python3上的模块导入不是python2

Question

I have a project with the following file structure:

test/
    test.py
    extra/
        stuff.py
        __init.py__

The code for test.py is simply:

import sys
sys.path.append("/path/to/test")
import extra

This happily runs using python3 test.py , but will not run in python 2.7, and I'm completely stumped having read a lot of questions on the topic.

The error given by python 2.7 is

 Traceback (most recent call last):
   File "test.py", line 3, in <module>
    import extra
ImportError: No module named extra

Thanks for the help!

Answer 1

This is a copy-paste from: http://python3porting.com/differences.html

In Python 2, if you have a package called mypackage and that contains a module called csv.py, it would hide the csv module from the standard library. The code import csv would within mypackage import the local file, and importing from the standard library would become tricky.

In Python 3, this has changed so that import csv would import from the standard library, and to import the local csv.py file you need to write from . import csv and from csv import my_csv needs to be changed to from .csv import my_csv. These are called “relative imports”, and there is also a syntax to import from one level up module above; from .. import csv.

If you to support both Python 2 and Python 3 without 2to3 the from . and from .. syntax has been available since Python 2.5, together with a from future import absolute_import statement that changes the behavior to the Python 3 behavior.

If you need to support Python 2.4 or earlier you have to spell out the whole package name so import csv becomes from mypkg import csv and from csv import my_csv becomes from mypckg.csv import my_csv. For clarity and readability I would avoid relative imports if you can and always spell out the whole path.

2to3 will check if your imports are local and change them.

In short, Python2 allows you to get away with relative imports, python3 doesn't.