github.com/stretchr/testify v1.3.0: assert.Len

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: verify github.com/stretchr/testify/assert.Len in pkg:golang/github.com/stretchr/testify@v1.3.0
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/github.com/stretchr/testify@v1.3.0
Demonstrate these symbols/APIs:
  - github.com/stretchr/testify/assert.Len
Constraints:
  - executionContext: node
Required runtime conditions:
  - ecosystem: npm
  - language: javascript
  - moduleSystem: cjs
  - packageManager: npm@10.9.8
  - runtime: node@22.23.2

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

{"case":{"caseId":"case:sha256:1b7cba7f7015a513dddee1ec78753b9c07479b4e17482f74da3f3817498fc6da","contract":["assert.Len returns true and records no error when slice, map, string, or array matches expected length","assert.Len returns true and records no error for empty slice, string, or map when expected length is 0","assert.Len returns true and records no error for channel matching number of buffered elements","assert.Len returns false and records an error when length does not match expected length","assert.Len returns false and records an error when object type does not support builtin len or is nil"],"goal":"verify github.com/stretchr/testify/assert.Len in pkg:golang/github.com/stretchr/testify@v1.3.0","kind":"HOW","packages":["pkg:golang/github.com/stretchr/testify@v1.3.0"],"schemaVersion":1,"symbols":["github.com/stretchr/testify/assert.Len"]},"contractCommand":["go","run","./test"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"golang","language":"go","libc":"glibc","libcVersion":"2.39","os":"linux","osVersionBucket":"24","packageManager":"go","packageManagerVersion":"1.26.6","runtime":"go","runtimeVersion":"1.26.6","schemaVersion":1},"license":"MIT-0","packages":["pkg:golang/github.com/stretchr/testify@v1.3.0"],"schemaVersion":1,"subject":"pkg:golang/github.com/stretchr/testify@v1.3.0","symbols":["github.com/stretchr/testify/assert.Len"],"verifierAdapter":"golang@1"}

module sample

go 1.26.6

require github.com/stretchr/testify v1.3.0

require (
	github.com/davecgh/go-spew v1.1.0 // indirect
	github.com/pmezard/go-difflib v1.0.0 // indirect
)

github.com/davecgh/go-spew v1.1.0 h1:ZDRjVQ15GmhC3fiQ8ni8+OwkZQO4DARzQgrnXU1Liz8=
github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
github.com/stretchr/testify v1.3.0 h1:TivCn/peBQ7UY8ooIcPgZFpTNSz0Q2U6UrFlUfqbe0Q=
github.com/stretchr/testify v1.3.0/go.mod h1:M5WIy9Dh21IEIfnGCwXGc5bZfKNJtfHm1UVUgZn+9EI=

package main

import (
	"fmt"

	"github.com/stretchr/testify/assert"
)

// Reporter implements assert.TestingT to record assertion outcomes.
type Reporter struct {
	errors []string
	failed bool
}

func (r *Reporter) Errorf(format string, args ...interface{}) {
	r.failed = true
	r.errors = append(r.errors, fmt.Sprintf(format, args...))
}

func main() {
	reporter := &Reporter{}

	// Example 1: Verifying slice length
	items := []string{"alpha", "beta", "gamma"}
	if assert.Len(reporter, items, 3) {
		fmt.Println("items slice confirmed to have length 3.")
	}

	// Example 2: Verifying map length
	scores := map[string]int{"alice": 95, "bob": 88}
	if assert.Len(reporter, scores, 2) {
		fmt.Println("scores map confirmed to have length 2.")
	}

	// Example 3: Verifying string length
	greeting := "hello world"
	if assert.Len(reporter, greeting, 11) {
		fmt.Println("greeting string confirmed to have length 11.")
	}

	// Example 4: Verifying buffered channel length
	queue := make(chan int, 5)
	queue <- 1
	queue <- 2
	if assert.Len(reporter, queue, 2) {
		fmt.Println("queue channel confirmed to have 2 buffered items.")
	}

	if !reporter.failed {
		fmt.Println("All assert.Len demonstrations succeeded.")
	}
}

{
  "schemaVersion": 1,
  "goal": "verify github.com/stretchr/testify/assert.Len in pkg:golang/github.com/stretchr/testify@v1.3.0",
  "kind": "HOW",
  "packages": [
    "pkg:golang/github.com/stretchr/testify@v1.3.0"
  ],
  "symbols": [
    "github.com/stretchr/testify/assert.Len"
  ],
  "constraints": {
    "executionContext": "node"
  },
  "runtimeConditions": {
    "ecosystem": "npm",
    "language": "javascript",
    "moduleSystem": "cjs",
    "packageManager": "npm@10.9.8",
    "runtime": "node@22.23.2"
  }
}

package main

import (
	"fmt"
	"os"

