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
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
其他回答
你可以简单地:
STRING_CONSTANT = "hi"
NUMBER_CONSTANT = 89
希望这能让一切变得简单
我使用冻结数据类声明常量值,如下所示:
from dataclasses import dataclass
@dataclass(frozen=True)
class _Const:
SOME_STRING = 'some_string'
SOME_INT = 5
Const = _Const()
# In another file import Const and try
print(Const.SOME_STRING) # ITS OK!
Const.SOME_INT = 6 # dataclasses.FrozenInstanceError: cannot assign to field 'SOME_INT'
在我的例子中,我需要不可变字节数组来实现包含许多文字数字的加密库,我想确保这些数字是常量。
这个答案是有效的,但是尝试重赋bytearray元素不会引发错误。
def const(func):
'''implement const decorator'''
def fset(self, val):
'''attempting to set a const raises `ConstError`'''
class ConstError(TypeError):
'''special exception for const reassignment'''
pass
raise ConstError
def fget(self):
'''get a const'''
return func()
return property(fget, fset)
class Consts(object):
'''contain all constants'''
@const
def C1():
'''reassignment to C1 fails silently'''
return bytearray.fromhex('deadbeef')
@const
def pi():
'''is immutable'''
return 3.141592653589793
常量是不可变的,但是常量bytearray赋值默默失败:
>>> c = Consts()
>>> c.pi = 6.283185307179586 # (https://en.wikipedia.org/wiki/Tau_(2%CF%80))
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "consts.py", line 9, in fset
raise ConstError
__main__.ConstError
>>> c.C1[0] = 0
>>> c.C1[0]
222
>>> c.C1
bytearray(b'\xde\xad\xbe\xef')
一种更强大、更简单,甚至可能更“python化”的方法涉及使用memoryview对象(<= python-2.6中的缓冲区对象)。
import sys
PY_VER = sys.version.split()[0].split('.')
if int(PY_VER[0]) == 2:
if int(PY_VER[1]) < 6:
raise NotImplementedError
elif int(PY_VER[1]) == 6:
memoryview = buffer
class ConstArray(object):
'''represent a constant bytearray'''
def __init__(self, init):
'''
create a hidden bytearray and expose a memoryview of that bytearray for
read-only use
'''
if int(PY_VER[1]) == 6:
self.__array = bytearray(init.decode('hex'))
else:
self.__array = bytearray.fromhex(init)
self.array = memoryview(self.__array)
def __str__(self):
return str(self.__array)
def __getitem__(self, *args, **kwargs):
return self.array.__getitem__(*args, **kwargs)
ConstArray项赋值是一个TypeError:
>>> C1 = ConstArray('deadbeef')
>>> C1[0] = 0
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'ConstArray' object does not support item assignment
>>> C1[0]
222
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
注意:这是一个糟糕的想法和糟糕的实现。此外,它只适用于最后的小例子,一个完整的实现将意味着大量的工作,这是我太懒了。而且,在Python 3.8之前,审计钩子可能是不可用的。
我基本上回答了另一个问题,结果和这个问题有关。它的思想是,你可以利用审计钩子来捕捉每一行的执行,解析代码对象,如果它满足某些条件(例如某个前缀并且已经定义过一次),你可以抛出一个错误。
你可能不得不支持其他赋值类型(例如,对于导入的东西,可能对于函数内部的局部变量,解包等),不使用全局变量,因为字典可以很容易地修改,实际上调查这是否安全,接受这个实现将对你的整个应用程序造成的性能损失,确保它在REPL之外工作,在ipython内部工作,等等等等。不管怎样,我们开始吧:
>>> import sys
>>> import ast
>>> import dis
>>> import types
>>>
>>>
>>> def hook(name, tup):
... if name == "exec" and tup:
... if tup and isinstance(tup[0], types.CodeType):
... code = tup[0]
... store_instruction_arg = None
... instructions = [dis.opname[op] for op in code.co_code]
...
... for i, instruction in enumerate(instructions):
... if instruction == "STORE_NAME":
... store_instruction_arg = code.co_code[i + 1]
... break
...
... if store_instruction_arg is not None:
... var_name = code.co_names[store_instruction_arg]
... if var_name in globals():
... raise Exception("Cannot re-assign variable")
...
>>>
>>> sys.addaudithook(hook)
>>>
>>> a = '123'
>>> a = 456
Traceback (most recent call last):
File "<stdin>", line 16, in hook
Exception: Cannot re-assign variable
>>>
>>> a
'123'
如果你以这种方式结束,你不应该,除了修复和泛化代码,你可能会想要找到一种方法,只让一些东西不变,例如,只有那些有特殊前缀的对象或只有对象有一些注释。