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Re: How to detect that a function argument is the default one

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Skip Montanaro <skip.montanaro@gmail.com>:

> On Wed, Dec 10, 2014 at 9:14 AM, ast <nomail@invalid.com> wrote:
>> I have the idea to write:
>>
>> def __init__(center=(0,0), radius=10, mass=None)):
>>
>>    if  mass == None:        self.mass = radius**2
>>    else:
>>        self.mass = mass
>>
>> but maybe Python provides something clever.
>
> This is almost the correct idiom. In general, pick defaults from
> values outside your valid domain. None is often perfect for
> this. Since you're comparing to a singleton value, "is" is the usual
> comparison, so your code becomes:
>
> def __init__(center=(0,0), radius=10, mass=None)):
>     if mass is None:
>         self.mass = radius**2
>     else:
>         self.mass = mass

In this case, None works well. However, sometimes None is a valid input
value. Then, a sentinel object is the way out:

        
    _OMITTED = object()

    class SomeClass:
        def __init__(self, center=(0,0), radius=10, mass=_OMITTED)):
            if mass is _OMITTED:
                self.mass = radius**2
            else:
                self.mass = mass


Marko

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Thread

How to detect that a function argument is the default one "ast" <nomail@invalid.com> - 2014-12-10 16:14 +0100
  Re: How to detect that a function argument is the default one Skip Montanaro <skip.montanaro@gmail.com> - 2014-12-10 09:29 -0600
    Re: How to detect that a function argument is the default one "ast" <nomail@invalid.com> - 2014-12-10 16:45 +0100
    Re: How to detect that a function argument is the default one Marko Rauhamaa <marko@pacujo.net> - 2014-12-10 18:33 +0200
  Re: How to detect that a function argument is the default one Chris Angelico <rosuav@gmail.com> - 2014-12-11 03:16 +1100
    Re: How to detect that a function argument is the default one wxjmfauth@gmail.com - 2014-12-10 12:08 -0800
    Re: How to detect that a function argument is the default one "ast" <nomail@invalid.com> - 2014-12-10 22:17 +0100
      Re: How to detect that a function argument is the default one Ian Kelly <ian.g.kelly@gmail.com> - 2014-12-11 11:03 -0700
      Re: How to detect that a function argument is the default one Ian Kelly <ian.g.kelly@gmail.com> - 2014-12-11 11:07 -0700
  Re: How to detect that a function argument is the default one Ned Batchelder <ned@nedbatchelder.com> - 2014-12-10 11:19 -0500
  Re: How to detect that a function argument is the default one Ian Kelly <ian.g.kelly@gmail.com> - 2014-12-10 09:20 -0700
  Re: How to detect that a function argument is the default one Tony the Tiger <tony@tiger.invalid> - 2014-12-11 19:45 +0000
    Re: How to detect that a function argument is the default one Ben Finney <ben+python@benfinney.id.au> - 2014-12-12 10:03 +1100

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