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?
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
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).