How to prevent a module from being imported twice?

Question

When writing python modules, is there a way to prevent it being imported twice by the client codes? Just like the c/c++ header files do:

#ifndef XXX #define XXX ... #endif 

Thanks very much!

Answer 1

Python modules aren't imported multiple times. Just running import two times will not reload the module. If you want it to be reloaded, you have to use the reload statement. Here's a demo

foo.py is a module with the single line

print("I am being imported") 

And here is a screen transcript of multiple import attempts.

   >>> import foo    Hello, I am being imported    >>> import foo # Will not print the statement    >>> reload(foo) # Will print it again    Hello, I am being imported 

Answer 2

Imports are cached, and only run once. Additional imports only cost the lookup time in sys.modules.