WIP: make printing output easier (approach 1)

cli/command/context/list.go

@@ -1,7 +1,7 @@
 package context
 
 import (
-	"fmt"
+	"io"
 	"os"
 	"sort"
 
@@ -29,7 +29,7 @@ func newListCommand(dockerCli command.Cli) *cobra.Command {
 		Short:   "List contexts",
 		Args:    cli.NoArgs,
 		RunE: func(cmd *cobra.Command, args []string) error {
-			return runList(dockerCli, opts)
+			return runList(cmd, dockerCli, opts)
 		},
 		ValidArgsFunction: completion.NoComplete,
 	}
@@ -40,7 +40,7 @@ func newListCommand(dockerCli command.Cli) *cobra.Command {
 	return cmd
 }
 
-func runList(dockerCli command.Cli, opts *listOptions) error {
+func runList(cmd *cobra.Command, dockerCli command.Cli, opts *listOptions) error {
 	if opts.format == "" {
 		opts.format = formatter.TableFormatKey
 	}
@@ -101,19 +101,23 @@ func runList(dockerCli command.Cli, opts *listOptions) error {
 	sort.Slice(contexts, func(i, j int) bool {
 		return sortorder.NaturalLess(contexts[i].Name, contexts[j].Name)
 	})
-	if err := format(dockerCli, opts, contexts); err != nil {
+	if err := format(cmd.OutOrStdout(), opts, contexts); err != nil {
 		return err
 	}
 	if os.Getenv(client.EnvOverrideHost) != "" {
-		_, _ = fmt.Fprintf(dockerCli.Err(), "Warning: %[1]s environment variable overrides the active context. "+
+		cmd.PrintErrf("Warning: %[1]s environment variable overrides the active context. "+
 			"To use a context, either set the global --context flag, or unset %[1]s environment variable.\n", client.EnvOverrideHost)
+
+		// Alternatives:
+		// - dockerCli.PrintErrf(.....)
+		// - dockerCli.Err().Printf(.....)
 	}
 	return nil
 }
 
-func format(dockerCli command.Cli, opts *listOptions, contexts []*formatter.ClientContext) error {
+func format(output io.Writer, opts *listOptions, contexts []*formatter.ClientContext) error {
 	contextCtx := formatter.Context{
-		Output: dockerCli.Out(),
+		Output: output,
 		Format: formatter.NewClientContextFormat(opts.format, opts.quiet),
 	}
 	return formatter.ClientContextWrite(contextCtx, contexts)

cli/command/context/list_test.go

@@ -4,6 +4,7 @@ import (
 	"testing"
 
 	"github.com/docker/cli/cli/command"
+	"github.com/spf13/cobra"
 	"gotest.tools/v3/assert"
 	"gotest.tools/v3/golden"
 )
@@ -26,7 +27,10 @@ func TestList(t *testing.T) {
 	createTestContext(t, cli, "unset")
 	cli.SetCurrentContext("current")
 	cli.OutBuffer().Reset()
-	assert.NilError(t, runList(cli, &listOptions{}))
+	cmd := &cobra.Command{}
+	cmd.SetOut(cli.Out())
+	cmd.SetErr(cli.Err())
+	assert.NilError(t, runList(cmd, cli, &listOptions{}))
 	golden.Assert(t, cli.OutBuffer().String(), "list.golden")
 }
 
@@ -36,7 +40,7 @@ func TestListQuiet(t *testing.T) {
 	createTestContext(t, cli, "other")
 	cli.SetCurrentContext("current")
 	cli.OutBuffer().Reset()
-	assert.NilError(t, runList(cli, &listOptions{quiet: true}))
+	assert.NilError(t, runList(&cobra.Command{}, cli, &listOptions{quiet: true}))
 	golden.Assert(t, cli.OutBuffer().String(), "quiet-list.golden")
 }
 

cmd/docker/docker.go

@@ -62,7 +62,6 @@ func newDockerCommand(dockerCli *command.DockerCli) *cli.TopLevelCommand {
 	setupHelpCommand(dockerCli, cmd, helpCmd)
 	setHelpFunc(dockerCli, cmd)
 
-	cmd.SetOut(dockerCli.Out())
 	commands.AddCommands(cmd, dockerCli)
 
 	cli.DisableFlagsInUseLine(cmd)