On Thu, Feb 17, 2011 at 8:44 AM, andrea crotti <andrea.crott...@gmail.com> wrote: > Suppose I have a function which uses a few external libraries, > and this function is only called once every 10 executions. > > Does it make sense to import these libraries for the whole module?
Yes. Having all the imports in one place keeps the code organized and makes figuring out dependencies easier. Also, IIRC, importing something within a function can lead to problems if threads are involved. Further, it's just plain conventional. PEP 8 (the Python style guide) states: "Imports are always put at the top of the file, just after any module comments and docstrings, and before module globals and constants." > import sys > > def fun(): > import x, y, z > > Something like this is acceptable/good practice in the scenario I was > talking about? I would argue no, per PEP 8. > Does it make any difference for performances too? Yes, of course. Importing a module multiple times (as would happen if fun() is called multiple times) is obviously slower than just importing it once, although the 2nd and subsequent imports will be faster as Python will just return another reference to the previously imported instance of the module rather than importing it again from scratch. There are also local variable vs. module-global variable speed differences involved. However, "Premature optimization is the root of all evil." Have you checked whether the imports are taking a significant amount of execution time? Odds are that they aren't. When optimizing, do so on the basis of empirical data; run your program under a profiler, and let the results guide your optimization process. Cheers, Chris -- http://blog.rebertia.com -- http://mail.python.org/mailman/listinfo/python-list
