不正确地使用__new__来生成类实例？
问题描述
我正在创建一些类来处理不同类型的文件共享(nf、afp、s3、本地磁盘)等中的文件名。我得到一个标识数据源(即"nfs://192.168.1.3"或"s3://mybucket/data")等的字符串作为用户输入。
我从一个具有公共代码的基类中子类化特定的文件系统。我感到困惑的地方是对象的创建。我拥有的是以下内容：
import os
class FileSystem(object):
class NoAccess(Exception):
pass
def __new__(cls,path):
if cls is FileSystem:
if path.upper().startswith('NFS://'):
return super(FileSystem,cls).__new__(Nfs)
else:
return super(FileSystem,cls).__new__(LocalDrive)
else:
return super(FileSystem,cls).__new__(cls,path)
def count_files(self):
raise NotImplementedError
class Nfs(FileSystem):
def __init__ (self,path):
pass
def count_files(self):
pass
class LocalDrive(FileSystem):
def __init__(self,path):
if not os.access(path, os.R_OK):
raise FileSystem.NoAccess('Cannot read directory')
self.path = path
def count_files(self):
return len([x for x in os.listdir(self.path) if os.path.isfile(os.path.join(self.path, x))])
data1 = FileSystem('nfs://192.168.1.18')
data2 = FileSystem('/var/log')
print type(data1)
print type(data2)
print data2.count_files()
我认为这将是__new__的一个很好的用法，但我读到的大多数关于它的用法的帖子都不鼓励这样做。有没有更容易被接受的方法来解决这个问题？
解决方案
我不认为使用__new__()做您想做的事情是不合适的。换句话说，我不同意accepted answer对此question的说法，即工厂职能始终是实现这一目标的最佳方式。
如果您真的想避免使用它，那么唯一的选择是元类或单独的factory函数/方法(但是请参阅下面的Python3.6+更新)。如果有可用的选择，使__new__()方法成为一个方法--因为它在默认情况下是静态的--是非常明智的方法。
也就是说，下面是我认为的代码的改进版本。我已经添加了几个类方法来帮助自动查找所有子类。它们支持最重要的更好的方式--现在添加子类不需要修改__new__()方法。这意味着它现在很容易扩展，因为它有效地支持您所说的虚拟构造函数。
还可以使用类似的实现将实例的创建从__new__()方法移到单独的(静态)工厂方法中-因此，从某种意义上讲，所示的技术只是对可扩展的泛型工厂函数进行编码的一种相对简单的方式，而不管它被赋予什么名称。
# Works in Python 2 and 3.
import os
import re
class FileSystem(object):
class NoAccess(Exception): pass
class Unknown(Exception): pass
# Regex for matching "xxx://" where x is any non-whitespace character except for ":".
_PATH_PREFIX_PATTERN = re.compile(r's*([^:]+)://')
@classmethod
def _get_all_subclasses(cls):
""" Recursive generator of all class' subclasses. """
for subclass in cls.__subclasses__():
yield subclass
for subclass in subclass._get_all_subclasses():
yield subclass
@classmethod
def _get_prefix(cls, s):
""" Extract any file system prefix at beginning of string s and
return a lowercase version of it or None when there isn't one.
"""
match = cls._PATH_PREFIX_PATTERN.match(s)
return match.group(1).lower() if match else None
def __new__(cls, path):
""" Create instance of appropriate subclass using path prefix. """
path_prefix = cls._get_prefix(path)
for subclass in cls._get_all_subclasses():
if subclass.prefix == path_prefix:
# Using "object" base class method avoids recursion here.
return object.__new__(subclass)
else: # No subclass with matching prefix found (& no default defined)
raise FileSystem.Unknown(
'path "{}" has no known file system prefix'.format(path))
def count_files(self):
raise NotImplementedError
class Nfs(FileSystem):
prefix = 'nfs'
def __init__ (self, path):
pass
def count_files(self):
pass
class LocalDrive(FileSystem):
prefix = None # Default when no file system prefix is found.
