why doesn't this python-like ruby script work?

Question

I'm kind of confused about how scoping works in Ruby. Which Ruby scoping rules prevent the following code from printing out the value of x?

#!/usr/bin/ruby

x = 5
def test
    puts "x = #{x}"
end

test    # => undefined local variable or method `x' for main:Object

The equivalent Python script does print the value of x:

#!/usr/bin/python

x = 5
def test():
    print "x = %s" % x

test()  # => x = 5

score 5 · Answer 1 · edited Jul 23 '12 at 22:03

5

This is so called "scope gate". There are three gates in ruby: definition of a method, a class or a module. Meaning that inside that definition locals from outer scope are not visible.

This is so because when scope changes, current set of bindings gets replaced by a new set of bindings and cannot be easily accessed.

See "Metaprogramming Ruby: Program Like the Ruby Pros" for more information.

edited Jul 23 '12 at 22:03

the Tin Man

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answered Jul 23 '12 at 21:26

Sergio Tulentsev

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The new version of this book is [Metaprogramming Ruby 2: Program Like the Ruby Pros](https://pragprog.com/book/ppmetr2/metaprogramming-ruby) – Ben Fulton Jun 04 '15 at 16:21

score 3 · Accepted Answer · edited May 23 '17 at 12:06

See "Ruby classes, include, and scope" for more information about Ruby's scoping and scope gating.

There are a few ways to provide visibility to your test function. One is to make the variable an instance variable:

#!/usr/bin/ruby

@x = 5
def test
    puts "x = #{@x}"
end

And another is to use a closure with lambda:

#!/usr/bin/ruby

x = 5
test = lambda do
    puts "x = #{x}"
end

test.call

or Proc.new()

#!/usr/bin/ruby

x = 5
test = Proc.new do
    puts "x = #{x}"
end

test.call

For the differences between lambda and Proc.new, see "When to use lambda, when to use Proc.new?".

why doesn't this python-like ruby script work?

2 Answers2