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123 lines
3.5 KiB
Markdown
123 lines
3.5 KiB
Markdown
# 13.4 自定义包中的错误处理和 panicking
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这是所有自定义包实现者应该遵守的最佳实践：
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1）*在包内部，总是应该从 panic 中 recover*：不允许显式的超出包范围的 `panic()`
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2）*向包的调用者返回错误值（而不是 panic）。*
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在包内部，特别是在非导出函数中有很深层次的嵌套调用时，将 panic 转换成 `error` 来告诉调用方为何出错，是很实用的（且提高了代码可读性）。
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下面的代码则很好地阐述了这一点。我们有一个简单的 `parse` 包（示例 13.4）用来把输入的字符串解析为整数切片；这个包有自己特殊的 `ParseError`。
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当没有东西需要转换或者转换成整数失败时，这个包会 `panic()`（在函数 `fields2numbers()` 中）。但是可导出的 `Parse()` 函数会从 `panic()` 中 `recover()` 并用所有这些信息返回一个错误给调用者。为了演示这个过程，在 [panic_recover.go](examples/chapter_13/panic_recover.go) 中 调用了 `parse` 包（示例 13.5）；不可解析的字符串会导致错误并被打印出来。
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示例 13.4 [parse.go](examples/chapter_13/parse/parse.go)：
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```go
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// parse.go
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package parse
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import (
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"fmt"
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"strings"
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"strconv"
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)
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// A ParseError indicates an error in converting a word into an integer.
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type ParseError struct {
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Index int // The index into the space-separated list of words.
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Word string // The word that generated the parse error.
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Err error // The raw error that precipitated this error, if any.
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}
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// String returns a human-readable error message.
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func (e *ParseError) String() string {
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return fmt.Sprintf("pkg parse: error parsing %q as int", e.Word)
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}
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// Parse parses the space-separated words in in put as integers.
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func Parse(input string) (numbers []int, err error) {
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defer func() {
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if r := recover(); r != nil {
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var ok bool
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err, ok = r.(error)
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if !ok {
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err = fmt.Errorf("pkg: %v", r)
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}
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}
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}()
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fields := strings.Fields(input)
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numbers = fields2numbers(fields)
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return
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}
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func fields2numbers(fields []string) (numbers []int) {
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if len(fields) == 0 {
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panic("no words to parse")
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}
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for idx, field := range fields {
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num, err := strconv.Atoi(field)
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if err != nil {
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panic(&ParseError{idx, field, err})
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}
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numbers = append(numbers, num)
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}
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return
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}
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```
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示例 13.5 [panic_package.go](examples/chapter_13/panic_package.go)：
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```go
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// panic_package.go
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package main
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import (
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"fmt"
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"./parse/parse"
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)
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func main() {
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var examples = []string{
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"1 2 3 4 5",
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"100 50 25 12.5 6.25",
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"2 + 2 = 4",
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"1st class",
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"",
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}
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for _, ex := range examples {
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fmt.Printf("Parsing %q:\n ", ex)
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nums, err := parse.Parse(ex)
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if err != nil {
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fmt.Println(err) // here String() method from ParseError is used
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continue
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}
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fmt.Println(nums)
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}
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}
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```
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输出：
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```
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Parsing "1 2 3 4 5":
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[1 2 3 4 5]
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Parsing "100 50 25 12.5 6.25":
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pkg: pkg parse: error parsing "12.5" as int
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Parsing "2 + 2 = 4":
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pkg: pkg parse: error parsing "+" as int
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Parsing "1st class":
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pkg: pkg parse: error parsing "1st" as int
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Parsing "":
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pkg: no words to parse
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```
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## 链接
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- [目录](directory.md)
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- 上一节：[从 panic 中恢复 (recover)](13.3.md)
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- 下一节：[一种用闭包处理错误的模式](13.5.md)
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