Check if module exists in ruby

Question

I'm dynamically defining a module name from an argument passed on the cli, for example Required::Module::#{ARGV.first}

Is there any way to check if that module exists? Also, how would I run methods on it not knowing it's exact name?

Interesting. There is no straight answer here, and noteworthy machination is required. Good Question. — New Alexandria, Sep 21 '12 at 19:45

score 37 · Answer 1 · answered Aug 24 '10 at 15:21

37

Use const_defined? for this.

Required::Module.const_defined?(:ModuleName)

returns true or false.

answered Aug 24 '10 at 15:21

horseyguy

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Now can it: `Module.const_defined?(:Name)` it return true or false – SunTechnique Aug 28 '20 at 03:46

Andrew Grimm · Answer 2 · 2010-08-19T23:22:22.367

14

defined?(Required::Module)

gives "constant" if it exists, and nil if it doesn't.

Update: Sorry, didn't read your question properly.

defined?(eval("Required::Module::"+string))

should give you what you're after.

edited Aug 19 '10 at 23:22

answered Aug 19 '10 at 13:20

Andrew Grimm

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But this doesn't seem to work with dynamically named modules, for example `defined?("Parade::Procedure::#{procedure.capitalize}")` returns `"expression"`, even though the module I'm calling doesn't exist. – Andrei Serdeliuc ॐ Aug 19 '10 at 14:16
I'd want to avoid using `eval`, especially if `string` is a user entered value. You could cause some serious damage. (read more about it https://stackoverflow.com/questions/637421/is-eval-supposed-to-be-nasty) – Andrew K Oct 15 '18 at 20:19

score 11 · Accepted Answer · answered Aug 19 '10 at 08:00

11

Check for module existence using the const_get method:

begin
    mod = Required::Module::const_get "ModuleName"
    #It exists
rescue NameError
    #Doesn't exist
end

answered Aug 19 '10 at 08:00

Daniel O'Hara

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I do not believe exception handling is the best approach, it's both slow and bloated (code-wise). Use `const_defined?` instead. – horseyguy Aug 24 '10 at 15:19
You should edit the answer to do something like `mod = Required::Module.const_defined?("ModuleName") ? Required::Module::const_get "ModuleName" : nil` – Leo Correa Jun 02 '13 at 00:15
1

Note that if the module is known statically it's easier to just use `Required::Module::ModuleName` - both will through `NameError`. This is useful if the module name is only known at runtime though. – Aram Kocharyan Aug 18 '13 at 03:56
1

Do not forget to pass inherited = false as a second argument, it is important. Otherwise you will conflict with ancestors. – puchu Sep 02 '18 at 11:47

siefca · Answer 4 · 2012-08-24T10:07:54.407

You have to check if:

a constant referring to a module exists,
an object that the constant holds reference to is a module.

Try that:

 def module_exists?(name, base = self.class)
   base.const_defined?(name) && base.const_get(name).instance_of?(::Module)
 end

Then in your code:

 module_exists?(ARGV.first, Required::Module)

It will return true if there is a module of the given name within a given namespace base. The difference from the examples given in other answers is that it will return false if the queried name reffers to a class, not to a module.

Include classes in a test

If you want to change that behavior and force the method to also return true for classes (not just modules) change instance_of? to is_a?.

OOP way

You can also code it in a more object-oriented way if your Required::Module module is the only module you're testing for submodules:

 module Required::Module
   def submodule_exists?(name)
     const_defined?(name) && const_get(name).instance_of?(::Module)
   end
 end
 module_function :submodule_exists?

Then in your code:

 Required::Module.submodule_exists?(ARGV.first)

score 2 · Answer 5 · answered Jun 02 '13 at 00:00

2

If you got ActiveSupport

mod = ("Required::Module::#{ARGV.first}".constantize rescue nil)

answered Jun 02 '13 at 00:00

ybs

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I don’t think this makes sense as there’s cleaner native way to do this. – Smar Jun 29 '14 at 19:32

score 1 · Answer 6 · answered Jun 29 '14 at 19:20

In the case where you require something that extends something else, you can't base your test on a constant because the extension may not define a new one. Instead, base it on the presence of something else, like a new method.

I use the below to test if open_uri_redirections has been required:

if OpenURI.methods.include?(:redirectable_safe?)
  # extension loaded
else
  # extension not loaded
fi

score 1 · Answer 7 · answered Dec 04 '14 at 20:28

1

The currently selected answer is not correct. const_get and const_defined look for any constant name, regardless of the object calling that method. For example, if I wanted to check for MyModule::Rails inside a Rails application, using const_get would return the normal Rails module.

To check for a constant within a specific namespace, use the constants method and check for your class:

MyModule.constants.include?("Rails") # => false

answered Dec 04 '14 at 20:28

saquino88

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Kudos. In the manner of `instance_methods` etc., `const_defined?` includes a second parameter of `true` (default, inheritance is considered) and `false` (only search the receiver). I think I prefer using that even though it's a rather ambiguous boolean property, because there's a good chance that the Ruby interpreter will have a more optimised search than creating a Ruby array via `constants` and asking Ruby to search it with `include?`. – Andrew Hodgkinson Apr 12 '16 at 21:03

Eric Norcross · Answer 8 · 2021-08-06T22:19:49.760

0

Get the class if it exists:

dynamic_klass = "Required::Module::#{ARGV.first}".classify.safe_constantize

Call a method on the class if it does:

dynamic_klass.send("some_method") if dynamic_klass.present?

edited Aug 06 '21 at 22:19

answered Aug 06 '21 at 22:14

Eric Norcross

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Check if module exists in ruby

8 Answers8

Include classes in a test

OOP way