Scope in Ruby and Python

Question

I've been learning Ruby and Python concurrently and one of the things I noticed is that these 2 languages seem to treat scope differently. Here's an example of what I mean:

# Python
a = 5
def myfunc():
  print a

myfunc() # => Successfully prints 5

# Ruby
a = 5
def myfunc
  puts a
end

myfunc # => Throws a "NameError: undefined local variable or method `a' for main:Object"

It appears that def block can access variables declared outside of its immediate scope in Python but not in Ruby. Can someone confirm whether my understanding is correct? And if so, whether one of these ways of thinking of scope is more common in programming?

For the Python side, you're right (there's obviously more to scoping, but your phrasing does not contradict any of it). — user395760
– user395760, Commented Feb 23, 2013 at 16:24
Possible Python answer here: stackoverflow.com/questions/370357/… — Todd A. Jacobs
– Todd A. Jacobs, Commented Feb 23, 2013 at 16:27

Linuxios · Accepted Answer · 2013-02-24 21:49:57Z

5

Disclaimer: I'm no python expert

In python, where variables defined in a module are, by default, module variables and as such global to that module. In Ruby, when you define a lowercase variable, it is always a local variable. Local variables are only accessible in the block that defined them and in procs/lambdas defined in that block that wrap the variable.

In Ruby, for a variable to cross scopes, it needs to be either:

A constant (ALL_CAPS): Always accessible, if prefixed with the right scope
Class variable (@@double_at): Always accessible from the defining class and any subclasses, but not from outside
Instance variable (@single_at): Accessible only from within that object, and from outside with getter methods/get_instance_variable.
Global ($starts_with_dollar): A bad idea. Crosses all scopes, no scoping needed. Do not use!

edited Feb 24, 2013 at 21:49

answered Feb 23, 2013 at 16:47

Linuxios

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4 Comments

wmock Over a year ago

Ah I see, so for Python, when you define a variable outside of any class or block, by default it will be in the global scope. This contrasts with Ruby where Ruby treats each scope as its own separate entity and you define global variables accessible within all scopes via using $. Is that an accurate summary?

Linuxios Over a year ago

@WillsonMock: Pretty much.

steenslag Over a year ago

Minor nitpick: constants are not ALL_CAPS, they just start with a capitalized letter. Classnames for instance are constants.

Linuxios Over a year ago

@steenslag: True, but unless you're defining a class or module, it's good form to use ALL_CAPS.

Endre Simo · Accepted Answer · 2013-02-23 16:41:52Z

1

In Ruby if you want to access a variable defined outside the method you are invoking you need to define it as global. Ruby try to match the variable defined in the local scope, if does not find it, throw an exception.

You can define a global variable with $ sign.

$a = 5
def myfunc
  puts $a
end

myfunc

However as a general rule is not a good practice to define global variables otherwise will risk to pollute the global namespace.

edited Feb 23, 2013 at 16:41

answered Feb 23, 2013 at 16:35

Endre Simo

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Comments

Akavall · Accepted Answer · 2013-02-23 18:20:35Z

1

You can use dis module to see what Python is doing.

import dis

a = 5
def myfunc():
  print a

Result:

>>> dis.dis(myfunc)
 15           0 LOAD_GLOBAL              0 (a)
              3 PRINT_ITEM          
              4 PRINT_NEWLINE       
              5 LOAD_CONST               0 (None)
              8 RETURN_VALUE

So you can see that a is in the global scope.

answered Feb 23, 2013 at 18:20

Akavall

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