<div dir="ltr">So in such cases it should not subclassed `dict`, but `collections.MutableMapping`, for example?<br></div><div class="gmail_extra"><br clear="all"><div><div class="gmail_signature"><div dir="ltr"><div>---</div><div><i><font color="#38761d">Zaur Shibzukhov</font></i><br></div><br></div></div></div>
<br><div class="gmail_quote">2015-03-17 22:38 GMT+03:00 Brett Cannon <span dir="ltr"><<a href="mailto:brett@python.org" target="_blank">brett@python.org</a>></span>:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><br><br><div class="gmail_quote"><span class="">On Tue, Mar 17, 2015 at 3:29 PM Zaur Shibzukhov <<a href="mailto:szport@gmail.com" target="_blank">szport@gmail.com</a>> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div>Yes... But I expected that dict constructor will use `__getitem__` or `items` method of MyDict instance in order to retrieve items of the MyDict instance during construction of the dict instance... Instead it interpreted MyDict instance as the dict instance during construction of new dict.This exactly caused my confusion.<br></div></div></blockquote><div><br></div></span><div>It's because you subclassed dict. Copying is optimized to skip over using the methods you listed when the object is a dict and so we know the structure of the object at the C level. You can look at <a href="https://hg.python.org/cpython/file/22a0c925a7c2/Objects/dictobject.c#l1997" target="_blank">https://hg.python.org/cpython/file/22a0c925a7c2/Objects/dictobject.c#l1997</a> to see the actual code.</div><span class="HOEnZb"><font color="#888888"><div><br></div><div>-Brett</div></font></span><div><div class="h5"><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div></div></div><div class="gmail_extra"><br clear="all"><div><div><div dir="ltr"><div>---</div><div><i><font color="#38761d">Zaur Shibzukhov</font></i><br></div><br></div></div></div></div><div class="gmail_extra">
<br><div class="gmail_quote">2015-03-17 22:12 GMT+03:00 Brett Cannon <span dir="ltr"><<a href="mailto:brett@python.org" target="_blank">brett@python.org</a>></span>:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><br><br><div class="gmail_quote"><div><div>On Tue, Mar 17, 2015 at 3:05 PM Zaur Shibzukhov <<a href="mailto:szport@gmail.com" target="_blank">szport@gmail.com</a>> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">Hello!<br><br>In order to explain, let define subclass of dict:<br><br>class Pair:<br> def __init__(self, key, val):<br> self.key = key<br> self.val = val<br><br>class MyDict(dict):<br> #<br> def __init__(self, *args, **kwds):<br> if len(args) > 1:<br> raise TypeError('Expected at most 1 arguments, but got %d' % len(args))<br><br> for key, val in args[0]:<br> self[key] = val<br> <br> for key, val in kwds.items():<br> self[key] = val<br><br> def __getitem__(self, key):<br> pair = dict.__getitem__(key)<br> return pair.value<br><br> def __setitem__(self, key, val):<br> if key in self:<br> pair = dict.__getitem__(key)<br> pair.value = value<br> else:<br> pair = Pair(key, val)<br> dict.__setitem__(self, key, pair)<br><br> def values(self):<br> for key in self:<br> p = dict.__getitem__(self, key)<br> yield p.value<br><br> def items(self):<br> for key, p in dict.__iter__(self):<br> yield p.key, p.value<br><br><br>The simple test give me strange result:<br><br>>>> d = MyDict([('a', 1), ('b', 2), ('c', 3)])<br>>>> dict(d)<br>{'a': <__main__.Pair at 0x104ca9e48>,<br> 'b': <__main__.Pair at 0x104ca9e80>,<br> 'c': <__main__.Pair at 0x104ca9eb8>}<br><br>instead of {'a':1, 'b':2, 'c':3}.<br><br><br>Is this right behavior of the dict?<br></div></blockquote><div><br></div></div></div><div>Yes because in your __setitem__ call you are storing the value as the Pair. So when dict prints its repr it prints the key and value, and in this case the value is a Pair.</div></div></div>
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