[Python-ideas] Make "yield" inside a with statement a SyntaxError

[Oscar Benjamin]

On 8 August 2018 at 15:22, Ronald Oussoren via Python-ideas
<python-ideas at python.org> wrote:
>>
>> On 08/08/18 07:14, Ken Hilton wrote:
>>> Now, let's take a look at the following scenario:
>>>     def read_multiple(*filenames):
>>>         for filename in filenames:
>>>             with open(filename) as f:
>>>                 yield f.read()
>>> Can you spot the problem? The "with open(filename)" statement is supposed
>>> to ensure that the file object is disposed of properly. However, the "yield
>>> f.read()" statement suspends execution within the with block, so if this
>>> happened:
>>>     for contents in read_multiple('chunk1', 'chunk2', 'chunk3'):
>>>         if contents == 'hello':
>>>             break
>>> and the contents of "chunk2" were "hello" then the loop would exit, and
>>> "chunk2" would never be closed! Yielding inside a with block, therefore,
>>> doesn't make sense and can only lead to obscure bugs.
>
> It is also possible to fix the particular issue by using another with statement, that is use:
>
> with contextlib.closing(read_multiple(…)) as chunks:
>    for contents in chunks:
>        …

That's a very good point Ronald. Having seen this a few times I can't
think of any cases that wouldn't be solved by this.

If a language change was wanted for this then perhaps it should be to
do like file objects and add the __exit__ (calls close()) method to
generator objects so that you can omit the closing and just do:

    with read_multiple(…) as chunks:
        for contents in chunks:
            ...

--
Oscar