特征测试

概念定义

遗留代码就是没有测试的代码，未经证实就去改它等于抡锤考古。特征测试把观察到的行为变成可执行事实，让重构有绊线。

工作机制

选定将要改动的接缝；输入代表性用例、捕获真实输出、据此写断言——包括令人意外的那些。被钉住的 bug 断言要显式标记，日后作为“有意的行为变更”单独修复。在网内重构；脚手架测试不再挣得维护成本就删除。

使用场景

对未测代码做任何结构性变更之前：抽取依赖、性能重写、框架迁移。替代方案“徒手重构加肉眼 diff”正是清理期间行为静默漂移的方式。

常见误区

特征测试验证的是稳定不是正确——断言观察到的行为（含缺陷）本来就是要点。把所有特征测试当永久回归测试保留，等于从救援行动继承一批维护欠账。过度钉化会把怪癖冻成契约；钉在实现内部而非可观察边界，会让每次合法重构都触网；快照式批量特征化会同时放大两个问题。

代码示例

import tempfile
from contextlib import contextmanager
from dataclasses import dataclass
from pathlib import Path
from typing import Iterator, Optional


class SandboxEscapeError(ValueError):
    """Raised when a requested path would leave the sandbox root."""


@dataclass(frozen=True)
class Sandbox:
    root: Path

    def resolve(self, *parts: str) -> Path:
        target = self.root.joinpath(*parts).resolve()
        if target != self.root and self.root not in target.parents:
            raise SandboxEscapeError("path escapes the sandbox: " + str(target))
        return target

    def write(self, relative_path: str, content: str) -> Path:
        target = self.resolve(relative_path)
        target.parent.mkdir(parents=True, exist_ok=True)
        target.write_text(content, encoding="utf-8")
        return target

    def read(self, relative_path: str) -> str:
        return self.resolve(relative_path).read_text(encoding="utf-8")

    def exists(self, relative_path: str) -> bool:
        return self.resolve(relative_path).exists()


@contextmanager
def sandbox(prefix: Optional[str] = None) -> Iterator[Sandbox]:
    """Yield a private temporary sandbox and always remove it afterwards."""
    with tempfile.TemporaryDirectory(prefix=prefix or "devfactory-") as raw:
        yield Sandbox(root=Path(raw).resolve())

package main

import (
	"fmt"
	"strings"
	"testing"
)

func normalizeEmail(raw string) (string, error) {
	email := strings.TrimSpace(strings.ToLower(raw))
	at := strings.IndexByte(email, '@')
	if at <= 0 || at == len(email)-1 || strings.Count(email, "@") != 1 {
		return "", fmt.Errorf("invalid email %q", raw)
	}
	return email, nil
}

func TestNormalizeEmail(t *testing.T) {
	t.Parallel()
	cases := []struct {
		name    string
		input   string
		want    string
		wantErr bool
	}{
		{"lowercases and trims", "  [email protected] ", "[email protected]", false},
		{"rejects missing at", "ada.example.com", "", true},
		{"rejects empty local part", "@example.com", "", true},
		{"rejects double at", "ada@@example.com", "", true},
	}

	for _, testCase := range cases {
		testCase := testCase
		t.Run(testCase.name, func(t *testing.T) {
			t.Parallel()
			got, err := normalizeEmail(testCase.input)
			if testCase.wantErr {
				if err == nil {
					t.Fatalf("expected error, got %q", got)
				}
				return
			}
			if err != nil {
				t.Fatalf("unexpected error: %v", err)
			}
			if got != testCase.want {
				t.Fatalf("got %q, want %q", got, testCase.want)
			}
		})
	}
}

func ExampleNormalizeEmail() {
	email, _ := normalizeEmail("  [email protected] ")
	fmt.Println(email)
	// Output: [email protected]
}