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 parameter

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]]