with 的原理和用法

### with 的原理
如果一个对象需要使用with，那么它必须实现__enter__, __exit__这两个方法。
```
class Sample:
	def __init__(self):
		pass
		
    def __enter__(self):
        print("in __enter__")
		return self
	
    def __exit__(self, ex_type, ex_val, ex_tb):
        print("in __exit__")
```
- 当执行 `with Sample() as obj` 语句时，对象会被创建，并且__enter__方法被调用，`obj`就是该方法的返回值；
- 在退出with语句时，__exit__方法被调用，可以用来保证一些资源的释放；

### with 的使用
#### 场景一：限制同时上传文件的数量
因为服务器的内存有限，如果很多人同时上传文件，会导致服务器内存崩溃，因此需要限制同时上传的数量。
```
class UploadCountHandler(object):
    def __init__(self, cache: Cache, user_uid):
        self.cache = cache
        self.user_uid = user_uid

def __enter__(self):
		# 增加上传数量
        self.cache.incr_upload_count_all()
        self.cache.incr_upload_count_user(user_uid=self.user_uid)

def __exit__(self, ex_type, ex_val, ex_tb):
		# 减少上传数量
        self.cache.decr_upload_count_all()
        self.cache.decr_upload_count_user(user_uid=self.user_uid)
```

#### 场景二：在指定目录中执行命令，执行完退出该目录
```
class EnterFolder(object):
    def __init__(self, folder, make=False):
        if not make:
            if not os.path.isdir(folder):
                raise NotADirectoryError("folder {} not found".format(folder))
        self.folder = folder

def __enter__(self):
        cd(self.folder)

def __exit__(self, ex_type, ex_val, ex_tb):
        cd("../")

with EnterFolder(project.name):
	project.buildProject()
	project.buildImage(tag="1.0")
with EnterFolder("images", make=True):
	project.saveImage(tag="1.0")
```

##### 场景三：对于耗时较长的任务，记录时间和状态
```
class DB(object):
    def __init__(self):
        self.path = ".mkzip.db"
        self.data = {}
        self.load()

@property
    def build_status(self):
        return self.data.get("build_status", 0)

def load(self):
        with open(self.path, "r+") as f:
            content = f.read()
        if not content:
            return
        self.data = json.loads(content)

def save(self):
        with open(self.path, "w+") as f:
            f.write(json.dumps(self.data))

def __enter__(self):
        if self.data.get("build_status") == 1:
            raise Exception("已经在构建了，请稍等")
        self.data["build_status"] = 1
        self.data["build_time"] = datetime.now().strftime("%m/%d %H:%M")
        self.save()

def __exit__(self, ex_type, ex_val, ex_tb):
        print(ex_type, ex_val)
        if not ex_val:
            self.data["build_status"] = 2
            self.save()

with DB():
	folder_path = PROJECT_HOME + project_name
	tar(["czf", f"{project_name}.tgz", f"{project_name}"], cwd=folder_path)
	zip("-r", f"{project_name}.zip", f"{project_name}", cwd=PROJECT_HOME)
```