如何在Python中声明常量?
在Java中,我们做:
public static final String CONST_NAME = "Name";
如何在Python中声明常量?
在Java中,我们做:
public static final String CONST_NAME = "Name";
当前回答
您可以使用namedtuple作为一种变通方法来有效地创建一个常量,其工作方式与Java中的静态final变量(Java“常量”)相同。作为一种变通方法,它是优雅的。(更优雅的方法是简单地改进Python语言——什么样的语言可以让您重新定义math.pi?——但我跑题了。)
(当我写这篇文章时,我意识到这个问题的另一个答案提到了namedtuple,但我将在这里继续,因为我将展示一种更接近于您在Java中所期望的语法,因为不需要像namedtuple那样创建命名类型。)
跟着你的例子,你会记得在Java中,我们必须在某个类中定义常量;因为你没有提到类名,我们称它为Foo。下面是Java类:
public class Foo {
public static final String CONST_NAME = "Name";
}
这里是等效的Python。
from collections import namedtuple
Foo = namedtuple('_Foo', 'CONST_NAME')('Name')
我想在这里补充的关键点是,您不需要单独的Foo类型(“匿名命名元组”会很好,尽管这听起来有点矛盾),所以我们将命名元组命名为_Foo,这样它就不会转义到导入模块中。
这里的第二点是,我们立即创建了nametuple的一个实例,称其为Foo;没有必要在单独的步骤中执行此操作(除非您想这样做)。现在你可以做你在Java中可以做的事情:
>>> Foo.CONST_NAME
'Name'
但你不能给它赋值:
>>> Foo.CONST_NAME = 'bar'
…
AttributeError: can't set attribute
确认:我认为我发明了命名元组方法,但后来我看到其他人给出了类似的答案(尽管不那么紧凑)。然后我还注意到Python中的“命名元组”是什么?,这就指出了sys。version_info现在是一个命名元组,所以可能Python标准库早就提出了这个想法。
注意,不幸的是(这仍然是Python),你可以完全删除整个Foo赋值:
>>> Foo = 'bar'
(facepalm)
但至少我们阻止了福星。CONST_NAME值,这比什么都没有好。祝你好运。
其他回答
在Python中,不能将变量或值声明为常量。
为了让程序员知道变量是常量,通常用大写:
CONST_NAME = "Name"
要在常量发生变化时引发异常,请参阅Alex Martelli的《Python中的常量》。注意,这在实践中并不常用。
从Python 3.8开始,有一个类型。最后一个变量注释,它将告诉静态类型检查器(如myypy)您的变量不应该被重新分配。这是最接近于Java的final。然而,它实际上并不能阻止重新分配:
from typing import Final
a: Final[int] = 1
# Executes fine, but mypy will report an error if you run mypy on this:
a = 2
在Python中,人们使用命名约定,例如私有方法使用__method,受保护方法使用_method。
所以用同样的方式,你可以简单地将常量声明为全大写,例如:
MY_CONSTANT = "one"
如果你想让这个常量永远不变,你可以挂钩到属性访问并做一些技巧,但更简单的方法是声明一个函数:
def MY_CONSTANT():
return "one"
唯一的问题是,在任何地方都必须执行MY_CONSTANT(),但MY_CONSTANT = "one"在Python中是正确的方式(通常)。
你也可以使用namedtuple()来创建常量:
>>> from collections import namedtuple
>>> Constants = namedtuple('Constants', ['pi', 'e'])
>>> constants = Constants(3.14, 2.718)
>>> constants.pi
3.14
>>> constants.pi = 3
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: can't set attribute
我为python const写了一个util lib: Kkconst - pypi 支持str, int, float, datetime
const字段实例将保持其基类型行为。
例如:
from __future__ import print_function
from kkconst import (
BaseConst,
ConstFloatField,
)
class MathConst(BaseConst):
PI = ConstFloatField(3.1415926, verbose_name=u"Pi")
E = ConstFloatField(2.7182818284, verbose_name=u"mathematical constant") # Euler's number"
GOLDEN_RATIO = ConstFloatField(0.6180339887, verbose_name=u"Golden Ratio")
magic_num = MathConst.GOLDEN_RATIO
assert isinstance(magic_num, ConstFloatField)
assert isinstance(magic_num, float)
print(magic_num) # 0.6180339887
print(magic_num.verbose_name) # Golden Ratio
更多详细用法,你可以阅读pypi url: Pypi或github
我们可以创建一个描述符对象。
class Constant:
def __init__(self,value=None):
self.value = value
def __get__(self,instance,owner):
return self.value
def __set__(self,instance,value):
raise ValueError("You can't change a constant")
1)如果我们想在实例级使用常量,那么:
class A:
NULL = Constant()
NUM = Constant(0xFF)
class B:
NAME = Constant('bar')
LISTA = Constant([0,1,'INFINITY'])
>>> obj=A()
>>> print(obj.NUM) #=> 255
>>> obj.NUM =100
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: You can't change a constant
2)如果我们只想在类级别上创建常量,我们可以使用元类作为常量(描述符对象)的容器;所有下降的类将继承我们的常量(我们的描述符对象),没有任何可以修改的风险。
# metaclass of my class Foo
class FooMeta(type): pass
# class Foo
class Foo(metaclass=FooMeta): pass
# I create constants in my metaclass
FooMeta.NUM = Constant(0xff)
FooMeta.NAME = Constant('FOO')
>>> Foo.NUM #=> 255
>>> Foo.NAME #=> 'FOO'
>>> Foo.NUM = 0 #=> ValueError: You can't change a constant
如果我创建一个Foo的子类,这个类将继承常量,而不可能修改它们
class Bar(Foo): pass
>>> Bar.NUM #=> 255
>>> Bar.NUM = 0 #=> ValueError: You can't change a constant
不幸的是,Python还没有常数,这是耻辱。ES6已经为JavaScript添加了支持常量(https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Statements/const),因为它在任何编程语言中都非常有用。 正如在Python社区的其他回答中所回答的那样,使用约定的用户大写变量作为常量,但它不能防止代码中的任意错误。 如果您愿意,您可能会发现一个有用的单文件解决方案 (参见文档字符串如何使用它)。
文件constants.py
import collections
__all__ = ('const', )
class Constant(object):
"""
Implementation strict constants in Python 3.
