Python內建函數——__import__ 的使用方法，python__import_

Python內建函數——__import__ 的使用方法，python__import_

__import__() 函數用於動態載入類和函數 。

如果一個模組經常變化就可以使用 __import__() 來動態載入。

文法

__import__ 文法：

__import__(name[, globals[, locals[, fromlist[, level]]]])

參數說明：

name -- 模組名

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. 函數功能用於動態匯入模組，主要用於反射或者消極式載入模組。

　　2. __import__(module)相當於import module

先定義兩個模組mian.py和index.py，兩個檔案在同一目錄下：

#index.pyprint ('index')def sayHello():  print('hello index')def sayHelloZhCn():  print('你好 index')

#mian.pyprint ('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__返回的模組也是index模組

C:\Users\Admin\Documents\Python3\importtest>python main.pymainindexhello 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對應的模組，如果fromlist傳入值，則返回package.module對應的模組。

先定義archives包，其中包含user和role兩個模組：

#__index__.pyprint ('archives.__index__')def sayHello():  print('hello archives')

#user.pyprint ('user')def sayHello():  print('hello user')

#role.pyprint ('role')def sayHello():  print('hello role')

結構如下：

修改mian.py：

#main.pyprint ('main')archives = __import__('archives')archives.sayHello()archives.user

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

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.pyprint ('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模組，__import__返回的模組也是archives模組

C:\Users\Admin\Documents\Python3\importtest>python main.pymainarchives.__index__userhello archives<module 'archives.user' from 'C:\\Users\\Admin\\Documents\\Python3\\importtest\\archives\\user.py'>

修改mian.py：

#main.pyprint ('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模組，__import__返回的模組是user模組

C:\Users\Admin\Documents\Python3\importtest>python main.pymainarchives.__index__userhello user<module 'archives.user' from 'C:\\Users\\Admin\\Documents\\Python3\\importtest\\archives\\user.py'>

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

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