Dynamic constant assignment

Question

class MyClass   def mymethod     MYCONSTANT    blah    end end   gives me the error      SyntaxError  dynamic constant assignment error   Why is this considered a dynamic constant  I m just assigning a string to it

User · Answer

You can t name a variable with capital letters or Ruby will asume its a constant and will want it to keep it s value constant  in which case changing it s value would be an error an  dynamic constant assignment error   With lower case should be fine  class MyClass   def mymethod     myconstant    blah    end end

User · Answer

Ruby doesn t like that you are assigning the constant inside of a method because it risks re-assignment  Several SO answers before me give the alternative of assigning it outside of a method--but in the class  which is a better place to assign it

User · Answer

Because constants in Ruby aren t meant to be changed  Ruby discourages you from assigning to them in parts of code which might get executed more than once  such as inside methods   Under normal circumstances  you should define the constant inside the class itself   class MyClass   MY CONSTANT    foo  end  MyClass  MY CONSTANT    gt   foo    If for some reason though you really do need to define a constant inside a method  perhaps for some type of metaprogramming   you can use const set   class MyClass   def my method     self class const set  MY CONSTANT   foo     end end  MyClass  MY CONSTANT    gt  NameError  uninitialized constant MyClass  MY CONSTANT  MyClass new my method MyClass  MY CONSTANT    gt   foo    Again though  const set isn t something you should really have to resort to under normal circumstances  If you re not sure whether you really want to be assigning to constants this way  you may want to consider one of the following alternatives   Class variables  Class variables behave like constants in many ways  They are properties on a class  and they are accessible in subclasses of the class they are defined on   The difference is that class variables are meant to be modifiable  and can therefore be assigned to inside methods with no issue   class MyClass   def self my class variable       my class variable   end   def my method       my class variable    foo    end end class SubClass  lt  MyClass end  MyClass my class variable    gt  NameError  uninitialized class variable   my class variable in MyClass SubClass my class variable    gt  NameError  uninitialized class variable   my class variable in MyClass  MyClass new my method MyClass my class variable    gt   foo  SubClass my class variable    gt   foo    Class attributes  Class attributes are a sort of  instance variable on a class   They behave a bit like class variables  except that their values are not shared with subclasses   class MyClass   class  lt  lt  self     attr accessor  my class attribute   end   def my method     self class my class attribute    blah    end end class SubClass  lt  MyClass end  MyClass my class attribute    gt  nil SubClass my class attribute    gt  nil  MyClass new my method MyClass my class attribute    gt   blah  SubClass my class attribute    gt  nil  SubClass new my method SubClass my class attribute    gt   blah    Instance variables  And just for completeness I should probably mention  if you need to assign a value which can only be determined after your class has been instantiated  there s a good chance you might actually be looking for a plain old instance variable   class MyClass   attr accessor  instance variable   def my method      instance variable    blah    end end  my object   MyClass new my object instance variable    gt  nil my object my method my object instance variable    gt   blah   MyClass new instance variable    gt  nil

User · Answer

Constants in ruby cannot be defined inside methods  See the notes at the bottom of this page  for example

User · Answer

In Ruby  any variable whose name starts with a capital letter is a constant and you can only assign to it once   Choose one of these alternatives   class MyClass   MYCONSTANT    blah     def mymethod     MYCONSTANT   end end  class MyClass   def mymethod     my constant    blah    end end

User · Answer

Your problem is that each time you run the method you are assigning a new value to the constant  This is not allowed  as it makes the constant non-constant  even though the contents of the string are the same  for the moment  anyhow   the actual string object itself is different each time the method is called  For example   def foo   p  bar  object id end  foo    gt  15779172 foo    gt  15779112   Perhaps if you explained your use case   why you want to change the value of a constant in a method   we could help you with a better implementation   Perhaps you d rather have an instance variable on the class   class MyClass   class  lt  lt  self     attr accessor  my constant   end   def my method     self class my constant    blah    end end  p MyClass my constant    gt  nil MyClass new my method  p MyClass my constant    gt   blah    If you really want to change the value of a constant in a method  and your constant is a String or an Array  you can  cheat  and use the  replace method to cause the object to take on a new value without actually changing the object   class MyClass   BAR    blah     def cheat new bar      BAR replace new bar   end end  p MyClass  BAR              gt   blah  MyClass new cheat  whee  p MyClass  BAR              gt   whee

User · Answer

Many thanks to Dorian and Phrogz for reminding me about the array  and hash  method  replace  which can  replace the contents of an array or hash    The notion that a CONSTANT s value can be changed  but with an annoying warning  is one of Ruby s few conceptual mis-steps -- these should either be fully immutable  or dump the constant idea altogether   From a coder s perspective  a constant is declarative and intentional  a signal to other that  this value is truly unchangeable once declared assigned    But sometimes an  obvious declaration  actually forecloses other  future useful opportunities   For example     There are legitimate use cases where a  constant s  value might really need to be changed  for example  re-loading ARGV from a REPL-like prompt-loop  then rerunning ARGV thru more  subsequent  OptionParser parse  calls -- voila   Gives  command line args  a whole new dynamic utility   The practical problem is either with the presumptive assumption that  ARGV must be a constant   or in optparse s own initialize method  which hard-codes the assignment of ARGV to the instance var  default argv for subsequent processing -- that array  ARGV  really should be a parameter  encouraging re-parse and re-use  where appropriate   Proper parameterization  with an appropriate default  say  ARGV  would avoid the need to ever change the  constant  ARGV   Just some 2  -worth of thoughts

[ruby] Dynamic constant assignment

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