打印time.Time类型别名时出现意外输出

huangapple
go评论118阅读模式
英文:

Unexpected output when printing time.Time type alias

问题

我正在尝试为自定义类型编写一个解组函数。考虑下面的代码（playground）

  1. package main
  3. import (
  4. 	"encoding/json"
  5. 	"fmt"
  6. 	"strings"
  7. 	"time"
  8. )
  10. type Time time.Time
  12. func (st *Time) UnmarshalJSON(b []byte) error {
  13. 	// "2021-05-21T03:10:20.958450" -> "2021-05-21T03:10:20.958450Z"
  14. 	s := strings.Trim(string(b), "\"")
  15. 	t, err := time.Parse(time.RFC3339, fmt.Sprintf("%s%s", s, "Z"))
  16. 	if err != nil {
  17. 		return fmt.Errorf("parse time: %w", err)
  18. 	}
  19. 	*st = Time(t)
  20. 	return nil
  21. }
  23. type User struct {
  24. 	Name string
  25. 	TS   Time
  26. }
  28. const data = `{"id":3, "name":"Name", "ts":"2021-05-21T03:10:20.958450"}`
  30. func main() {
  31. 	user := new(User)
  32. 	json.Unmarshal([]byte(data), &user)
  33. 	fmt.Printf("%v\n", user)
  34. }

我成功地从time.Parse()中获得了一个有效的time.Time值，但是我不太明白为什么*st = Time(t)会给出如此奇怪的值？

目前上面的代码输出为：

  1. &{Name {958450000 63757163420 <nil>}}

但我希望输出更类似于：

  1. &{Name 2021-05-21 03:10:20.95845 +0000 UTC}

我在这里理解错了什么？

英文:

I am trying to write a unmarshal function for a custom type.
Consider the below code (playground)

  1. package main
  3. import (
  4. 	&quot;encoding/json&quot;
  5. 	&quot;fmt&quot;
  6. 	&quot;strings&quot;
  7. 	&quot;time&quot;
  8. )
  10. type Time time.Time
  12. func (st *Time) UnmarshalJSON(b []byte) error {
  13. 	// &quot;2021-05-21T03:10:20.958450&quot; -&gt; &quot;2021-05-21T03:10:20.958450Z&quot;
  14. 	s := strings.Trim(string(b), &quot;\&quot;&quot;)
  15. 	t, err := time.Parse(time.RFC3339, fmt.Sprintf(&quot;%s%s&quot;, s, &quot;Z&quot;))
  16. 	if err != nil {
  17. 		return fmt.Errorf(&quot;parse time: %w&quot;, err)
  18. 	}
  19. 	*st = Time(t)
  20. 	return nil
  21. }
  23. type User struct {
  24. 	Name string
  25. 	TS Time
  26. }
  28. const data = `{&quot;id&quot;:3, &quot;name&quot;:&quot;Name&quot;, &quot;ts&quot;:&quot;2021-05-21T03:10:20.958450&quot;}`
  30. func main() {
  31. 	user := new(User)
  32. 	json.Unmarshal([]byte(data), &amp;user)
  33. 	fmt.Printf(&quot;%v\n&quot;, user)
  34. }

I am successfully able to get a valid time.Time value from my time.Parse() but I am not quite grasping why *st = Time(t) gives such a wierd value?

Currently the above prints out:

  1. &amp;{Name {958450000 63757163420 &lt;nil&gt;}}

But I would like to be more similar to:

  1. &amp;{Name 2021-05-21 03:10:20.95845 +0000 UTC}

What am I missunderstanding here?

答案1

得分: 5

与time.Time相反，您的类型没有实现fmt.Stringer接口，因此fmt.Print*函数只能使用它们的默认格式化逻辑，这种情况下是打印包含在花括号中的底层time.Time值的字段。

为了获得所需的行为，请添加一个String方法，该方法委托给time.Time.String：

  1. func (t Time) String() string {
  2.     return time.Time(t).String()
  3. }

https://go.dev/play/p/5PwOwa49B5X

或者，将您的Time类型更改为嵌入time.Time。这将自动提升String方法以及所有其他方法（例如Marshal*方法）：

  1. type Time struct {
  2.     time.Time
  3. }
  5. func (st *Time) UnmarshalJSON(b []byte) error {
  6.     // ...
  7.     st.Time = t // 简单赋值，无需类型转换
  8.     return nil
  9. }

https://go.dev/play/p/0H5qyCO22gu

此外，您不应手动解析JSON。strings.Trim(string(b), "\"")不足以完全解码JSON字符串值。始终使用json.Unmarshal。您还可以使用time.ParseInLocation进行简化。

  1. func (st *Time) UnmarshalJSON(b []byte) error {
  2.     var s string
  3.     if err := json.Unmarshal(b, &s); err != nil {
  4.         return err
  5.     }
  7.     t, err := time.ParseInLocation("2006-01-02T15:04:05", s, time.UTC)
  8.     if err != nil {
  9.         return err
  10.     }
  12.     // ...
  13. }
英文:

Contrary to time.Time your type does not implement fmt.Stringer, so the fmt.Print* functions have no choice but to use their default formatting logic, which in this case is to print the fields of the underlying time.Time value enclosed in curly braces.

Add a String method which delegates to time.Time.String to your type to get the desired behavior:

  1. func (t Time) String() string {
  2.     return time.Time(t).String()
  3. }

https://go.dev/play/p/5PwOwa49B5X

Alternatively, change your Time type to embed time.Time. This will automatically promote the String method along with all other methods (such as the Marshal* methods):

  1. type Time struct {
  2. 	time.Time
  3. }
  5. func (st *Time) UnmarshalJSON(b []byte) error {
  6. 	// ...
  7. 	st.Time = t // simple assignment without type conversion
  8. 	return nil
  9. }

https://go.dev/play/p/0H5qyCO22gu

Also, you should never parse JSON by hand. strings.Trim(string(b), &quot;\&quot;&quot;) is not enough to fully decode a JSON string value. Always use json.Unmarshal. And you can simplify by using time.ParseInLocation.

  1. func (st *Time) UnmarshalJSON(b []byte) error {
  2.     var s string
  3.     if err := json.Unmarshal(b, &amp;s); err != nil {
  4.         return err
  5.     }
  7.     t, err := time.ParseInLocation(&quot;2006-01-02T15:04:05&quot;, s, time.UTC)
  8.     if err != nil {
  9.         return err
  10.     }
  12.     // ...
  13. }

huangapple
  • 本文由 发表于 2022年6月27日 23:58:14
  • 转载请务必保留本文链接：https://go.coder-hub.com/72775047.html
  • formatting
  • go
  • time
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