動態實例化python類的對象，類似於PHP new $ classname？

Question

如何實例化一個類，如果它的名字是作爲字符串變量給出的（即動態實例化類的對象）。或者，下面的PHP 5.3+代碼如何？

<?php 

namespace Foo; 

class Bar {}; 

$classname = 'Foo\Bar'; 
$bar = new $classname(); 

可以用python拼寫嗎？

另見

Does python have an equivalent to Java Class.forName()?

Answer 1

下面是我用來從完整課程路徑（例如foo.bar.baz.TheClass）類本身：

def get_class(class_path): 
    module_path, class_name = class_path.rsplit(".", 1) 

    try: 
     module = __import__(module_path, fromlist=[class_name]) 
    except ImportError: 
     raise ValueError("Module '%s' could not be imported" % (module_path,)) 

    try: 
     cls = getattr(module, class_name) 
    except AttributeError: 
     raise ValueError("Module '%s' has no class '%s'" % (module_path, class_name,)) 

    return cls 

用法：

>>> get_class('twisted.internet.nonexistant.Deferred') 

Traceback (most recent call last): 
    File "<pyshell#8>", line 1, in <module> 
    get_class('twisted.internet.nonexistant.Deferred') 
    File "<pyshell#1>", line 7, in get_class 
    raise ValueError("Module '%s' could not be imported" % (module_path,)) 
ValueError: Module 'twisted.internet.nonexistant' could not be imported 
>>> get_class('twisted.internet.defer.NoClass') 

Traceback (most recent call last): 
    File "<pyshell#14>", line 1, in <module> 
    get_class('twisted.internet.defer.NoClass') 
    File "<pyshell#13>", line 12, in get_class 
    raise ValueError("Module '%s' has no class '%s'" % (module_path, class_name,)) 
ValueError: Module 'twisted.internet.defer' has no class 'NoClass' 
>>> get_class('twisted.internet.defer.Deferred') 
<class twisted.internet.defer.Deferred at 0x02B25DE0> 

注意，這不一定返回一個類，它只是返回導入的模塊的屬性：

>>> get_class('twisted.internet') 
<module 'twisted.internet' from 'C:\Python26\lib\site-packages\twisted\internet\__init__.pyc'> 

Answer 2

foo.py

class Bar(object): 
    pass 

test.py

from importlib import import_module 

module = import_module('foo') 
BarCls = getattr(module, 'Bar') 
bar = BarCls()