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".
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.
2a. Quoting from "Learning Python: Mark Lutz" : In Python, cross-file module
linking
is not resolved until such import statements are **executed at runtime**.
import being a statement, can come anywhere a statement can, so a module can be
selectively imported depending upon the conditions at runtime.
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.
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).
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.
But wouldn't that violate the *same picture* requirement?
Probably my example is not that perfect.
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