Is there a way to know how many parameters are needed for a method?

Question

Using irb, we can list methods for particular object by doing following:

"Name".methods 

But if I want to know how many parameters are needed for a particular method, how can I achieve this? I mean is there any way (by hitting some command on irb), we can get number of parameters for particular method (instead of referring to docs)?

.methods returns only method names, not list of parameters for method.

Answer 1

You can use the method Method#arity:

"string".method(:strip).arity # => 0 

From the Ruby documentation:

Returns an indication of the number of arguments accepted by a method. Returns a nonnegative integer for methods that take a fixed number of arguments. For Ruby methods that take a variable number of arguments, returns -n-1, where n is the number of required arguments. For methods written in C, returns -1 if the call takes a variable number of arguments.

So, for example:

# Variable number of arguments, one is required def foo(a, *b); end method(:foo).arity # => -2  # Variable number of arguments, none required def bar(*a); end method(:bar).arity # => -1  # Accepts no argument, implemented in C "0".method(:to_f).arity # => 0  # Variable number of arguments (0 or 1), implemented in C "0".method(:to_i).arity # => -1 

Update I've just discovered the exitence of Method#parameters, it could be quite useful:

def foo(a, *b); end method(:foo).parameters # => [[:req, :a], [:rest, :b]]  

Answer 2

You can use arity

Returns an indication of the number of arguments accepted by a method. Returns a nonnegative integer for methods that take a fixed number of arguments. For Ruby methods that take a variable number of arguments, returns -n-1, where n is the number of required arguments. For methods written in C, returns -1 if the call takes a variable number of arguments.

Example from ruby-doc

class C   def one;    end   def two(a); end   def three(*a);  end   def four(a, b); end   def five(a, b, *c);    end   def six(a, b, *c, &d); end end  c = C.new c.method(:one).arity     #=> 0  c.method(:two).arity     #=> 1 c.method(:three).arity   #=> -1 c.method(:four).arity    #=> 2 c.method(:five).arity    #=> -3 c.method(:six).arity     #=> -3