导入语句python3中的更改

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.py

Now, your derived.py requires something from base.py. In Python 2, you could do it like this (in derived.py):

from base import BaseThing

Python 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 BaseThing

The 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 sqrt

A 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
from . import brother
from ..aunt import bar
from .. import uncle

Added another case to Michał Górny’s answer:

Note that relative imports are based on the name of the current module. Since the name of the main module is always “__main__“, modules intended for use as the main module of a Python application must always use absolute imports.