Using types for variables in for loops?

I don't see any obvious way to set the type for variable in a for loop.

For example, if I have an array of type Dictionary, the following is not legal:

# not valid Godot syntax
for value:Dictionary in my_array:
    # do_stuff expects a parameter of type Dictionary, so it's an error
   do_stuff(dictionary_value)

I can cast the type on the variable when I use it:

# iterate through my_array
for value in my_array:
    # do_stuff expects a Dictionary parameter, so perform explicit cast
   do_stuff(Dictionary(value))

Or I can put it in another variable that declares the type:

# iterate through my_array, putting value in proxy
for value in my_array:
    # put in a variable of type Dictionary for convenience
    var dictionary_value:Dictionary = value_tmp
    # can now call do_stuff without issue, because type is Dictionary
   do_stuff(dictionary_value)

How expensive is theDictionary() operation? If I do it multiple times, is it better to put it in an explicitly declared variable?