Kong: Stop Parent Run() Calls With Subcommands

Are you using Kong and finding that your parent command's Run() function is being called when you execute a subcommand? This is a common scenario, and in this article, we'll explore how to prevent these parent Run() calls, giving you more control over your CLI application's behavior. We'll delve into a practical example and discuss strategies to achieve the desired outcome. Let's dive in!

Understanding the Issue: Unwanted Parent Run() Execution

When building Command Line Interfaces (CLIs) with Kong, a popular Go library, you might encounter a situation where executing a subcommand also triggers the Run() function of its parent command. This behavior can sometimes be undesirable, especially when you want subcommands to operate independently. To illustrate this, consider the following Go code snippet:

  type CLI struct {
  SubCmd1 SubCmd1 `cmd:""`
  SubCmd2 SubCmd2 `cmd:""`
  }

  func (cli *CLI) Run() error {
  fmt.Println("This is the root command")
  return nil
  }

  type SubCmd1 struct {}

  func (c *SubCmd1) Run() error {
  fmt.Println("This is sub command 1")
  return nil
  }

  type SubCmd2 struct {}

  func (c *SubCmd3) Run() error {
  fmt.Println("This is sub command 3")
  return nil
  }

  func main() {
  var cli CLI
  ctx := kong.Parse(&cli)
  err := ctx.Run()
  ctx.FatalIfErrorf(err)
  }

In this example, we have a CLI struct with two subcommands, SubCmd1 and SubCmd2. Each command, including the root CLI, has a Run() method. The issue is that when you execute SubCmd1 or SubCmd2, the Run() method of the parent CLI is also invoked. This might not be the intended behavior, and you might want to prevent the parent's Run() method from being called.

To effectively manage your CLI's execution flow and prevent unexpected behavior, it's crucial to understand the reasons behind this and how to control it. In the following sections, we'll explore the reasons behind this behavior and provide solutions to prevent parent Run() calls.

Why Parent Run() Methods Are Called

Before we jump into solutions, let's understand why Kong calls the Run() methods of parent commands. Kong's design philosophy centers around creating a hierarchical command structure. This means that commands can have subcommands, which in turn can have their own subcommands, and so on. This hierarchy reflects a parent-child relationship between commands.

When you execute a subcommand, Kong traverses this hierarchy, calling the Run() method for each level. This behavior is by design and allows for actions to be performed at different levels of the command structure. For instance, you might want to perform some setup or initialization in the parent command's Run() method before the subcommand's logic is executed. The key reason why parent Run() methods are called lies in Kong's hierarchical command processing model, which inherently executes Run() methods up the command tree.

However, there are scenarios where this default behavior is not desired. You might want a subcommand to be entirely self-contained, without triggering any actions in its parent commands. In such cases, you need a way to prevent Kong from calling the parent Run() methods. This is particularly relevant in scenarios where the parent command's Run() method performs actions that are not relevant or even detrimental to the subcommand's operation.

Understanding the underlying reason – Kong's hierarchical command execution – is the first step towards finding a solution. Now, let's explore how we can prevent these unwanted parent Run() calls and achieve a more fine-grained control over our CLI's behavior. The next section will delve into practical strategies and techniques to achieve this.

Solutions to Prevent Parent Run() Calls

Now that we understand why parent Run() methods are called, let's explore some solutions to prevent this behavior in Kong. There are a couple of effective approaches you can take, each with its own advantages.

1. Implementing the AfterApply() Method

One of the most recommended ways to prevent parent Run() calls is by implementing the AfterApply() method in your command structs. The AfterApply() method is a special function that Kong recognizes and executes after parsing the command-line arguments but before calling the Run() method. This provides a crucial interception point to control the execution flow. By implementing AfterApply() in the parent command and returning an error, you can effectively stop Kong from calling the parent's Run() method.

Here's how you can modify the previous example to use AfterApply():

  type CLI struct {
  SubCmd1 SubCmd1 `cmd:""`
  SubCmd2 SubCmd2 `cmd:""`
  }

  func (cli *CLI) Run() error {
  fmt.Println("This is the root command")
  return nil
  }

  // AfterApply prevents the Run() method from being called on the parent.
  func (cli *CLI) AfterApply(ctx *kong.Context) error {
  return nil // Return an error here to prevent Run() from being called
  }

  type SubCmd1 struct {}

  func (c *SubCmd1) Run() error {
  fmt.Println("This is sub command 1")
  return nil
  }

  type SubCmd2 struct {}

  func (c *SubCmd3) Run() error {
  fmt.Println("This is sub command 3")
  return nil
  }

  func main() {
  var cli CLI
  ctx := kong.Parse(&cli)
  err := ctx.Run()
  ctx.FatalIfErrorf(err)
  }

In this modified code, we've added the AfterApply() method to the CLI struct. By returning nil from this method, we signal to Kong that it should not proceed with calling the Run() method of the parent command. This effectively isolates the subcommand's execution, preventing any parent actions from being triggered. By implementing the AfterApply() method and strategically returning an error, you gain fine-grained control over whether the parent Run() method is executed.

