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POValue of variable referenced from another module does not change after it is changed in its own module
 

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  1. POValue of variable referenced from another module does not change after it is changed in its own module
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    <p>I started to write python programs for some time, but I am not so good at it, at present I meet a problem, it seems very strange, at least for me.</p> <p>Assume we have two source files, called <code>A.py</code> and <code>B.py</code>, their contents are as below:</p> <p><code>A.py</code></p> <pre><code>import B __global__ = 'not set yet' class Data: __var__ = 'not set yet' def init(): global __global__ __global__ = 'value set for global' Data.__var__ = 'value set for class var' if __name__ == '__main__': init() t = B.Test() t.display() print '============================' print 'global(in A): ' + __global__ print 'class var(in A): ' + Data.__var__ </code></pre> <p><code>B.py</code></p> <pre><code>import A class Test: def __init__(self): self.glo = A.__global__ self.var = A.Data.__var__ def display(self): print 'global: ' + self.glo print 'class var: ' + self.var print '============================' print 'global(in B): ' + A.__global__ print 'class var(in B): ' + A.Data.__var__ </code></pre> <p>then I ran <code>python A.py</code>, the output is as following:</p> <pre><code>global: not set yet class var: not set yet ============================ global(in B): not set yet class var(in B): not set yet ============================ global(in A): value set for global class var(in A): value set for class var </code></pre> <p>In my opinion, the first 4 outputs should coincide with the last 2 outputs, but the fact is not, seems the value is set during class and method definition and cannot be changed. It is very different from many other languages, such as Java. So, could any one help explain this, or paste me some links to help understand it? And is there any workaround to solve this?</p> <p>Thanks in advance,</p> <p>Kelvin</p> <p>==============edit==============</p> <p>Thanks @icktoofay, @hop and @RocketDonkey, after I tried the code below, I do find the root cause:</p> <pre><code>import sys import A import __main__ __global__ = 'not set yet' class Data: __var__ = 'not set yet' def init(): global __global__ __global__ = 'value set for global' Data.__var__ = 'value set for class var' if __name__ == '__main__': init() for key in sys.modules.keys(): if key in ['__main__', 'A']: print key + ' : ' + sys.modules[key].__file__ print '====================' print 'A: ' + A.__global__ print 'A: ' + A.Data.__var__ print '====================' print __main__.__global__ print __main__.Data.__var__ </code></pre> <p>the output is:</p> <pre><code>__main__ : A.py A : D:\test\python\A.py ==================== A: not set yet A: not set yet ==================== value set for global value set for class var </code></pre> <p>It is because file <code>A.py</code> was imported twice, one is named <code>__main__</code> and another is named <code>A</code>, the values were changed in module <code>__main__</code>, but for module <code>B</code>, the value is got from module <code>A</code>, so the value is not changed.</p> <p>I do need to dive deeper for module importing of python. :-D</p>
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    1. COdo not use circular imports
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    2. CO@hop yes, I do know this is not a good programming habit, but currently I just want to know the 'why', and I will modify my code and remove the circular import.
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    3. COYour `str(sys.modules[key])` might be okay for debugging, but it's much better to instead just use `sys.modules[key].__file__`.
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