In Python 2.6:

    def acc(x):
        foo = x
        def closures_are_broken():
            foo += 1
            return foo
        return closures_are_broken
    
    
    def acc2(x):
        foo = [x]
        def closure_with_workaround():
            foo[0] += 1
            return foo[0]
        return closure_with_workaround

The former is an error ("UnboundLocalError: local variable 'foo' referenced before assignment"), the latter is a workaround by manually boxing the variable in a list. I'm not sure why that works, but it's rather ugly.

The same, in Lua:

    function acc(x)
       local foo = x
       return function()
                 foo = foo + 1
                 return foo
              end
    end

Oh, yes. You are mutating variables. You shouldn't do that anyway in a functional setting.

Python's syntax forces the language to guess whether you want to make a new variable or use the variable of the same name from the outer scope. The heuristic they have, says that if you assign something to the variable, you get a new one by default. You can declare

  global x

before to assign to the variable in the outerscope.