On 24 May 2007, at 15:42, Martin Blais wrote:

On 5/23/07, Collin Winter <collinw@gmail.com> wrote:

...

[moving to python-ideas, where it belongs]

...

I think Python needs a builtin for this very purpose. I propose 'nodef', a unique object whose sole purpose is to serve as a default value. It should be unique, in the same sense that 'None' is unique.

I've run into this situation several times before, and I've never felt that defining my own sentinel was unduly taxing. Having a built-in would only save a single line, the initial "sentinel = object()".

There is another benefit to a builtin 'nodef', BTW, which is that of a consistent meaning for the word. Having a consistent name for this concept--across all programs, if it were in the language--makes it more obvious what you're trying to achieve. Otherwise, you decide to use 'sentinel', I choose 'NoDef', Barry uses 'missing', and we're all speaking a different "language".

Just as you find that 'None' is sometimes a valid value for a function parameter (and this is the reason why you would like 'nodef'), if there was a 'nodef' builtin you would sometimes find that it is a meaningful value for a function parameter. Would you then advocate the creation of a 'really_nodef' builtin? And then a 'this_time_i_really_mean_it_nodef' builtin? This is why a 'nodef' keyword is not a good idea. 'None' is useful because often you expect an argument of a certain type (e.g. a list, so you know it should not be 'None'). -- Arnaud

On 24 May 2007, at 15:42, Martin Blais wrote:

On 5/23/07, Collin Winter <collinw@gmail.com> wrote:

...

[moving to python-ideas, where it belongs]

...

I think Python needs a builtin for this very purpose. I propose 'nodef', a unique object whose sole purpose is to serve as a default value. It should be unique, in the same sense that 'None' is unique.

I've run into this situation several times before, and I've never felt that defining my own sentinel was unduly taxing. Having a built-in would only save a single line, the initial "sentinel = object()".

There is another benefit to a builtin 'nodef', BTW, which is that of a consistent meaning for the word. Having a consistent name for this concept--across all programs, if it were in the language--makes it more obvious what you're trying to achieve. Otherwise, you decide to use 'sentinel', I choose 'NoDef', Barry uses 'missing', and we're all speaking a different "language".

Just as you find that 'None' is sometimes a valid value for a function parameter (and this is the reason why you would like 'nodef'), if there was a 'nodef' builtin you would sometimes find that it is a meaningful value for a function parameter. Would you then advocate the creation of a 'really_nodef' builtin? And then a 'this_time_i_really_mean_it_nodef' builtin? This is why a 'nodef' keyword is not a good idea. 'None' is useful because often you expect an argument of a certain type (e.g. a list, so you know it should not be 'None'). -- Arnaud