A constant can be set up, but can not be changed or deleted.
Value of constant may any immutable type, as well as list or set.
Besides if value of a constant is list or set, it will be converted in an immutable type as next:
list -> tuple
set -> frozenset
Dict as value of a constant has no support.
>>> const = Constant()
>>> del const.temp
Traceback (most recent call last):
NameError: name 'temp' is not defined
>>> const.temp = 1
>>> const.temp = 88
Traceback (most recent call last):
...
TypeError: Constanst can not be changed
>>> del const.temp
Traceback (most recent call last):
...
TypeError: Constanst can not be deleted
>>> const.I = ['a', 1, 1.2]
>>> print(const.I)
('a', 1, 1.2)
>>> const.F = {1.2}
>>> print(const.F)
frozenset([1.2])
>>> const.D = dict()
Traceback (most recent call last):
...
TypeError: dict can not be used as constant
>>> del const.UNDEFINED
Traceback (most recent call last):
...
NameError: name 'UNDEFINED' is not defined
>>> const()
{'I': ('a', 1, 1.2), 'temp': 1, 'F': frozenset([1.2])}
"""
def __setattr__(self, name, value):
"""Declaration a constant with value. If mutable - it will be converted to immutable, if possible.
If the constant already exists, then made prevent againt change it."""
if name in self.__dict__:
raise TypeError('Constanst can not be changed')
if not isinstance(value, collections.Hashable):
if isinstance(value, list):
value = tuple(value)
elif isinstance(value, set):
value = frozenset(value)
elif isinstance(value, dict):
raise TypeError('dict can not be used as constant')
else:
raise ValueError('Muttable or custom type is not supported')
self.__dict__[name] = value
def __delattr__(self, name):
"""Deny against deleting a declared constant."""
if name in self.__dict__:
raise TypeError('Constanst can not be deleted')
raise NameError("name '%s' is not defined" % name)
def __call__(self):
"""Return all constans."""
return self.__dict__
const = Constant()
if __name__ == '__main__':
import doctest
doctest.testmod()
如果这还不够,请参阅完整的测试用例。
import decimal
import uuid
import datetime
import unittest
from ..constants import Constant
class TestConstant(unittest.TestCase):
"""
Test for implementation constants in the Python
"""
def setUp(self):
self.const = Constant()
def tearDown(self):
del self.const
def test_create_constant_with_different_variants_of_name(self):
self.const.CONSTANT = 1
self.assertEqual(self.const.CONSTANT, 1)
self.const.Constant = 2
self.assertEqual(self.const.Constant, 2)
self.const.ConStAnT = 3
self.assertEqual(self.const.ConStAnT, 3)
self.const.constant = 4
self.assertEqual(self.const.constant, 4)
self.const.co_ns_ta_nt = 5
self.assertEqual(self.const.co_ns_ta_nt, 5)
self.const.constant1111 = 6
self.assertEqual(self.const.constant1111, 6)
def test_create_and_change_integer_constant(self):
self.const.INT = 1234
self.assertEqual(self.const.INT, 1234)
with self.assertRaisesRegexp(TypeError, 'Constanst can not be changed'):
self.const.INT = .211
def test_create_and_change_float_constant(self):
self.const.FLOAT = .1234
self.assertEqual(self.const.FLOAT, .1234)
with self.assertRaisesRegexp(TypeError, 'Constanst can not be changed'):
self.const.FLOAT = .211
def test_create_and_change_list_constant_but_saved_as_tuple(self):
self.const.LIST = [1, .2, None, True, datetime.date.today(), [], {}]
self.assertEqual(self.const.LIST, (1, .2, None, True, datetime.date.today(), [], {}))
self.assertTrue(isinstance(self.const.LIST, tuple))
with self.assertRaisesRegexp(TypeError, 'Constanst can not be changed'):
self.const.LIST = .211
def test_create_and_change_none_constant(self):
self.const.NONE = None
self.assertEqual(self.const.NONE, None)
with self.assertRaisesRegexp(TypeError, 'Constanst can not be changed'):
self.const.NONE = .211
def test_create_and_change_boolean_constant(self):
self.const.BOOLEAN = True
self.assertEqual(self.const.BOOLEAN, True)
with self.assertRaisesRegexp(TypeError, 'Constanst can not be changed'):
self.const.BOOLEAN = False
def test_create_and_change_string_constant(self):
self.const.STRING = "Text"
self.assertEqual(self.const.STRING, "Text")
with self.assertRaisesRegexp(TypeError, 'Constanst can not be changed'):
self.const.STRING += '...'
