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POHow to detect or prevent multiple instances of the same modules in Python?
 

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  1. POHow to detect or prevent multiple instances of the same modules in Python?
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    <p>In short: in Python it is too easy to create multiple instances of the same module, each instance having its own set of global variables.</p> <p>I need to add a check to the module to detect such multiple instantiation and raise an exception.</p> <p>My problem is the same as this one: <a href="https://stackoverflow.com/questions/4897232/a-module-is-imported-multiple-times">Module imported multiple times</a></p> <p>This is the smallest directory structure to reproduce the problem:</p> <pre><code>/test/a/__init__.py /test/a/aa.py: print "aa: __name__: ", __name__ /test/b/b.py: from a import aa import aa </code></pre> <p>Then</p> <pre><code>export PYTHONPATH=/test:/test/a python /test/b/b.py </code></pre> <p>prints:</p> <pre><code>aa: __name__: a.aa aa: __name__: aa </code></pre> <p>So, module aa.py was imported twice with different names.</p> <p>Needless to say that module aa.py will get 2 sets of global variables, which screws up all the logic inside this module.</p> <p>Of course, in the trivial example above it is easy to detect error by eyes, but in complex project with multiple subdirectories, these errors keep popping up regularly.</p> <p>So, I need some really global variable or a process-wide store or something like that. Any idea?</p> <p>Edit: Bibhas asked for an example of multiple instances of the same global variable. Here it is:</p> <pre><code>/test/a/__init__.py /test/a/aa.py: print "aa: __name__: ", __name__ import thread import time test_var = __name__ def test(): for i in range(0,5): print "aa thread: test_var: ", test_var time.sleep(1) thread.start_new_thread( test, () ) /test/b/b.py: print "b: __name__: ", __name__ from a import aa import aa import time time.sleep(10) </code></pre> <p>Now running</p> <pre><code>export PYTHONPATH=/test:/test/a python /test/b/b.py </code></pre> <p>prints:</p> <pre><code>aa: __name__: a.aa aa: __name__: aa aa thread: test_var: aa aa thread: test_var: a.aa aa thread: test_var: aa aa thread: test_var: a.aa ... </code></pre> <p>So, it is clear that there are 2 instances of variable test_var. If I will try to implement a singleton in this module, there will be 2 instances of this singleton, etc.</p> <p>Edit2: So, solution suggested by Guy L is something like:</p> <pre><code>/test/a/aa.py: import os if "aa.py" in os.environ: raise Exception("Duplicate instantiation of aa.py") os.environ["aa.py"] = __name__ </code></pre> <p>It seems to work OK (as long as I do not call import on multiple threads). Anybody have a better one?</p>
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    1. CO`aa.py will get 2 sets of global variables`. Can you give an example of this? From what I see Python reads aa.py twice, hence executed the print statement twice, b.py still got just one global variable `aa`. `__name__` just shows the module name while being imported. It might as well be `__main__` depending on how you execute it.
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    2. COMaybe to use def setV(x): global v, v =x and getV(): global v, return v. I'm pretty sure it will not work since the module is imported twice
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