Is it possible to have an optional with/as statement in python?

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别跟我提以往
别跟我提以往 2021-01-04 03:25

Instead of this:

FILE = open(f)
do_something(FILE)
FILE.close()

it\'s better to use this:

with open(f) as FILE:
    do_some         


        
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5条回答
  • 2021-01-04 03:40

    If you were to just write it like this:

    if f is not None:
        with open(f) as FILE:
            do_something(FILE)
    else:
        do_something(f)
    

    (file is a builtin btw )

    Update

    Here is a funky way to do an on-the-fly context with an optional None that won't crash:

    from contextlib import contextmanager
    
    none_context = contextmanager(lambda: iter([None]))()
    # <contextlib.GeneratorContextManager at 0x1021a0110>
    
    with (open(f) if f is not None else none_context) as FILE:
        do_something(FILE)
    

    It creates a context that returns a None value. The with will either produce FILE as a file object, or a None type. But the None type will have a proper __exit__

    Update

    If you are using Python 3.7 or higher, then you can declare the null context manager for stand-in purposes in a much simpler way:

    import contextlib
    none_context = contextlib.nullcontext()
    

    You can read more about these here:

    https://docs.python.org/3.7/library/contextlib.html#contextlib.nullcontext

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  • 2021-01-04 03:48

    While all of the other answers are excellent, and preferable, note that the with expression may be any expression, so you can do:

    with (open(file) if file is not None else None) as FILE:
        pass
    

    Note that if the else clause were evaluated, to yield None this would result in an exception, because NoneType does not support the appropriate operations to be used as a context manager.

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  • 2021-01-04 03:53

    This seems to solve all of your concerns.

    if file_name is not None:
        with open(file_name) as fh:
            do_something(fh)
    else:
            do_something(None)
    
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  • 2021-01-04 03:56

    Since Python 3.7, you can also do

    from contextlib import nullcontext
    
    with (open(file) if file else nullcontext()) as FILE:
        # Do something with `FILE`
        pass
    

    See the official documentation for more details.

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  • 2021-01-04 03:59

    something like:

    if file:      #it checks for None,false values no need of "if file is None"
        with open(file) as FILE:
            do_something(FILE)
    else:
        FILE=None
    
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