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Date: Wed, 29 May 2013 14:10:58 -0400
From: Ned Batchelder <ned@nedbatchelder.com>
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To: Croepha <croepha@gmail.com>
Subject: Re: Getting a callable for any value?
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On 5/29/2013 1:46 PM, Croepha wrote:
> Is there anything like this in the standard library?
>
> class AnyFactory(object):
> def __init__(self, anything):
> self.product = anything
> def __call__(self):
> return self.product
> def __repr__(self):
> return "%s.%s(%r)" % (self.__class__.__module__, 
> self.__class__.__name__, self.product)
>
> my use case is: 
> collections.defaultdict(AnyFactory(collections.defaultdict(AnyFactory(None))))
>
> And I think lambda expressions are not preferable...
>

It's not clear to me that this does what you want.  Won't your 
defaultdict use the same defaultdict for all of its default values? This 
is hard to describe in English but:

     d = collections.defaultdict(AnyFactory(collections.defaultdict(AnyFactory(None))))

     d[1][1] = 1
     d[2][2] = 2

     print d[1]
     #->  defaultdict(__main__.AnyFactory(None), {1: 1, 2: 2})
     print d[1] is d[2]
     #->  True

It might not be possible to get this right without lambdas:

     d = collections.defaultdict(lambda: collections.defaultdict(lambda: None))

     d[1][1] = 1
     d[2][2] = 2

     print d[1]
     #->  defaultdict(<function <lambda> at 0x02091D70>, {1: 1})
     print d[1] is d[2]
     #->  False


--Ned.

> I found itertools.repeat(anything).next and 
> functools.partial(copy.copy, anything)
>
> but both of those don't repr well... and are confusing...
>
> I think AnyFactory is the most readable, but is confusing if the 
> reader doesn't know what it is, am I missing a standard implementation 
> of this?
>
>
>


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    On 5/29/2013 1:46 PM, Croepha wrote:<br>
    <blockquote
cite="mid:CANm61jeHSZL1FQvTr9FS0QKEhFPXug0fibXBEA-4guYF1Lf7UQ@mail.gmail.com"
      type="cite">
      <div dir="ltr">Is there anything like this in the standard
        library?
        <div><br>
        </div>
        <div>
          <div>class AnyFactory(object):</div>
          <div><span class="" style="white-space:pre"> </span>def
            __init__(self, anything):</div>
          <div><span class="" style="white-space:pre"> </span>self.product
            = anything</div>
          <div><span class="" style="white-space:pre"> </span>def
            __call__(self):</div>
          <div><span class="" style="white-space:pre"> </span>return
            self.product</div>
          <div><span class="" style="white-space:pre"> </span>def
            __repr__(self):</div>
          <div><span class="" style="white-space:pre"> </span>return
            "%s.%s(%r)" % (self.__class__.__module__,
            self.__class__.__name__, self.product)</div>
        </div>
        <div><br>
        </div>
        <div style="">my use case is:
collections.defaultdict(AnyFactory(collections.defaultdict(AnyFactory(None))))</div>
        <div style=""><br>
        </div>
        <div style="">And I think lambda expressions are
          not&nbsp;preferable...</div>
        <div style=""><br>
        </div>
      </div>
    </blockquote>
    <br>
    It's not clear to me that this does what you want.&nbsp; Won't your
    defaultdict use the same defaultdict for all of its default values?&nbsp;
    This is hard to describe in English but:<br>
    <pre>&nbsp;&nbsp;&nbsp; d = collections.defaultdict(AnyFactory(collections.defaultdict(AnyFactory(None))))

&nbsp;&nbsp;&nbsp; d[1][1] = 1
&nbsp;&nbsp;&nbsp; d[2][2] = 2

&nbsp;&nbsp;&nbsp; print d[1]
&nbsp;&nbsp;&nbsp; #-&gt;&nbsp; defaultdict(__main__.AnyFactory(None), {1: 1, 2: 2})
&nbsp;&nbsp;&nbsp; print d[1] is d[2]
&nbsp;&nbsp;&nbsp; #-&gt;&nbsp; True</pre>
    It might not be possible to get this right without lambdas:<br>
    <pre>&nbsp;&nbsp;&nbsp; d = collections.defaultdict(lambda: collections.defaultdict(lambda: None))

&nbsp;&nbsp;&nbsp; d[1][1] = 1
&nbsp;&nbsp;&nbsp; d[2][2] = 2

&nbsp;&nbsp;&nbsp; print d[1]
&nbsp;&nbsp;&nbsp; #-&gt;&nbsp; defaultdict(&lt;function &lt;lambda&gt; at 0x02091D70&gt;, {1: 1})
&nbsp;&nbsp;&nbsp; print d[1] is d[2]
&nbsp;&nbsp;&nbsp; #-&gt;&nbsp; False</pre>
    <br>
    --Ned.<br>
    <br>
    <blockquote
cite="mid:CANm61jeHSZL1FQvTr9FS0QKEhFPXug0fibXBEA-4guYF1Lf7UQ@mail.gmail.com"
      type="cite">
      <div dir="ltr">
        <div style="">I found itertools.repeat(anything).next and
          functools.partial(copy.copy, anything)</div>
        <div style="">
          <br>
        </div>
        <div style="">but both of those don't repr well... and are
          confusing... &nbsp;</div>
        <div style=""><br>
        </div>
        <div style="">I think AnyFactory is the most readable, but is
          confusing if the reader doesn't know what it is, am I missing
          a standard implementation of this?</div>
        <div style=""><br>
        </div>
      </div>
      <br>
      <fieldset class="mimeAttachmentHeader"></fieldset>
      <br>
    </blockquote>
    <br>
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