多个构造函数:Python方式? [英] Multiple constructors: the Pythonic way?
问题描述
我有一个容纳数据的容器类。创建容器时,有多种传递数据的方法。
- 传递包含数据的文件
- 直接通过参数传递数据
- 不传递数据;只需创建一个空容器
在Java中,我将创建三个构造函数。如果可能的话,这就是Python的样子:
类容器:
def __init__ (自我):
self.timestamp = 0
self.data = []
self.metadata = {}
def __init __(self,file):
f = file.open()
self.timestamp = f.get_timestamp()
self.data = f.get_data()
self.metadata = f.get_metadata()
def __init __(self,timestamp,data,metadata):
self.timestamp =时间戳记
self.data =数据
self.metadata =元数据
在Python中,我看到了三个明显的解决方案,但它们都不是很漂亮的:
A :使用关键字参数:
def __init __(self,** kwargs ):
如果在kwargs中使用'file':
...
elif在kwargs中使用'timestamp'在kwargs中使用'data'而在kwargs中使用'metadata':
...
其他:
...创建空容器
B :使用默认参数:
def __init __(self,file = None,timestamp = None,data = None,metadata = None):
如果文件:
...
elif时间戳以及数据和元数据:
...
其他:
...创建空容器
C :仅提供构造函数来创建空容器。提供使用不同来源的数据填充容器的方法。
def __init __(self):
self.timestamp = 0
self.data = []
self.metadata = {}
def add_data_from_file(file):
...
def add_data(时间戳,数据,元数据):
...
解决方案A和B是基本相同。我不喜欢if / else,尤其是因为我必须检查是否提供了此方法所需的所有参数。如果要通过第四种方法扩展代码以添加数据,则A比B灵活。
解决方案C似乎是最好的方法,但是用户必须知道他需要哪种方法。例如:如果他不知道 args
是什么,他就不能做 c = Container(args)
。
最Python化的解决方案是什么?
不能有多个 Python
中具有相同名称的方法。不支持函数重载-与 Java
不同-
使用默认参数或 ** kwargs
和 * args
参数。
您可以创建静态方法或类 @staticmethod
或 @classmethod
装饰器的方法返回类的实例,或添加其他构造函数。 / p>
我建议您这样做:
F类:
def __init __(self,timestamp = 0,data = None,metadata = None):
self.timestamp = timestamp
self.data = list()如果data为none,则数据
self.metadata = dict()如果元数据为其他元数据
@classmethod
def from_file(cls,path):
_file = cls.get_file(path)
时间戳= _file.get_timestamp()
数据= _file.get_data()
元数据= _file.get_metadata()
return cls(时间戳,数据,元数据)
@classmet hod
def from_metadata(cls,时间戳,数据,元数据):
return cls(时间戳,数据,元数据)
@staticmethod
def get_file(path):
#...
通过
⚠永远不要将可变类型作为python中的默认值。 ⚠
>
参见此处。
I have a container class that holds data. When the container is created, there are different methods to pass data.
- Pass a file which contains the data
- Pass the data directly via arguments
- Don't pass data; just create an empty container
In Java, I would create three constructors. Here's how it would look like if it were possible in Python:
class Container:
def __init__(self):
self.timestamp = 0
self.data = []
self.metadata = {}
def __init__(self, file):
f = file.open()
self.timestamp = f.get_timestamp()
self.data = f.get_data()
self.metadata = f.get_metadata()
def __init__(self, timestamp, data, metadata):
self.timestamp = timestamp
self.data = data
self.metadata = metadata
In Python, I see three obvious solutions, but none of them is pretty:
A: Using keyword arguments:
def __init__(self, **kwargs):
if 'file' in kwargs:
...
elif 'timestamp' in kwargs and 'data' in kwargs and 'metadata' in kwargs:
...
else:
... create empty container
B: Using default arguments:
def __init__(self, file=None, timestamp=None, data=None, metadata=None):
if file:
...
elif timestamp and data and metadata:
...
else:
... create empty container
C: Only provide constructor to create empty containers. Provide methods to fill containers with data from different sources.
def __init__(self):
self.timestamp = 0
self.data = []
self.metadata = {}
def add_data_from_file(file):
...
def add_data(timestamp, data, metadata):
...
Solutions A and B are basically the same. I don't like doing the if/else, especially since I have to check if all arguments required for this method were provided. A is a bit more flexible than B if the code is ever to be extended by a fourth method to add data.
Solution C seems to be the nicest, but the user has to know which method he requires. For example: he cant do c = Container(args)
if he doesn't know what args
is.
Whats the most Pythonic solution?
You can't have multiple methods with same name in Python
. Function overloading - unlike in Java
- isn't supported.
Use default parameters or **kwargs
and *args
arguments.
You can make static methods or class methods with the @staticmethod
or @classmethod
decorator to return an instance of your class, or to add other constructors.
I advise you to do:
class F:
def __init__(self, timestamp=0, data=None, metadata=None):
self.timestamp = timestamp
self.data = list() if data is None else data
self.metadata = dict() if metadata is None else metadata
@classmethod
def from_file(cls, path):
_file = cls.get_file(path)
timestamp = _file.get_timestamp()
data = _file.get_data()
metadata = _file.get_metadata()
return cls(timestamp, data, metadata)
@classmethod
def from_metadata(cls, timestamp, data, metadata):
return cls(timestamp, data, metadata)
@staticmethod
def get_file(path):
# ...
pass
⚠ Never have mutable types as defaults in python. ⚠ See here.
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