with self.assertRaisesRegexp(TypeError, 'Constanst can not be changed'):
self.const.STRING = 'TEst1'
def test_create_dict_constant(self):
with self.assertRaisesRegexp(TypeError, 'dict can not be used as constant'):
self.const.DICT = {}
def test_create_and_change_tuple_constant(self):
self.const.TUPLE = (1, .2, None, True, datetime.date.today(), [], {})
self.assertEqual(self.const.TUPLE, (1, .2, None, True, datetime.date.today(), [], {}))
with self.assertRaisesRegexp(TypeError, 'Constanst can not be changed'):
self.const.TUPLE = 'TEst1'
def test_create_and_change_set_constant(self):
self.const.SET = {1, .2, None, True, datetime.date.today()}
self.assertEqual(self.const.SET, {1, .2, None, True, datetime.date.today()})
self.assertTrue(isinstance(self.const.SET, frozenset))
with self.assertRaisesRegexp(TypeError, 'Constanst can not be changed'):
self.const.SET = 3212
def test_create_and_change_frozenset_constant(self):
self.const.FROZENSET = frozenset({1, .2, None, True, datetime.date.today()})
self.assertEqual(self.const.FROZENSET, frozenset({1, .2, None, True, datetime.date.today()}))
with self.assertRaisesRegexp(TypeError, 'Constanst can not be changed'):
self.const.FROZENSET = True
def test_create_and_change_date_constant(self):
self.const.DATE = datetime.date(1111, 11, 11)
self.assertEqual(self.const.DATE, datetime.date(1111, 11, 11))
with self.assertRaisesRegexp(TypeError, 'Constanst can not be changed'):
self.const.DATE = True
def test_create_and_change_datetime_constant(self):
self.const.DATETIME = datetime.datetime(2000, 10, 10, 10, 10)
self.assertEqual(self.const.DATETIME, datetime.datetime(2000, 10, 10, 10, 10))
with self.assertRaisesRegexp(TypeError, 'Constanst can not be changed'):
self.const.DATETIME = None
def test_create_and_change_decimal_constant(self):
self.const.DECIMAL = decimal.Decimal(13123.12312312321)
self.assertEqual(self.const.DECIMAL, decimal.Decimal(13123.12312312321))
with self.assertRaisesRegexp(TypeError, 'Constanst can not be changed'):
self.const.DECIMAL = None
def test_create_and_change_timedelta_constant(self):
self.const.TIMEDELTA = datetime.timedelta(days=45)
self.assertEqual(self.const.TIMEDELTA, datetime.timedelta(days=45))
with self.assertRaisesRegexp(TypeError, 'Constanst can not be changed'):
self.const.TIMEDELTA = 1
def test_create_and_change_uuid_constant(self):
value = uuid.uuid4()
self.const.UUID = value
self.assertEqual(self.const.UUID, value)
with self.assertRaisesRegexp(TypeError, 'Constanst can not be changed'):
self.const.UUID = []
def test_try_delete_defined_const(self):
self.const.VERSION = '0.0.1'
with self.assertRaisesRegexp(TypeError, 'Constanst can not be deleted'):
del self.const.VERSION
def test_try_delete_undefined_const(self):
with self.assertRaisesRegexp(NameError, "name 'UNDEFINED' is not defined"):
del self.const.UNDEFINED
def test_get_all_defined_constants(self):
self.assertDictEqual(self.const(), {})
self.const.A = 1
self.assertDictEqual(self.const(), {'A': 1})
self.const.B = "Text"
self.assertDictEqual(self.const(), {'A': 1, 'B': "Text"})
优点: 1. 访问整个项目的所有常量 2. 严格控制常数值
缺乏: 1. 不支持自定义类型和'dict'类型
注:
使用Python3.4和Python3.5进行测试(我使用“tox”进行测试) 测试环境:
.
$ uname -a
Linux wlysenko-Aspire 3.13.0-37-generic #64-Ubuntu SMP Mon Sep 22 21:28:38 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux