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

[Rhodri James]

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.

An incomplete analysis and therefore an incorrect conclusion.  Until the 
garbage collector comes out to play, read_multiple() will keep the file 
open, keep the tuple of filenames in memory and of course keep the 
environment of the generator around.  That's all leakage caused by the 
_break_, and following your logic the obvious solution would be to ban 
breaks in loops that are reading from generators.  But that wouldn't be 
helpful, obviously.

-- 
Rhodri James *-* Kynesim Ltd