翻译强力驱动I don't understand the following from pep-0404
In Python 3, implicit relative imports within packages are no longer available - only absolute imports and explicit relative imports are supported. In addition, star imports (e.g. from x import *) are only permitted in module level code.
What is a relative import? In what other places star import was allowed in python2? Please explain with examples.
Relative import happens whenever you are importing a package relative to the current script/package.
Consider the following tree for example:
mypkg ├―― base.py └―― derived.pyNow, your derived.py requires something from base.py. In Python 2, you could do it like this (in derived.py):
from base import BaseThingPython 3 no longer supports that since it's not explicit whether you want the 'relative' or 'absolute' base. In other words, if there was a Python package named base installed in the system, you'd get the wrong one.
Instead it requires you to use explicit imports which explicitly specify location of a module on a path-alike basis. Your derived.py would look like:
from .base import BaseThingThe leading . says 'import base from module directory'; in other words, .base maps to ./base.py.
Similarly, there is .. prefix which goes up the directory hierarchy like ../ (with ..mod mapping to ../mod.py), and then ... which goes two levels up (../../mod.py) and so on.
Please however note that the relative paths listed above were relative to directory where current module (derived.py) resides in, not the current working directory.
@BrenBarn has already explained the star import case. For completeness, I will have to say the same ;).
For example, you need to use a few math functions but you use them only in a single function. In Python 2 you were permitted to be semi-lazy:
def sin_degrees(x): from math import * return sin(degrees(x))Note that it already triggers a warning in Python 2:
a.py:1: SyntaxWarning: import * only allowed at module level def sin_degrees(x):In modern Python 2 code you should and in Python 3 you have to do either:
def sin_degrees(x): from math import sin, degrees return sin(degrees(x))or:
from math import * def sin_degrees(x): return sin(degrees(x))For relative imports see the documentation. A relative import is when you import from a module relative to that module's location, instead of absolutely from sys.path.
As for import *, Python 2 allowed star imports within functions, for instance:
>>> def f(): ... from math import * ... print sqrtA warning is issued for this in Python 2 (at least recent versions). In Python 3 it is no longer allowed and you can only do star imports at the top level of a module (not inside functions or classes).
To support both Python 2 and Python 3, use explicit relative imports as below. They are relative to the current module. They have been supported starting from 2.5.
from .sister import foo import .brother from ..aunt import bar import ..uncle