import bpy

class WM_OT_my_props_dialog(bpy.types.Operator):
    bl_idname = "wm.my_props_dialog"
    bl_label = "Props Dialog"

    bool_prop = bpy.props.BoolProperty()
    str_prop = bpy.props.StringProperty()

    def check(self, context):
        return True

    def draw(self, context):
        layout = self.layout

        layout.prop(self, "bool_prop", toggle=True)
        if self.bool_prop:
            layout.prop(self, "str_prop")

    def execute(self, context):
        return {'FINISHED'}

    def invoke(self, context, event):
        return context.window_manager.invoke_props_dialog(self)


if not hasattr(bpy.types, "WM_OT_my_props_dialog"):
    bpy.utils.register_class(WM_OT_my_props_dialog)

bpy.ops.wm.my_props_dialog('INVOKE_DEFAULT')