import可以有一下几种形式：
将A导入到当前的namespace
变种可以时 import A.B.C.D 如果是这种情况 ABC必须是Package 而D只能是一个模块 或者时一个Pakcage
将B导入到当前的namespace 当改变B时 对B作出的修改不会影响到A.B
- X = __import__('X') (运行时才知道X的名称)
将A中所有的public name引入到当前的namespace public name在这里指的是name明称不以_(下划线)开头的那些name
import vs loading
一个模块不管被imoprt多少次 它只会被load一次 如果有执行代码在这个模块里面 那么只有第一次被加载的时候会运行 之后import则不会去运行未包装的执行代码
What does python do to import a Module?
1, check sys.modules to see if module is already imported, if that's the case, python will use the existing module object as is.
2, otherwise, create a new , empty module object
3, insert that module in sys.modules
4, Load the module code object
5, Execute the module code object in the new module's namespace. All variables assigned by the code will be available via the module object.
Importing * From a Package
The only solution is for the package author to provide an explicit index of the
package. The import statement uses the following convention: if a package’s
__init__.py code defines a list named __all__, it is taken to be the
list of module names that should be imported when from package import * is
encountered. It is up to the package author to keep this list up-to-date when a
new version of the package is released. Package authors may also decide not to
support it, if they don’t see a use for importing * from their package.
If
__all__ is not defined, the statement
from sound.effects import *
does
not import all submodules from the package
sound.effects into the
current namespace; it only ensures that the package
sound.effects has
been imported (possibly running any initialization code in
__init__.py)
and then imports whatever names are defined in the package. This includes any
names defined (and submodules explicitly loaded) by
__init__.py. It
also includes any submodules of the package that were explicitly loaded by
previous
import statements. Consider this code:
import sound.effects.echo
import sound.effects.surround
from sound.effects import *
In this example, the
echo and
surround modules are imported in the
current namespace because they are defined in the
sound.effects package
when the
from...import statement is executed. (This also works when
__all__ is defined.)
