Check variable defined and not nil

Rails
1

Does anyone know of a way to write a function (or the name of an
existing function?) that will both check if a variable is defined, and
ensure that it is non-nil?

This has to be an uber-common problem, but hard to figure out the
right Google term for.

Situation:
I have partials that use optional local variables. If an optional
variable is not passed in, and has never been passed in since my
mongrel instance started, then defined?(variable) will be false.
However, if the partial has ever been called before, the local
variable has the value of “nil” (even if it wasn’t passed in on this
call), so defined?(variable) will be true.

Thus, I am left with ugly code (defined?(variable) && variable)
littered throughout my partials.

What’s worse, I can’t seem to write a method to combine these two
checks because I get an error in my partial when I try to call a
method with a variable that isn’t defined. Bleck.

Ideas?

Bill

2

Hi –

On Fri, 11 Apr 2008, [email protected] wrote:

variable is not passed in, and has never been passed in since my
method with a variable that isn’t defined. Bleck.
I’d be inclined to try to bypass the defined? thing and arrange the
code so that all you needed was to test for nil. Can you pass in the
variable no matter what, but just have it set to nil if it doesn’t
have a useful value?

David

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3

I could, but that would be putting extra responsibility on the
caller. I guess that a decent option would be to have an options hash
for the optional options, and just pluck the variables out of there,
so options[:variable] will always be defined, just as nil.

That wouldn’t be an awful solution, but it would only work for
partials, and I’m pretty sure someone out there knows a better way…

B

4

On Apr 11, 2:54 pm, “[email protected]”
[email protected] wrote:

Does anyone know of a way to write a function (or the name of an
existing function?) that will both check if a variable is defined, and
ensure that it is non-nil?
This has to be an uber-common problem,

Not that common, but I’ve encountered it :slight_smile:

:
:
Ideas?

Might not be what you’re looking for but at least it’s shorter,

(variable rescue nil)

You’re probably aware, that defined? is not an ordinary function:
http://redhanded.hobix.com/inspect/methodCheckDefined.html
So addressing the problem directly without built-in support looks
unlikely.

Stephan

5

Interestingly, I’m running into this same problem.

I discovered a fascinating fact. The rescue only happens the very
first time for that partial. Page reloads or other calls to the
partial on the same page somehow make the local variable defined. I
can understand multiple calls on the same page I guess (maybe “local”
is scoped for the whole view?) but on reloads?

Anyway, I had a simple test partial that looked like this:

<h1><%= (text || "DEFAULT") rescue "RESCUED DEFAULT" %></h1>

And in my view I have the following:

<%= render :partial => 'test' %>
<%= render :partial => 'test', :locals => {:text => "LOCAL TEXT" }%>
<%= render :partial => 'test' %>
<%= render :partial => 'test', :locals => {:text => "ANOTHER LOCAL
TEXT" }%>

If I run it from a freshly started server, I get:

RESCUED DEFAULT
LOCAL TEXT
DEFAULT
ANOTHER LOCAL TEXT

and if I simply reload the page (either reload or shift-reload), I
get:

DEFAULT
LOCAL TEXT
DEFAULT
ANOTHER LOCAL TEXT

Which tells me that the rescue truly only happens the very first time.
Weird.

Anyway, by combining a default and a rescue, I think you can do what
you need, mainly: have a partial with some default functionality that
can be overridden by locals and then do the right thing whether the
local variable is nil or undefined.

For me, this is really useful to know because I have some partials
that render logos via attachment_fu using a default sized thumbnail.
Then, I can set it so I can pass in a different thumbnail size as a
local and it’ll use that instead. Whee!

-Danimal

6

Here’s the solution that I use to set a default value for a partial
local variable if one is not passed by the calling view.

Add something like this to the start of your partial:

<%- if local_assigns[:local_variable_name].nil? then
local_variable_name = your_default_value end -%>

You could probably clean it up a bit, depending on your ability to
read Ruby:

<%- local_variable_name = your_default_value if
local_assigns[:local_variable_name].nil? -%>
  • GG Crew
7

Hi
Is there anything wrong with
<% if !defined? variable_name%>

8

What’s wrong with:

your_variable ||= “Default”?