如何知道何时在Python中管理资源

class FileManager:
    def __init__(self, file_path):
        self.file_path = file_path

    def __enter__(self):
        self.file = open(self.file_path, 'w')
        return self.file

    def __exit__(self, exc_type, exc_value, traceback):
        self.file.close()

with FileManager('example.txt') as file:
    file.write('Hello, world!')