def __init__(self, path):
if not os.access(path, os.R_OK):
raise FileSystem.NoAccess('Cannot read directory')
self.path = path
def count_files(self):
return sum(os.path.isfile(os.path.join(self.path, filename))
for filename in os.listdir(self.path))
if __name__ == '__main__':
data1 = FileSystem('nfs://192.168.1.18')
data2 = FileSystem('c:/') # Change as necessary for testing.
print(type(data1).__name__) # -> Nfs
print(type(data2).__name__) # -> LocalDrive
print(data2.count_files()) # -> <some number>
Python3.6+更新
上面的代码可以在Python2和3.x中运行。然而，在Python3.6中，名为__init_subclass__()的object中添加了一个新的类方法，它可以使用它自动创建一个子类的注册表，从而使查找子类变得更简单，而不必像上面的_get_all_subclasses()方法那样递归地检查每个子类。
我从PEP 487 -- Simpler customisation of class creation提案中的Subclass registration部分得到了使用__init_subclass__()来完成此操作的想法。由于该方法将由所有基类的子类继承，因此子类的注册也将自动完成(与仅直接子类相反)-它完全不需要像_get_all_subclasses()这样的方法。
# Requires Python 3.6+
import os
import re
class FileSystem(object):
class NoAccess(Exception): pass
class Unknown(Exception): pass
# Pattern for matching "xxx://" # x is any non-whitespace character except for ":".
_PATH_PREFIX_PATTERN = re.compile(r's*([^:]+)://')
_registry = {} # Registered subclasses.
@classmethod
def __init_subclass__(cls, /, path_prefix, **kwargs):
super().__init_subclass__(**kwargs)
cls._registry[path_prefix] = cls # Add class to registry.
@classmethod
def _get_prefix(cls, s):
""" Extract any file system prefix at beginning of string s and
return a lowercase version of it or None when there isn't one.
"""
match = cls._PATH_PREFIX_PATTERN.match(s)
return match.group(1).lower() if match else None
def __new__(cls, path):
""" Create instance of appropriate subclass. """
path_prefix = cls._get_prefix(path)
subclass = cls._registry.get(path_prefix)
if subclass:
return object.__new__(subclass)
else: # No subclass with matching prefix found (and no default).
raise cls.Unknown(
f'path "{path}" has no known file system prefix')
def count_files(self):
raise NotImplementedError
class Nfs(FileSystem, path_prefix='nfs'):
def __init__ (self, path):
pass
def count_files(self):
pass
class Ufs(Nfs, path_prefix='ufs'):
def __init__ (self, path):
pass
def count_files(self):
pass
class LocalDrive(FileSystem, path_prefix=None): # Default file system.
def __init__(self, path):
if not os.access(path, os.R_OK):
raise self.NoAccess(f'Cannot read directory {path!r}')
self.path = path
def count_files(self):
return sum(os.path.isfile(os.path.join(self.path, filename))
for filename in os.listdir(self.path))
if __name__ == '__main__':
data1 = FileSystem('nfs://192.168.1.18')
data2 = FileSystem('c:/') # Change as necessary for testing.
data4 = FileSystem('ufs://192.168.1.18')
print(type(data1)) # -> <class '__main__.Nfs'>
print(type(data2)) # -> <class '__main__.LocalDrive'>
print(f'file count: {data2.count_files()}') # -> file count: <some number>
try:
data3 = FileSystem('c:/foobar') # A non-existent directory.
except FileSystem.NoAccess as exc:
print(f'{exc} - FileSystem.NoAccess exception raised as expected')
else:
raise RuntimeError("Non-existent path should have raised Exception!")
try:
data4 = FileSystem('foobar://42') # Unregistered path prefix.
except FileSystem.Unknown as exc:
print(f'{exc} - FileSystem.Unknown exception raised as expected')
else:
raise RuntimeError("Unregistered path prefix should have raised Exception!")
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