# How to test monkey patching code?

**URL:** <https://www.ruby-forum.com/t/how-to-test-monkey-patching-code/128412>\
**Category:** Ruby\
**Created:** [February 7, 2008, 3:00pm UTC](https://www.ruby-forum.com/t/how-to-test-monkey-patching-code/128412 "2008-02-07T15:00:50Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![pluskid](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/ba8739/32.png) [@pluskid](https://www.ruby-forum.com/u/pluskid)\
**Post date:** [February 7, 2008, 3:00pm UTC](https://www.ruby-forum.com/t/how-to-test-monkey-patching-code/128412/1 "2008-02-07T15:00:50Z")

</div>

Hi, all!

I’m wondering how to test some monkey patching code. Take the standard  
library ‘jcode’  
in Ruby 1.8 for example. It do some monkey patching for the core  
String class when I  
require it. If I want to write some unit-test or spec for it, how to  
avoid it from affecting other  
tests? In other words, how can I “uninstall” those monkey patching  
after the test case  
is finished or how can I constrain the monkey patching inside a …,  
say, namespace?

I tried the following code:

```
orig_string = Object::String.clone

$KCODE = 'u'
require 'jcode'

# do testings

Object::String = orig_string

str1 = "foo"
str2 = String.new("foo")

# jlength are monkey patching
puts str1.jlength # => 3
puts str2.jlength # => raise NoMethodError

```

As I expected, str2.jlength raises NoMethodError, but str1 which is  
built  
by a string literal, returned 3, still being patched.

Further more, when I type those code directly in IRB, the irb fire out  
some exception and exited immediately:

C:/ruby/lib/ruby/1.8/irb/ruby-token.rb:101:in `Token': undefined method`ancesto  
rs’ for “=”:String (NoMethodError)  
from C:/ruby/lib/ruby/1.8/irb/ruby-lex.rb:400:in `lex_init' from C:/ruby/lib/ruby/1.8/irb/slex.rb:237:in`call’  
from C:/ruby/lib/ruby/1.8/irb/slex.rb:237:in `match_io' from C:/ruby/lib/ruby/1.8/irb/slex.rb:222:in`match\_io’  
from C:/ruby/lib/ruby/1.8/irb/slex.rb:76:in `match' from C:/ruby/lib/ruby/1.8/irb/ruby-lex.rb:287:in`token’  
from C:/ruby/lib/ruby/1.8/irb/ruby-lex.rb:263:in `lex' from C:/ruby/lib/ruby/1.8/irb/ruby-lex.rb:234:in`each\_top\_level\_stateme  
nt’  
from C:/ruby/lib/ruby/1.8/irb/ruby-lex.rb:230:in `loop' from C:/ruby/lib/ruby/1.8/irb/ruby-lex.rb:230:in`each\_top\_level\_stateme  
nt’  
from C:/ruby/lib/ruby/1.8/irb/ruby-lex.rb:229:in `catch' from C:/ruby/lib/ruby/1.8/irb/ruby-lex.rb:229:in`each\_top\_level\_stateme  
nt’  
from C:/ruby/lib/ruby/1.8/irb.rb:146:in `eval_input' from C:/ruby/lib/ruby/1.8/irb.rb:70:in`start’  
from C:/ruby/lib/ruby/1.8/irb.rb:69:in `catch' from C:/ruby/lib/ruby/1.8/irb.rb:69:in`start’  
from C:/ruby/bin/irb:13

I suppose IRB itself is doing some monkey patching to String? So, is  
there any  
way to …, return to my original question, to test monkey patching  
code?

Thanks!

---

<div class="post-metadata">

**Author:** ![pluskid](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/ba8739/32.png) [@pluskid](https://www.ruby-forum.com/u/pluskid)\
**Post date:** [February 7, 2008, 3:27pm UTC](https://www.ruby-forum.com/t/how-to-test-monkey-patching-code/128412/2 "2008-02-07T15:27:32Z")

</div>

On Feb 7, 2008 2:59 PM, pluskid [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> is finished or how can I constrain the monkey patching inside a …,  
> # do testings
> 
> ```
> Object::String = orig_string
> 
> ```

I would try

```
 orig_string = Object::String
 Object::String = Object::String.clone

 require 'jcode'

 # do testings

 Object::String = orig_string

```

instead.

Ie, let the modifications happen to a clone, rather than the original.  
That might stop “” from generating a monkey-patched string during  
testing, though.

Eivind.