即使存在值，Go Map也会返回nil值

Question

Assume answers below is a map[string]interface{} unmarshalled from a JSON string.

if baths, ok := answers["bathrooms"]; ok {
  bathsFloat := baths.(float64)
}

Somehow I'm getting a panic with interface conversion: interface {} is nil, not float64. How can this be possible when the presence check is true?

Answer 1

ok only tells if the key is in the map, whether the value associated with it is nil (or in general the zero value of he value type) or or not is another thing.

See this example:

answers := map[string]interface{}{
    "isnil":  nil,
    "notnil": 1.15,
}

if v, ok := answers["isnil"]; ok {
    fmt.Printf("Value: %v, type: %T
", v, v)
}
if v, ok := answers["notnil"]; ok {
    fmt.Printf("Value: %v, type: %T
", v, v)
}

Output (try it on the Go Playground):

Value: <nil>, type: <nil>
Value: 1.15, type: float64

If answers is the result of a JSON unmarshaling, then the value associated with a key in it will be nil if the value in the source is JSON null.

See this example:

var m map[string]interface{}

err := json.Unmarshal([]byte(`{"isnil":null,"notnil":1.15}`), &m)
if err != nil {
    panic(err)
}
fmt.Println(m)

Output (try it on the Go Playground):

map[notnil:1.15 isnil:<nil>]