Python內置函數(68)——__import__

英文文檔：

__import__(name, globals=None, locals=None, fromlist=(), level=0)

This function is invoked by the import statement. It can be replaced (by importing the builtins module and assigning to builtins.__import__) in order to change semantics of the import statement, but doing so is strongly discouraged as it is usually simpler to use import hooks (see PEP 302) to attain the same goals and does not cause issues with code which assumes the default import implementation is in use. Direct use of __import__() is also discouraged in favor of importlib.import_module().

The function imports the module name, potentially using the given globals and locals to determine how to interpret the name in a package context. The fromlist gives the names of objects or submodules that should be imported from the module given by name. The standard implementation does not use its locals argument at all, and uses its globals only to determine the package context of the import statement.

level specifies whether to use absolute or relative imports. 0 (the default) means only perform absolute imports. Positive values for level indicate the number of parent directories to search relative to the directory of the module calling __import__() (see PEP 328 for the details).

When the name variable is of the form package.module, normally, the top-level package (the name up till the first dot) is returned, not the module named by name. However, when a non-empty fromlist argument is given, the module named by name is returned.

說明：

　　1. 函數功能用於動態的導入模塊，主要用於反射或者延遲載入模塊。

　　2. __import__(module)相當於import module

　　先定義兩個模塊mian.py和index.py，兩個文件在同一目錄下：

#index.py

print ('index')

def sayHello():
    print('hello index')

def sayHelloZhCn():
    print('你好 index')

#mian.py
print ('main')

index = __import__('index')
dir(index)
index.sayHello()
index.sayHelloZhCn()

　　執行main.py，可以證實動態載入了index.py，__import__返回的模塊也是index模塊

C:\Users\Admin\Documents\Python3\importtest>python main.py
main
index
hello index
你好 index

　　3. __import__(package.module)相當於from package import name，如果fromlist不傳入值，則返回package對應的模塊，如果fromlist傳入值，則返回package.module對應的模塊。

　　先定義archives包，其中包含user和role兩個模塊：

#__index__.py
print ('archives.__index__')

def sayHello():
    print('hello archives')

#user.py
print ('user')

def sayHello():
    print('hello user')

#role.py
print ('role')

def sayHello():
    print('hello role')

　　結構如下：

　　修改mian.py：

#main.py
print ('main')

archives = __import__('archives')
archives.sayHello()
archives.user

　　執行main.py，可以證實動態載入了archives包，__import__返回的模塊也是archives模塊

C:\Users\Admin\Documents\Python3\importtest>python main.py
main
archives.__index__
hello archives
Traceback (most recent call last):
  File "main.py", line 5, in <module>
    archives.user
AttributeError: module 'archives' has no attribute 'user'

　　修改mian.py：

#main.py
print ('main')

archives = __import__('archives.user')
archives.sayHello()
print(archives.user)

　　執行main.py，可以證實動態載入了archives包的user模塊，__import__返回的模塊也是archives模塊

C:\Users\Admin\Documents\Python3\importtest>python main.py
main
archives.__index__
user
hello archives
<module 'archives.user' from 'C:\\Users\\Admin\\Documents\\Python3\\import
test\\archives\\user.py'>

　　修改mian.py：

#main.py
print ('main')

archives = __import__('archives.user',fromlist = ('user',))
archives.sayHello()
print(archives)

　　執行main.py，可以證實動態載入了archives包的user模塊，__import__返回的模塊是user模塊

C:\Users\Admin\Documents\Python3\importtest>python main.py
main
archives.__index__
user
hello user
<module 'archives.user' from 'C:\\Users\\Admin\\Documents\\Python3\\import
test\\archives\\user.py'>

　　4. level參數，指定是使用絕對導入還是相對導入。 0(預設值)表示只執行絕對導入。