with EXPRESSION as VARIABLE:
    BLOCK
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with open("timer.py") as fp:
    print("".join(ln for ln in fp if ":" in ln))

class TimerError(Exception):
class Timer:
    timers: ClassVar[Dict[str, float]] = {}
    name: Optional[str] = None
    text: str = "Elapsed time: {:0.4f} seconds"
    logger: Optional[Callable[[str], None]] = print
    _start_time: Optional[float] = field(default=None, init=False, repr=False)
    def __post_init__(self) -> None:
        if self.name is not None:
    def start(self) -> None:
        if self._start_time is not None:
    def stop(self) -> float:
        if self._start_time is None:
        if self.logger:
        if self.name:
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fp.closed
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True
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# studio.py
class Studio:
    def __init__(self, name):
        self.name = name

    def __enter__(self):
        print(f"你好 {self.name}")
        return self

    def __exit__(self, exc_type, exc_value, exc_tb):
        print(f"一会儿见, {self.name}")
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from studio import Studio
with Studio("云朵君"):
    print("正在忙 ...")
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你好 云朵君
正在忙 ...
一会儿见, 云朵君
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from greeter import Greeter
with Greeter("云朵君") as grt:
  print(f"{grt.name} 正在忙 ...")
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你好 云朵君
云朵君 正在忙 ...
一会儿见, 云朵君
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from greeter import Greeter
with Greeter("云朵君") as grt:
    print(f"{grt.age} does not exist")
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你好 云朵君
一会儿见, 云朵君
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
AttributeError: 'Greeter' object has no attribute 'age'
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import contextlib

@contextlib.contextmanager
def open_func(file_name):
    # __enter__方法
    print('open file:', file_name, 'in __enter__')
    file_handler = open(file_name, 'r')
 
    # 【重点】：yield
    yield file_handler

    # __exit__方法
    print('close file:', file_name, 'in __exit__')
    file_handler.close()
    return

with open_func('test.txt') as file_in:
    for line in file_in:
        print(line)
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import contextlib

@contextlib.contextmanager
def open_func(file_name):
    # __enter__方法
    print('open file:', file_name, 'in __enter__')
    file_handler = open(file_name, 'r')

    try:
        yield file_handler
    except Exception as exc:
        # deal with exception
        print('the exception was thrown')
    finally:
        print('close file:', file_name, 'in __exit__')
        file_handler.close()
        return

with open_func('test.txt') as file_in:
    for line in file_in:
        1/0
        print(line)
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# timer.py
@dataclass
class Timer:
    # 其他代码保持不变

    def __enter__(self):
        """Start a new timer as a context manager"""
        self.start()
        return self

    def __exit__(self, *exc_info):
        """Stop the context manager timer"""
        self.stop()
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from timer import Timer
import time
with Timer():
    time.sleep(0.7)
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Elapsed time: 0.7012 seconds
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from timer import Timer
with Timer():
    for num in range(-3, 3):
        print(f"1 / {num} = {1 / num:.3f}")
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1 / -3 = -0.333
1 / -2 = -0.500
1 / -1 = -1.000
Elapsed time: 0.0001 seconds
Traceback (most recent call last):
  File "<stdin>", line 3, in <module>
ZeroDivisionError: division by zero
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# download_data.py
import requests
from timer import Timer
def main():
    t = Timer()
    t.start()
    source_url = 'https://cloud.tsinghua.edu.cn/d/e1ccfff39ad541908bae/files/?p=%2Fall_six_datasets.zip&dl=1'
    headers = {'User-Agent': 'Mozilla/5.0'}
    res = requests.get(source_url, headers=headers) 
    t.stop()
    with open('dataset/datasets.zip', 'wb') as f:
        f.write(res.content)

if __name__ == "__main__":
    main()
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# download_data.py
import requests
from timer import Timer
def main():
    source_url = 'https://cloud.tsinghua.edu.cn/d/e1ccfff39ad541908bae/files/?p=%2Fall_six_datasets.zip&dl=1'
    headers = {'User-Agent': 'Mozilla/5.0'}
    with Timer():
        res = requests.get(source_url, headers=headers)
        
    with open('dataset/datasets.zip', 'wb') as f:
        f.write(res.content)

if __name__ == "__main__":
    main()
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