Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.python > #82366

Re: When do default parameters get their values set?

Show all headers | View raw

[Multipart message — attachments visible in raw view] - view raw

On Wed, Dec 10, 2014 at 7:15 PM, Steven D'Aprano <steve@pearwood.info>
wrote:

> On Wed, 10 Dec 2014 18:18:44 -0800, Rustom Mody wrote:
>
>
> > And going the other way -- no defs only lambdas its this:
> >
> >
> >>>> f = lambda : (lambda x= {}: x)
> >>>> f()() is f()()
> > False
> >>>> d = f()
> >>>> d() is d()
> > True
> >>>>
> >>>>
> >
> > But I have a different question -- can this be demonstrated without the
> > 'is'?
>
>
> Can *what* be demonstrated? That the functions returned are different
> objects, or that the dicts are different objects? Both? Something else?
>
> Using "is" you are demonstrating that calling the function twice returns
> two distinct objects. That is the purpose of "is", to compare object
> identity. Without "is", you can compare object IDs directly:
>
> id(f()()) == id(f()())
>
> but that's ugly and less efficient. Using "is" is the more idiomatic and
> natural way to do this.
>

In CPython, that does not work, as the dictionary will be garbage collected
after each call to id:
>>> f = lambda : (lambda x= {}: x)
>>> f()() is f()()
False
>>> id(f()()) == id(f()())
True


>
> Other than that, you could do something to demonstrate the consequences
> of the two values being distinct objects:
>
> a = f()()  # Call twice to get a dict.
> b = f()()
> a['key'] = 23
> b['key']  # raises KeyError
>
>
> or
>
> a = f()  # Call once to get a function.
> b = f()
> a.attribute = 23
> b.attribute  # raises AttributeError
>
>
> Or you could inspect the byte-code of f and try to understand it.
>
>
Another way would be to to demonstrate the behavior, without using is,
would be to use a global counter variable:

a = 1
def f():
    global a
    def g(b=[a]):
        return b
    a += 1
    return g

 >>> f()()
[1]
>>> f()()
[2]
>>> a = f()
>>> a()
[3]
>>> a()
[3]
>>> b = f()
>>> b()
[4]
>>> a()
[3]

Back to comp.lang.python | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

When do default parameters get their values set? "bSneddon" <bsneddon@1:249/999.remove-cdt-this> - 2014-12-09 11:32 -0500
  Re: When do default parameters get their values set? "Chris Angelico" <chris.angelico@1:249/999.remove-cdt-this> - 2014-12-09 11:32 -0500
  Re: When do default parameters get their values set? "Dave Angel" <dave.angel@1:249/999.remove-cdt-this> - 2014-12-09 11:32 -0500
    Re: When do default parameters get their values set? Duncan Booth <duncan.booth@invalid.invalid> - 2014-12-09 21:18 +0000
      Re: When do default parameters get their values set? random832@fastmail.us - 2014-12-09 18:07 -0500
        Re: When do default parameters get their values set? Rustom Mody <rustompmody@gmail.com> - 2014-12-09 18:44 -0800
          Re: When do default parameters get their values set? random832@fastmail.us - 2014-12-10 13:38 -0500
            Re: When do default parameters get their values set? Rustom Mody <rustompmody@gmail.com> - 2014-12-10 18:18 -0800
              Re: When do default parameters get their values set? Chris Angelico <rosuav@gmail.com> - 2014-12-11 13:24 +1100
                Re: When do default parameters get their values set? Rustom Mody <rustompmody@gmail.com> - 2014-12-10 18:50 -0800
              Re: When do default parameters get their values set? Steven D'Aprano <steve@pearwood.info> - 2014-12-11 03:15 +0000
                Re: When do default parameters get their values set? Rustom Mody <rustompmody@gmail.com> - 2014-12-10 19:26 -0800
                Re: When do default parameters get their values set? Ben Finney <ben+python@benfinney.id.au> - 2014-12-11 14:49 +1100
                Re: When do default parameters get their values set? Chris Kaynor <ckaynor@zindagigames.com> - 2014-12-11 09:10 -0800
                Re: When do default parameters get their values set? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-12-12 09:51 +1100
              Re: When do default parameters get their values set? random832@fastmail.us - 2014-12-11 15:44 -0500
  Re: When do default parameters get their values set? "Tim Chase" <tim.chase@1:249/999.remove-cdt-this> - 2014-12-09 11:32 -0500

csiph-web