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POUnexpected relative import behavior in Python
 

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  1. POUnexpected relative import behavior in Python
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    <p>I ran into a very surprising relative import behavior today (unfortantely after nearly 4 hours of pulling my hair out).</p> <p>I have always been under the impression that if you have "Class A" inside of a module name "module_a.py" within a package named "package" that you could equivalently use either:</p> <pre><code>from package.module_a import ClassA </code></pre> <p>or</p> <pre><code>from module_a import ClassA </code></pre> <p>as long as you were importing from a module within "package". I understood this to be a relative import.</p> <p>I never had a problem until today when I needed to check the instance of an object against Class A and I was surprised to find a very unusual behavior.</p> <p>Consider the following:</p> <h2>package/module_a.py</h2> <pre><code>class ClassA(object): pass def check_from_module_a(obj): print 'from module_a' print '-------------' print 'class is:', ClassA print 'object is', type(obj) print 'is obj a ClassA:', isinstance(obj, ClassA) </code></pre> <h2>package/module_b.py</h2> <pre><code>from package.module_a import ClassA from module_a import check_from_module_a a = ClassA() check_from_module_a(a) print ' ' print 'from module_b' print '-------------' print 'class is:', ClassA print 'object is', type(a) print 'is obj a ClassA:', isinstance(a, ClassA) </code></pre> <p>Now when executing module_b.py you get:</p> <pre><code>from module_a ------------- class is: &lt;class 'module_a.ClassA'&gt; object is &lt;class 'package.module_a.ClassA'&gt; is obj a ClassA: False from module_b ------------- class is: &lt;class 'package.module_a.ClassA'&gt; object is &lt;class 'package.module_a.ClassA'&gt; is obj a ClassA: True </code></pre> <p>I have followed the logic and now see why this occurs - it was not real obvious as I assumed that the binding for ClassA was the same regardless of an absolute or relative import. This caused a very nasty bug for me that was very hard to isolate.</p> <p>My questions:</p> <ol> <li><p>Is this the behavior that would be expected? </p></li> <li><p>If this is the logical way that it should work - it is then not clear to me why I would use relative imports if they are not compatible (in the above sense) with absolute imports. Is there a good explanation here that I am missing?</p></li> <li><p>I always assume that relative imports provided additional ease in large refactors when sub package structure might be moved around. Is this the main benefit of a relative import?</p></li> </ol>
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