```Python
with open("./somefile.txt") as f:
    contents = f.read()
    print(contents)
```

Por baixo dos panos, o código `open("./somefile.txt")` cria um objeto que é chamado de "Gerenciador de Contexto".

```Python
with open("./somefile.txt") as f:
    contents = f.read()
    print(contents)
```

Underneath, the `open("./somefile.txt")` creates an object that is called a "Context Manager".

When the `with` block finishes, it makes sure to close the file, even if there were exceptions.

		},
	}

	for testID, tt := range tests {
		rng := rand.New(rand.NewSource(0))
		// Run for a number of times, shuffling the input to ensure that output is consistent.
		for i := range 10 {
			t.Run(fmt.Sprintf("test-%d-%s-run-%d", testID, tt.name, i), func(t *testing.T) {
				if i > 0 {
					rng.Shuffle(len(tt.m), func(i, j int) {
						tt.m[i], tt.m[j] = tt.m[j], tt.m[i]
					})
				}

			case int, int8, int16, int32, int64, uint, uint8, uint16, uint32, uint64:
				field.ReflectValueOf(ctx, value).SetString(utils.ToString(data))
			case float64, float32:
				field.ReflectValueOf(ctx, value).SetString(fmt.Sprintf("%."+strconv.Itoa(field.Precision)+"f", data))
			default:
				return fallbackSetter(ctx, value, v, field.Set)
			}
			return err
		}
	default:
		fieldValue := reflect.New(field.FieldType)

				if *lp.Name == "api" && toLowerAPILabels {
					labels[*lp.Name] = strings.ToLower(*lp.Value)
				} else {
					labels[*lp.Name] = *lp.Value
				}
			}
			labels["le"] = fmt.Sprintf("%.3f", *b.UpperBound)
			metric := MetricV2{
				Description:    desc,
				VariableLabels: labels,
				Value:          float64(b.GetCumulativeCount()),
			}
			if limitBuckets && labels["bucket"] != "" {

////

### 在 Python 中读取环境变量

您还可以在 Python 之外的地方（例如终端中或使用任何其他方法）创建环境变量，然后在 Python 中读取它们。

例如，您可以有一个名为 `main.py` 的文件，其中包含以下内容：

```Python hl_lines="3"
import os

name = os.getenv("MY_NAME", "World")
print(f"Hello {name} from Python")
```

/// tip

<a href="https://docs.python.org/3.8/library/os.html#os.getenv" class="external-link" target="_blank">`os.getenv()`</a> 的第二个参数是要返回的默认值。

		unmarshaled:                true,
	}
	for n := range 11 {
		f := openTestFile(b, "nyc-taxi-data-100k.csv")
		want := 309
		for i := 0; i < n; i++ {
			f = append(f, f...)
			want *= 2
		}
		b.Run(fmt.Sprint(len(f)/(1<<10), "K"), func(b *testing.B) {
			b.ReportAllocs()
			b.SetBytes(int64(len(f)))
			b.ResetTimer()
			var record sql.Record
			for b.Loop() {

```Python
with open("./somefile.txt") as f:
    contents = f.read()
    print(contents)
```

在底层，`open("./somefile.txt")`创建了一个被称为"上下文管理器"的对象。

当 `with` 代码块结束时，它会确保关闭文件，即使发生了异常也是如此。

当你使用 `yield` 创建一个依赖项时，**FastAPI** 会在内部将其转换为上下文管理器，并与其他相关工具结合使用。

### 在使用 `yield` 的依赖项中使用上下文管理器

type auditObjectOp struct {
	Name string `json:"name"`
	Pool int    `json:"poolId"`
	Set  int    `json:"setId"`
}

func (op auditObjectOp) String() string {
	// Flatten the auditObjectOp
	return fmt.Sprintf("name=%s,pool=%d,set=%d", op.Name, op.Pool, op.Set)
}

// Add erasure set information to the current context
func auditObjectErasureSet(ctx context.Context, api, object string, set *erasureObjects) {

		}
	}

	// check paging works.
	ag := []string{"a", "b", "c", "d", "e", "f", "g"}
	checkObjCount := make(map[string]int)
	for i := range 7 {
		dirName := strings.Repeat(ag[i], 3)
		key := fmt.Sprintf("testPrefix/%s/obj%s", dirName, dirName)
		checkObjCount[key]++
		_, err = obj.PutObject(context.Background(), "bucket", key, mustGetPutObjReader(t, bytes.NewBufferString(uploadContent), int64(len(uploadContent)), "", ""), opts)