go-printers

A Golang module built on top of the spf13/pflag library to assist with abstracting output formatting.
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About the project

go-printers provides a set of helpers extending the spf13/pflag library to abstract away print formatting, making it easy to separate output formatting and support printing as text, table, json, or yaml.

Built with

• Golang 1.18

• spf13/pflag - "a drop-in replacement for Go's flag package, implementing POSIX/GNU-style --flags"

Getting started

To get a local copy up and running follow these simple steps.

Installation

go get https://github.com/davidalpert/go-printers

Usage

1. Import the package

   import (
   	"github.com/davidalpert/go-printers/v1"
   )
   

2. Define an Options struct and a factory function to set defaults

   type MyCmdOptions struct {
     *printers.PrinterOptions
     printers.IOStreams
   }
   
   func NewMyCmdOptions(ioStreams printers.IOStreams) *MyCmdOptions {
     return &MyCmdOptions{
       IOStreams:      ioStreams,
       PrinterOptions: printers.NewPrinterOptions(),
     }
   }
   

3. Create an instance of streams, probably in your startup code

   ioStreams := printers.DefaultOSStreams() 
   

   In a unit test you might create test streams which exposes each stream as a *bytes.Buffer:

   testStreams, testOutBuf, testErrBuf, testInBuf := printers.NewTestIOStreams()
   

   See the #NewTestIOStreams test scenario for an example of using these buffers in unit tests.

4. Configure flags inside a cobra.Command factory:

   func NewCmdMyCmd(ioStreams printers.IOStreams) *cobra.Command {
     o := NewMyCmdOptions(ioStreams)
     var cmd = &cobra.Command{
       Use:     "mycmd",
       Args:    cobra.NoArgs,
       RunE: func(cmd *cobra.Command, args []string) error {
        if err := o.Complete(cmd, args); err != nil {
         return err
        }
        if err := o.Validate(); err != nil {
         return err
        }
        return o.Run()
       },
      }
   
      o.PrinterOptions.AddPrinterFlags(cmd.Flags())
   
      return cmd
     }
   

5. Works well with the Complete/Validate/Run pattern:

   // Complete the options
   func (o *MyCmdOptions) Complete(cmd *cobra.Command, args []string) error {
     return nil
   }
   
   // Validate the options
   func (o *MyCmdOptions) Validate() error {
     return o.PrinterOptions.Validate()
   }
   

6. Send any interface{} or struct to the printer:

   type Car struct {
     Make    string `json:"make"`
     Model   string `json:"model"`
     Mileage int    `json:"mileage"`
   }
   
   // String implements Stringer
   func (c *Car) String() string {
     return fmt.Sprintf("%s %s [%d miles]", c.Make, c.Model, c.Mileage)
   }
   
   func (o *MyCmdOptions) Run() error {
     newCar := Car{
       make:    "Ford",
       model:   "Taurus",
       mileage: 200000,
     }
   
     return o.IOStreams.WriteOutput(newCar, o.PrinterOptions)
   }
   

This adds an -o/--output flag to your command:

Flags:
  -h, --help            help for mycmd
  -o, --output string   output format: one of json|text|yaml. (default "text")

which can then print the same output in different formats:

• with -o text and a type which implements Stringer:

  Ford Taurus [200000 miles]
  

• with -o json:

  {
    "make": "Ford",
    "model": "Taurus",
    "mileage": 200000
  }
  

• with -o yaml:

  ---
  make: Ford
  model: Taurus
  mileage: 200000
  

Roadmap

See open issues for a list of known issues and up-for-grabs tasks.

Contributing

See the CONTRIBUTING guide for local development setup and contribution guidelines.

License

Distributed under the GPU v3 License. See LICENSE for more information.

Contact

David Alpert - @davidalpert

Project Link: https://github.com/davidalpert/go-printers