“import *"到底是什么?进口?

问题描述

In Python, what exactly does import * import? Does it import __init__.py found in the containing folder?

For example, is it necessary to declare from project.model import __init__, or is from project.model import * sufficient?

解决方案

The "advantage" of from xyz import * as opposed to other forms of import is that it imports everything (well, almost... [see (a) below] everything) from the designated module under the current module. This allows using the various objects (variables, classes, methods...) from the imported module without prefixing them with the module's name. For example

>>> from math import *
>>>pi
3.141592653589793
>>>sin(pi/2)
>>>1.0

This practice (of importing * into the current namespace) is however discouraged because it

• provides the opportunity for namespace collisions (say if you had a variable name pi prior to the import)
• may be inefficient, if the number of objects imported is big
• doesn't explicitly document the origin of the variable/method/class (it is nice to have this "self documentation" of the program for future visit into the code)

Typically we therefore limit this import * practice to ad-hoc tests and the like. As pointed out by @Denilson-Sá-Maia, some libraries such as (e.g. pygame) have a sub-module where all the most commonly used constants and functions are defined and such sub-modules are effectively designed to be imported with import *. Other than with these special sub-modules, it is otherwise preferable to ...:

explicitly import a few objects only

>>>from math import pi
>>>pi
>>>3.141592653589793
>>> sin(pi/2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'sin' is not defined

or import the module under its own namespace (or an alias thereof, in particular if this is a long name, and the program references its objects many times)

  >>>import math
  >>>math.pi
  >>>3.141592653589793
  etc..


  >>>import math as m  #bad example math being so short and standard...
  >>>m.pi
  >>>3.141592653589793
  etc..

See the Python documentation on this topic

(a) Specifically, what gets imported with from xyz import * ?
if xyz module defines an __all__ variable, it will import all the names defined in this sequence, otherwise it will import all names, except these which start with an underscore.

Note Many libraries have sub-modules. For example the standard library urllib includes sub-modules like urllib.request, urllib.errors, urllib.response etc. A common point of confusion is that

from urllib import *

would import all these sub-modules. That is NOT the case: one needs to explicitly imports these separately with, say, from urllib.request import * etc. This incidentally is not specific to import *, plain import will not import sub-modules either (but of course, the * which is often a shorthand for "everything" may mislead people in thinking that all sub-modules and everything else would be imported).