[Python-ideas] Better repr() for sentinel objects

[Michael Foord]

On 15 February 2014 11:56, Chris Angelico <rosuav at gmail.com> wrote:

> A common pattern, both in the stdlib and in user code, is the
> dedicated sentinel object to recognize an omitted argument:
>
> no_timeout = object()
> def get_data(source, timeout=no_timeout):
>     if timeout is not no_timeout:
>         source.set_alarm(timeout)
>     return source.pop()
>
> This is a bit unclear in the function signature, as seen in help():
>
> """
> Help on function get_data in module __main__:
>
> get_data(source, timeout=<object object at 0x7f579fe53070>)
> """
>
> The stdlib does this:
> """
> Help on function create_connection in module socket:
>
> create_connection(address, timeout=<object object at 0x7fe52900b080>,
> source_address=None)
> """ # chomp docstring
>
> """
> Help on function urlopen in module urllib.request:
>
> urlopen(url, data=None, timeout=<object object at 0x7fe52900b080>, *,
> cafile=None, capath=None, cadefault=False)
> """ # no docstring to chomp, actually
>
> It's not particularly useful to get the object's address. Proposal: A
> self-documenting Sentinel class which serves the exact same purpose.
>
> class Sentinel:
>     def __init__(self, desc):
>         self.desc = desc
>     def __repr__(self):
>         return "Sentinel(" + repr(self.desc) + ")"
>
> This can then be used just like object(), only it retains something
> for the benefit of its repr:
>
> no_timeout = Sentinel("No timeout")
> # Same function definition
> """
> Help on function get_data in module __main__:
>
> get_data(source, timeout=Sentinel('No timeout'))
> """
>
> I don't know how this interacts with Argument Clinic and C-written
> functions. If there's something that's being done for those that would
> make sentinels come out a particular way in their help() info, ideally
> this should be displayed the same way (or at least similarly).
>
>
There's always this:

Python 3.3.4 (v3.3.4:7ff62415e426, Feb  9 2014, 00:29:34)

[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin

Type "help", "copyright", "credits" or "license" for more information.

>>> from unittest.mock import sentinel

>>> sentinel.FOO

sentinel.FOO

>>> sentinel.FOO is sentinel.FOO

True

>>> sentinel.BarBamBaz

sentinel.BarBamBaz


Michael


> Thoughts?
>
> ChrisA
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