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    <p>Notice that an <code>object</code> instance has no <code>__dict__</code> attribute:</p> <pre><code>&gt;&gt;&gt; dir(object()) ['__class__', '__delattr__', '__doc__', '__getattribute__', '__hash__', '__init__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__str__'] </code></pre> <p>An example to illustrate this behavior in a derived class:</p> <pre><code>&gt;&gt;&gt; class Foo(object): ... __slots__ = {} ... &gt;&gt;&gt; f = Foo() &gt;&gt;&gt; f.bar = 42 Traceback (most recent call last): File "&lt;stdin&gt;", line 1, in &lt;module&gt; AttributeError: 'Foo' object has no attribute 'bar' </code></pre> <p>Quoting from the docs on <a href="https://docs.python.org/2/reference/datamodel.html#slots" rel="nofollow noreferrer"><code>slots</code></a>:</p> <blockquote> <p>[...] The <code>__slots__</code> declaration takes a sequence of instance variables and reserves just enough space in each instance to hold a value for each variable. Space is saved because <code>__dict__</code> is not created for each instance.</p> </blockquote> <p>EDIT: To answer ThomasH from the comments, OP's test class is an "old-style" class. Try:</p> <pre><code>&gt;&gt;&gt; class test: pass ... &gt;&gt;&gt; getattr(test(), '__dict__') {} &gt;&gt;&gt; getattr(object(), '__dict__') Traceback (most recent call last): File "&lt;stdin&gt;", line 1, in &lt;module&gt; AttributeError: 'object' object has no attribute '__dict__' </code></pre> <p>and you'll notice there is a <code>__dict__</code> instance. The object class may not have a <code>__slots__</code> defined, but the result is the same: lack of a <code>__dict__</code>, which is what prevents dynamic assignment of an attribute. I've reorganized my answer to make this clearer (move the second paragraph to the top).</p>
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    1. COSo, you are saying that the *object* class is using \_\_slots\_\_ in one way or another, to prevent assignment of instance members. Is this an assumption or a fact (i.e. is it in the implementation of *object*)? Because the mere absence of \_\dict\_\_ from an *object* instance is not indicative. Instances of the OP's *test1* class show the same missing \_\_dict\_\_ when called with dir(), but allow nevertheless assignment of instance members.
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    2. CO@ThomasH, please see the note I added to the answer.
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