns.has_key? :headline %>
  Headline: <%= headline %>
<% end %>

Testing using defined? headline will not work. This is an implementation restriction.

In my case, I use:

<% variable ||= "" %>

in my partial.
I don't have idea if that is good but for my is OK

I know it's an old thread but here's my small contribution: i would use local_assigns[:foo].presence in a conditional inside the partial. Then i set foo only when needed in the render call:

<%= render 'path/to/my_partial', always_present_local_var: "bar", foo: "baz" %>

Have a look at te official Rails guide here. Valid from RoR 3.1.0.

This is a derivative of Pablo's answer. This allows me to set a default ('full'), and in the end, 'mode' is set in both local_assigns and an actual local variable.

haml/slim:

- mode ||= local_assigns[:mode] = local_assigns.fetch(:mode, 'full')

erb:

<% mode ||= local_assigns[:mode] = local_assigns.fetch(:mode, 'full') %>

I think a better option that allows for multiple default variables:

<% options = local_assigns.reverse_merge(:include_css => true, :include_js => true) %>
<%= include_stylesheets :national_header_css if options[:include_css] %>
<%= include_javascripts :national_header_js if options[:include_js] %>

Ruby 2.5

Erb

It's possible, but you must to declare your default values in the scope.

VARIABLE the word for replacement.

# index.html.erb
...
<%= render 'some_content', VARIABLE: false %>
...

# _some_content.html.erb
...
<% VARIABLE = true if local_assigns[:VARIABLE].nil? %>
<% if VARIABLE %>
    <h1>Do you see me?</h1>
<% end %>
...

More intuitive and compact:

<% some_local = default_value unless local_assigns[:some_local] %>

If you do not want to pass local variable to partial each time you call it you do this:

<% local_param = defined?(local_param) ? local_param : nil %>

This way you avoid undefined variable error. This will allow you to call your partial with/without local variables.

A helper can be created to look like this:

somearg = opt(:somearg) { :defaultvalue }

Implemented like:

module OptHelper
  def opt(name, &block)
    was_assigned, value = eval(
      "[ local_assigns.has_key?(:#{name}), local_assigns[:#{name}] ]", 
      block.binding)
    if was_assigned
      value
    else
      yield
    end
  end
end

See my blog for details on how and why.

Note that this solution does allow you to pass nil or false as the value without it being overridden.