[Tutor] s.insert(i, x) explanation in docs for Python 3.4 confusing to me

[Alex Kleider]

On 2016-01-17 02:18, Cameron Simpson wrote:
> On 16Jan2016 22:42, Alex Kleider <akleider at sonic.net> wrote:
>> On 2016-01-16 18:02, Cameron Simpson wrote:
>>> much like writing a function "def f(x, y=None)"; None is a sentinel
>>> value - specially recognised as nor in the normal domain for that
>>> value.
>> 
>> Can you please clarify the last bit:
>> "specially recognised as nor in the normal domain for that value."
> 
> s/nor/not/
> 
> The domain of a function (or value) is the set of valid values it may
> take. The range is the set of values it may produce.
> 
> A sentinel value is a special value you may encounter in a data set
> (or as a value) which is _not_ part of the normal domain; often it
> indicates the end of a sequence (hence the name, like a border guard).
>  In Python the special value None is often used as a sentinel value.
> Since all names have _some_ value, if you need to indicate that this
> name "isn't set" or "doesn't specify a valid value", you need a
> sentinal value, often "None".
> 
> So the common idiom for default values in Python function definitions:
> 
>  def func(a, b, c=None):
>    if c is None:
>      c = default-value-for-c
>    ...
> 
> indicates that the parameters "a" and "b must be supplied, and "c" is
> optional.  If not supplied it will have the value "None". This is a
> sentinel value so that "func" can distinguish a valid value from a
> value not supplied.
> 
> You can also use them to indicate the end of data in some sense. If
> you're looking at a text file as lines of printable characters, the
> newline character at the end of the line could be considered a
> sentinel. Also consider a Queue: there's no notion of "closed" in the
> stdlib version, so one might put a None on the queue to indicate to
> the consumer that there will be no more items.
> 
> It isn't always None. Sometimes you may want to pass None as normal
> data, or you have no control over what is passed around. In that case
> you might need to make a unique sentinel value entirely for your
> object. The normal way to do this is simply to make a new object:
> 
>  sentinel = object()
> 
> This is a minimal Python object which nobody else is using. Its value
> is nothing special (or even meaningful), so you merely want to check
> whether what you've got is that particular object, using "is".
> Untested example sketch:
> 
>  class SomethingLikeAQueue:
> 
>    def __init__(self,......):
>      self.things = []
>      # private value to use as a sentinel, unique per instance
>      self._sentinel = object()
>      ... whatever else ...
> 
>    def put(self, value):
>      # public method to put a new value
>      if value is self._sentinel:
>        raise ValueError("you may not put the sentinel value")
>      self._put(value)
> 
>    def _put(self, value):
>      # private method accepting any value including the sentinel
>      # we will use receipt of the sentinel to process the things and
> stop      # further acceptance of more things
>      if value is self._sentinel:
>        things = self.things
>        self.things = None  # will make the .append blow up
>        ... process things here maybe ...
>      else:
>        things.append(value)     #### ??? self.things.append(value)
> 
>    def close(self):
>      # send the sentinel to indicate no more things
>      self._put(self._sentinel)
> 
> Cheers,
> Cameron Simpson <cs at zip.com.au>

Thanks, Cameron; use of 'sentinel = object()' is a whole new (and 
useful) concept for me.
May I trouble you further by specifically asking about 's/nor/not/'- I 
don't get what that's about.
Has it to do with this 'nor': 
http://www.merriam-webster.com/dictionary/nor?
Sincerely,
Alex

ps Am I correct that towards the end of your code it should have been
     self.things.append(value)
(about 17 lines above?? )