Python importing

问题:

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?

回答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!-).

回答2:

Python doesn't automatically introduce anything into the namespace of myfile.py, but you can access everything that is in the namespaces of all the other modules.

That is to say, if in file1.py you did from file2 import SomeClass and in myfile.py you did import file1, then you can access it within myfile as file1.SomeClass. If in file1.py you did import file2 and in myfile.py you did import file1, then you can access the class from within myfile as file1.file2.SomeClass. (These aren't generally the best ways to do it, especially not the second example.)

This is easily tested.

回答3:

In the myfile module, you can either do from file import ClassFromFile2 or from file2 import ClassFromFile2 to access ClassFromFile2, assuming that the class is also imported in file.

This technique is often used to simplify the API a bit. For example, a db.py module might import various things from the modules mysqldb, sqlalchemy and some other helpers. Than, everything can be accessed via the db module.

回答4:

If you are using wildcard import, yes, wildcard import actually is the way of creating new aliases in your current namespace for contents of the imported module. If not, you need to use the namespace of the module you have imported as usual.