# Propagating exceptions from File.new

**URL:** <https://www.ruby-forum.com/t/propagating-exceptions-from-file-new/59786>\
**Category:** Ruby\
**Created:** [May 6, 2006, 4:33pm UTC](https://www.ruby-forum.com/t/propagating-exceptions-from-file-new/59786 "2006-05-06T16:33:26Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![eliben](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/3bc359/32.png) [@eliben](https://www.ruby-forum.com/u/eliben)\
**Post date:** [May 6, 2006, 4:33pm UTC](https://www.ruby-forum.com/t/propagating-exceptions-from-file-new/59786/1 "2006-05-06T16:33:26Z")

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Hello,

Suppose I have a class with a constructor that accepts a configuration  
file (in XML format). Part of an object’s initialization is to read some  
data from the XML file.

However, sometimes the file passed into the constructor doesn’t exist,  
and I wonder about the best idiomatic way in Ruby to handle it.

There are two main approaches (using File.new to open the file):

1. Rescue the File.new exception inside the constructor, and either  
throw some class-specific exception or return a class-specific return  
code

2. Ignore the exception inside the constructor and let the caller catch  
it

Approach (2) looks cleaner, but leads me to wonder how to document the  
exceptions my class is throwing (in C++ there’s a useful construct for  
this), can this only be done by means of non-code documentation (a  
comment) ?. Moreover, trying to test this solution, I got stuck in  
finding out which exception File.new actually throws when it doesn’t  
find a file. It says:

test.rb:12:in `initialize’: No such file or directory - test.rbgt  
(Errno::ENOENT)

Trying to rescue IOError doesn’t work, what does ? (except for the  
general catch-all ‘rescue Exception’, of course)

rescue Exception =\> details  
puts "IO Failed: " + $!  
p details.class  
end

the second print prints: “Errno::ENOENT” - is this a kind of exception ?

How can I find out which exceptions are actually thrown by standard Ruby  
classes, without looking at the source code or deciphering them with  
.class ?

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<div class="post-metadata">

**Author:** ![eliben](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/3bc359/32.png) [@eliben](https://www.ruby-forum.com/u/eliben)\
**Post date:** [May 8, 2006, 5:45pm UTC](https://www.ruby-forum.com/t/propagating-exceptions-from-file-new/59786/2 "2006-05-08T17:45:49Z")

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Eli B. wrote:

\> How can I find out which exceptions are actually thrown by standard Ruby \> classes, without looking at the source code or deciphering them with \> .class ?

Any ideas about this ?

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<div class="post-metadata">

**Author:** ![eliben](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/3bc359/32.png) [@eliben](https://www.ruby-forum.com/u/eliben)\
**Post date:** [May 8, 2006, 8:02pm UTC](https://www.ruby-forum.com/t/propagating-exceptions-from-file-new/59786/3 "2006-05-08T20:02:07Z")

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On 06/05/06, Eli B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> Hello,  
> […]  
> the second print prints: “Errno::ENOENT” - is this a kind of exception ?

Yes: Errno::ENOENT \< SystemCallError \< StandardError \< Exception

> How can I find out which exceptions are actually thrown by standard Ruby  
> classes, without looking at the source code or deciphering them with  
> .class ?

Well, you said it. Look up in the sources and (maybe) documentation or  
collect some empirical evidence. Note that what exceptions descend  
from SystemCallErrors depends on the OS, and are a subset of those  
defined in sys/errno.h.

Ciao,  
Stefano