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))
python derived.py
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
import .brother is invalid syntax for both python 2 and 3
import ..uncle. Fixed. Oh, what have I been thinking... probably got distracted by the knights who say Ni!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.
__package__. And the module level __package__ attribute is writable, so even when __name__ == "__main__" you can specify where you want relative imports to resolve by setting a __package__ attribute.