import - Python importing

Question

I have a file, myfile.py, which imports Class1 from file.py and file.py contains imports to different classes in file2.py, file3.py, file4.py.

In my myfile.py, can I access these classes or do I need to again import file2.py, file3.py, etc.?

Does Python automatically add all the imports included in the file I imported, and can I use them automatically?

Answer 1

Best practice is to import every module that defines identifiers you need, and use those identifiers as qualified by the module's name; I recommend using from only when what you're importing is a module from within a package. The question has often been discussed on SO.

Importing a module, say moda, from many modules (say modb, modc, modd, ...) that need one or more of the identifiers moda defines, does not slow you down: moda's bytecode is loaded (and possibly build from its sources, if needed) only once, the first time moda is imported anywhere, then all other imports of the module use a fast path involving a cache (a dict mapping module names to module objects that is accessible as sys.modules in case of need... if you first import sys, of course!-).