On Fri, Jul 15, 2011 at 10:05 AM, Don Spaulding <donspauldingii@gmail.com> wrote:

When I come across a situation where I want a sentinel that doesn't preclude None from being a valid value, I've always spelled it like this:     SENTINEL = object()  # Guaranteed to be unique and not None.

    def f(arg=SENTINEL):         if arg is SENTINEL:             arg = 'foo'         x(arg) It's always worked well, but that little voice in my head tells me there's probably something I'm not thinking of that will come back to bite me in the future.  I don't think this helps the OP in any meaningful way, but I was wondering if you bright people might shed some light on the drawbacks of this approach for me.  What are the better alternatives?

No, this approach isn't error-prone. For non-idea questions you might consider comp.lang.python in the future. =) Mike

On Sat, Jul 16, 2011 at 12:05 AM, Don Spaulding <donspauldingii@gmail.com> wrote:

When I come across a situation where I want a sentinel that doesn't preclude None from being a valid value, I've always spelled it like this:     SENTINEL = object()  # Guaranteed to be unique and not None.

    def f(arg=SENTINEL):         if arg is SENTINEL:             arg = 'foo'         x(arg) It's always worked well, but that little voice in my head tells me there's probably something I'm not thinking of that will come back to bite me in the future.  I don't think this helps the OP in any meaningful way, but I was wondering if you bright people might shed some light on the drawbacks of this approach for me.  What are the better alternatives?

Explicit sentinel objects *are* the better alternative :) As they rely on object identity, they typically don't play nicely with serialisation, though. You need a value based sentinel for that use case. Cheers, Nick. -- Nick Coghlan   |   ncoghlan@gmail.com   |   Brisbane, Australia

On Fri, Jul 15, 2011 at 10:05 AM, Don Spaulding <donspauldingii@gmail.com> wrote:

When I come across a situation where I want a sentinel that doesn't preclude None from being a valid value, I've always spelled it like this:     SENTINEL = object()  # Guaranteed to be unique and not None.

    def f(arg=SENTINEL):         if arg is SENTINEL:             arg = 'foo'         x(arg) It's always worked well, but that little voice in my head tells me there's probably something I'm not thinking of that will come back to bite me in the future.  I don't think this helps the OP in any meaningful way, but I was wondering if you bright people might shed some light on the drawbacks of this approach for me.  What are the better alternatives?

No, this approach isn't error-prone. For non-idea questions you might consider comp.lang.python in the future. =) Mike

On Sat, Jul 16, 2011 at 12:05 AM, Don Spaulding <donspauldingii@gmail.com> wrote:

When I come across a situation where I want a sentinel that doesn't preclude None from being a valid value, I've always spelled it like this:     SENTINEL = object()  # Guaranteed to be unique and not None.

    def f(arg=SENTINEL):         if arg is SENTINEL:             arg = 'foo'         x(arg) It's always worked well, but that little voice in my head tells me there's probably something I'm not thinking of that will come back to bite me in the future.  I don't think this helps the OP in any meaningful way, but I was wondering if you bright people might shed some light on the drawbacks of this approach for me.  What are the better alternatives?

Explicit sentinel objects *are* the better alternative :) As they rely on object identity, they typically don't play nicely with serialisation, though. You need a value based sentinel for that use case. Cheers, Nick. -- Nick Coghlan   |   ncoghlan@gmail.com   |   Brisbane, Australia