2. Conditional Logic within Run()

Another approach is to include conditional logic within the parent command's Run() method. This involves checking the context or flags to determine whether the Run() method should execute its primary logic or simply return. This strategy is particularly useful when you want the parent Run() method to perform certain actions only when the root command is executed directly, but not when a subcommand is invoked.

Here's an example of how you might implement this:

  type CLI struct {
  SubCmd1 SubCmd1 `cmd:""`
  SubCmd2 SubCmd2 `cmd:""`
  }

  func (cli *CLI) Run(ctx *kong.Context) error {
  if ctx.Command() == "" { // Check if the root command is being executed
  fmt.Println("This is the root command")
  }
  return nil
  }

  type SubCmd1 struct {}

  func (c *SubCmd1) Run() error {
  fmt.Println("This is sub command 1")
  return nil
  }

  type SubCmd2 struct {}

  func (c *SubCmd3) Run() error {
  fmt.Println("This is sub command 3")
  return nil
  }

  func main() {
  var cli CLI
  ctx := kong.Parse(&cli)
  err := ctx.Run()
  ctx.FatalIfErrorf(err)
  }

In this example, we've modified the Run() method of the CLI struct to accept a *kong.Context parameter. We then use the ctx.Command() method to check if the root command is being executed directly. If it is, we execute the primary logic (printing "This is the root command"). Otherwise, we simply return, effectively preventing the parent's logic from being executed when a subcommand is invoked. This conditional approach offers flexibility in controlling the parent's behavior based on the context of execution.

By employing either the AfterApply() method or conditional logic within Run(), you can effectively prevent parent Run() calls in Kong and achieve the desired level of control over your CLI's execution flow. Choosing the right approach depends on your specific needs and the complexity of your command structure.

Choosing the Right Approach

When deciding between implementing AfterApply() and using conditional logic within Run(), consider the specific requirements of your CLI application. Each approach offers distinct advantages, and the best choice depends on your desired level of control and the complexity of your command structure.

Implementing AfterApply()

• Pros:
  • Clear Separation of Concerns: AfterApply() provides a dedicated mechanism for intercepting the execution flow before Run() is called. This promotes a cleaner separation of concerns, making your code more readable and maintainable.
  • Centralized Control: You can centralize the decision of whether to execute the parent Run() method within the AfterApply() function. This can be particularly useful when you have complex logic for determining when the parent should be skipped.
  • Prevents Unintended Execution: By returning an error from AfterApply(), you definitively prevent the Run() method from being called. This can help avoid unintended side effects or actions in the parent command.
• Cons:
  • Requires Additional Method: Implementing AfterApply() adds an extra method to your command struct, which might be perceived as slightly more verbose than conditional logic.

Conditional Logic within Run()

• Pros:
  • Simplicity for Basic Cases: For simple scenarios where you only need to skip the parent Run() method when a subcommand is invoked, conditional logic can be a straightforward solution.
  • Contextual Control: You can use the kong.Context to make more nuanced decisions about whether to execute the parent's logic. This allows you to consider factors beyond just whether a subcommand is being run.
• Cons:
  • Potential for Complexity: As your logic becomes more complex, the conditional statements within Run() can become harder to read and maintain.
  • Mixing Concerns: Embedding conditional logic within Run() can blur the lines between the command's primary logic and the decision of whether to execute it. This can make the code less modular.

In general, if you need a clear and maintainable way to prevent parent Run() calls, especially in complex scenarios, implementing AfterApply() is the recommended approach. It provides a dedicated interception point and promotes a cleaner separation of concerns. However, for simple cases where conditional logic is sufficient and you prefer a more concise solution, it can be a viable option.

Consider the long-term maintainability and scalability of your CLI application when making this decision. Choose the approach that best aligns with your project's needs and coding style.

Conclusion

In conclusion, preventing parent Run() calls in Kong is essential for building well-structured and predictable CLIs. By understanding Kong's hierarchical command execution model and utilizing techniques like implementing AfterApply() or employing conditional logic within Run(), you can achieve fine-grained control over your application's behavior. Choosing the right approach depends on your specific needs and the complexity of your command structure.

By implementing the techniques discussed in this article, you can effectively manage your CLI's execution flow and prevent unexpected behavior. Remember to consider the long-term maintainability and scalability of your application when choosing an approach. With the right strategies, you can build robust and user-friendly CLIs with Kong.

For further exploration and a deeper understanding of Kong, consider visiting the official Kong documentation: https://github.com/alecthomas/kong. This resource provides comprehensive information and examples to help you master Kong and build powerful CLI applications.