Bug #19795
closed
attr_accessor leading to nil values for re-assignment
Description
Steps to reproduce:¶
- Create a class with an
attr_accessorfor an instance variable - Create an instance method that reassign this variable using the current value stored in the variable
- Show that the variable is set to nil during the evaluation
Code snippet:
# attr_accessor_nil.rb class A attr_accessor :a def initialize @a = 0 end def my_method puts "a is '#{a.inspect}' of class '#{a.class}'" a += 1 end end instance = A.new instance.my_method # output: # # a is '0' of class 'Integer' # attr_accessor_nil.rb:12:in `my_method': undefined method `+' for nil:NilClass (NoMethodError) # # a += 1 # ^ # from attr_accessor_nil.rb:17:in `<main>' Expected behavior¶
a += 1 should lead to a being equal to 1 at the end of the assignment, because a was storing 0 previously, as shown by the puts. Am I being wrong expecting this result?
Actual behavior¶
a += 1 raises an error about a being nil in the evaluation.
Further investigation¶
I checked if it was coming from the "instance variable", or about the "attr_accessor" by running the following snippet:
Code snippet:
# attr_accessor_nil.rb class A attr_accessor :a def initialize @a = 0 end def my_method puts "a is '#{a.inspect}' of class '#{a.class}'" @a += 1 # use the instance variable directly, instead of the accessor end end instance = A.new instance.my_method # output: # # a is '0' of class 'Integer' This snippet runs just fine, and no error is raised.
System configuration¶
Ruby version : 3.2.2
Updated by francktrouillez (Franck Trouillez) over 2 years ago
- Description updated (diff)
Updated by nobu (Nobuyoshi Nakada) over 2 years ago
- Status changed from Open to Closed
This is an assignment to the local variable, not the accessor.
a += 1 You have to write:
self.a += 1 Updated by francktrouillez (Franck Trouillez) over 2 years ago
I guess that it is a local variable, but is it the expected behavior?
As a developer, I expect that I can access a without the self.a.
Since the following code works as expected without self:
# attr_accessor_nil.rb class A attr_accessor :a def initialize @a = 0 end def my_method puts "a is '#{a.inspect}' of class '#{a.class}'" b = a + 1 a = b puts "b is '#{b.inspect}' of class '#{b.class}'" puts "a is '#{a.inspect}' of class '#{a.class}'" end end instance = A.new instance.my_method # output: # # a is '0' of class 'Integer' # b is '1' of class 'Integer' # a is '1' of class 'Integer' I expect that the following works as well:
# attr_accessor_nil.rb class A attr_accessor :a def initialize @a = 0 end def my_method puts "a is '#{a.inspect}' of class '#{a.class}'" a = a + 1 end end instance = A.new instance.my_method # output: # # a is '0' of class 'Integer' # Traceback (most recent call last): # 1: from attr_accessor_nil.rb:17:in `<main>' # attr_accessor_nil.rb:12:in `my_method': undefined method `+' for nil:NilClass (NoMethodError) Why can I access the instance variable (read and write) without the self, but as soon as I try to re-assign it using the current value, it switches to a local variable? This is something unexpected for me.
Is it the expected behavior for this? If so, is there a way of making this special case clearer? Or am I being wrong thinking the behavior is unexpected?
Updated by francktrouillez (Franck Trouillez) over 2 years ago
I got it in the end thanks to a peer. I misunderstood wrongly how this was working:
Running the following snippet helped me understand:
# attr_accessor_nil.rb class A attr_accessor :a def initialize @a = 0 end def my_method puts "a is '#{a.inspect}' of class '#{a.class}'" b = a + 1 a = b puts "b is '#{b.inspect}' of class '#{b.class}'" puts "a is '#{a.inspect}' of class '#{a.class}'" end def my_method_2 puts "a is '#{a.inspect}' of class '#{a.class}'" end end instance = A.new instance.my_method instance.my_method_2 # output: # # a is '0' of class 'Integer' # b is '1' of class 'Integer' # a is '1' of class 'Integer' # a is '0' of class 'Integer' So, we cannot modify the instance variable through the accessor directly. If it was, the last puts should show 1
Thanks for your help!