On Sun, 6 Mar 2016 07:20 pm, alien2u...@gmail.com wrote: > Based on the input from members, and my subsequent reading the > textbook/tutorials, let me summarize my understanding of "why subsequent > imports of same module are designed to be effect-less".
That is not correct. Imports are not a no-op, they always have an effect. The effect is that they bind the module to a name in the current namespace. What import does NOT do is *reload the module* from disk. But that's not the same thing as being "effect-less". > 1. Imports are costly affair, as it involves > - finding the module's file > - compile it to byte code (if needed) > - run the module's code to build the objects it defines. Correct. > 2a. Quoting from "Learning Python: Mark Lutz" : In Python, cross-file > module linking > is not resolved until such import statements are **executed at > runtime**. I don't understand that statement. > import being a statement, can come anywhere a statement can, so a module > can be selectively imported depending upon the conditions at runtime. Correct. > 2b. Inter-dependencies between modules: For example, A (main module) > imports B, C, D > and E. B again imports C and E, D imports B and C. > > If import statements are processed entirely from scratch each time, it > would amount to lot of **redundant (and costly)** work. > > Secondly there could be dangerous? side-effects of reprocessing imports > from scratch. Correct. > Let us say, in above example, module B does some initialization of > variables, or things like resetting a file for further work during the > session. These variables and file are updated during the course of > execution of A. > > Much later via some conditional path, module B is imported again, and BOOM > (were import processed from scratch again). Correct. This would be a bad thing. > In this case - we are respecting that every import of a module should give > same picture (assuming it is not modified across two different invocation > during the program execution). > > An argument here could be: to code module B in such a way that those > initializations don't happen again, but just once - check before resetting > etc. during the import. How would you do that? How could a .py file know if it has already been loaded? -- Steven -- https://mail.python.org/mailman/listinfo/python-list
