[Python-Dev] PEP 340: Breaking out.

[Shane Hathaway]

Alex Martelli wrote:
> Looking for a file with a certain magicnumber in its 1st two bytes...?
> 
> for name in filenames:
>      opening(name) as f:
>          if f.read(2) == 0xFEB0: break
> 
> This does seem to make real-life sense to me...

I'd like to suggest a small language enhancement that would fix this
example.  Allow the break and continue statements to use a keyword,
either "for" or "while", to state that the code should break out of both
the block statement and the innermost "for" or "while" statement.  The
example above would change to:

    for name in filenames:
        opening(name) as f:
            if f.read(2) == 0xFEB0:
                break for

This could be a separate PEP if necessary.  When a "break for" is used
in a block statement, it should raise a new kind of exception,
BreakForLoop, and the block statement should propagate the exception.
When used outside a block statement, "break for" can use existing Python
byte code to jump directly to the next appropriate statement.

Shane