	"github.com/stretchr/testify/assert"
)

type contractTestingT struct {
	errors []string
	failed bool
}

func (t *contractTestingT) Errorf(format string, args ...interface{}) {
	t.failed = true
	t.errors = append(t.errors, fmt.Sprintf(format, args...))
}

func main() {
	// Assertion 1: assert.Len returns true and records no error when slice, map, string, or array matches expected length
	{
		t := &contractTestingT{}
		sliceVal := []int{10, 20, 30}
		mapVal := map[string]int{"first": 1, "second": 2}
		strVal := "test"
		arrayVal := [3]int{1, 2, 3}

		if !assert.Len(t, sliceVal, 3) || t.failed || len(t.errors) != 0 {
			fmt.Fprintf(os.Stderr, "assertion 1 failed: slice of 3 items should have length 3\n")
			os.Exit(1)
		}
		if !assert.Len(t, mapVal, 2) || t.failed || len(t.errors) != 0 {
			fmt.Fprintf(os.Stderr, "assertion 1 failed: map of 2 items should have length 2\n")
			os.Exit(1)
		}
		if !assert.Len(t, strVal, 4) || t.failed || len(t.errors) != 0 {
			fmt.Fprintf(os.Stderr, "assertion 1 failed: string of 4 bytes should have length 4\n")
			os.Exit(1)
		}
		if !assert.Len(t, arrayVal, 3) || t.failed || len(t.errors) != 0 {
			fmt.Fprintf(os.Stderr, "assertion 1 failed: array of 3 items should have length 3\n")
			os.Exit(1)
		}
	}

	// Assertion 2: assert.Len returns true and records no error for empty slice, string, or map when expected length is 0
	{
		t := &contractTestingT{}
		emptySlice := []string{}
		var nilSlice []int
		emptyStr := ""
		emptyMap := map[string]int{}

		if !assert.Len(t, emptySlice, 0) || !assert.Len(t, nilSlice, 0) || t.failed || len(t.errors) != 0 {
			fmt.Fprintf(os.Stderr, "assertion 2 failed: empty or nil slice should have length 0\n")
			os.Exit(1)
		}
		if !assert.Len(t, emptyStr, 0) || t.failed || len(t.errors) != 0 {
			fmt.Fprintf(os.Stderr, "assertion 2 failed: empty string should have length 0\n")
			os.Exit(1)
		}
		if !assert.Len(t, emptyMap, 0) || t.failed || len(t.errors) != 0 {
			fmt.Fprintf(os.Stderr, "assertion 2 failed: empty map should have length 0\n")
			os.Exit(1)
		}
	}

	// Assertion 3: assert.Len returns true and records no error for channel matching number of buffered elements
	{
		t := &contractTestingT{}
		ch := make(chan int, 4)
		if !assert.Len(t, ch, 0) || t.failed || len(t.errors) != 0 {
			fmt.Fprintf(os.Stderr, "assertion 3 failed: empty channel should have length 0\n")
			os.Exit(1)
		}
		ch <- 100
		ch <- 200
		if !assert.Len(t, ch, 2) || t.failed || len(t.errors) != 0 {
			fmt.Fprintf(os.Stderr, "assertion 3 failed: channel with 2 elements should have length 2\n")
			os.Exit(1)
		}
	}

	// Assertion 4: assert.Len returns false and records an error when length does not match expected length
	{
		tSlice := &contractTestingT{}
		sliceVal := []int{1, 2, 3}
		if assert.Len(tSlice, sliceVal, 5) || !tSlice.failed || len(tSlice.errors) == 0 {
			fmt.Fprintf(os.Stderr, "assertion 4 failed: slice length mismatch should return false and record error\n")
			os.Exit(1)
		}

		tStr := &contractTestingT{}
		strVal := "hello"
		if assert.Len(tStr, strVal, 3) || !tStr.failed || len(tStr.errors) == 0 {
			fmt.Fprintf(os.Stderr, "assertion 4 failed: string length mismatch should return false and record error\n")
			os.Exit(1)
		}
	}

	// Assertion 5: assert.Len returns false and records an error when object type does not support builtin len or is nil
	{
		tInt := &contractTestingT{}
		if assert.Len(tInt, 42, 0) || !tInt.failed || len(tInt.errors) == 0 {
			fmt.Fprintf(os.Stderr, "assertion 5 failed: non-len type int should return false and record error\n")
			os.Exit(1)
		}

		tStruct := &contractTestingT{}
		type dummy struct{ val int }
		if assert.Len(tStruct, dummy{val: 1}, 1) || !tStruct.failed || len(tStruct.errors) == 0 {
			fmt.Fprintf(os.Stderr, "assertion 5 failed: struct should return false and record error\n")
			os.Exit(1)
		}

		tNil := &contractTestingT{}
		if assert.Len(tNil, nil, 0) || !tNil.failed || len(tNil.errors) == 0 {
			fmt.Fprintf(os.Stderr, "assertion 5 failed: untyped nil should return false and record error\n")
			os.Exit(1)
		}
	}

	fmt.Println("All contract assertions passed successfully.")
}