python是否有不可变列表？
假设我希望拥有有序元素集合的功能，但我想保证不会改变，如何实现？列表是有序的，但它们可以变异。
答案 0 :(得分：80)
是。它被称为tuple。
因此，[1,2]不是list而是(1,2)而且可以变异，tuple是tuple而不是。{/ p>
更多信息：
无法通过编写(1)来实例化单元素(1,)，而是需要编写1,2。这是因为解释器具有括号的各种其他用途。
您也可以完全取消括号：(1,2)与{{1}}相同
请注意，元组不是完全不可变列表。点击此处阅读有关differences between lists and tuples
的更多信息答案 1 :(得分：4)
但是如果有一个数组和元组的元组，则可以修改元组内的数组。
>>> a
([1, 2, 3], (4, 5, 6))
>>> a[0][0] = 'one'
>>> a
(['one', 2, 3], (4, 5, 6))
答案 2 :(得分：3)
这是一个ImmutableList实现。基础列表不会在任何直接数据成员中公开。仍然可以使用成员函数的闭包属性访问它。如果我们遵循不使用上述属性修改闭包内容的约定，则此实现将用于此目的。这个ImmutableList类的实例可以在预期的普通python列表的任何地方使用。
from functools import reduce
__author__ = 'hareesh'
class ImmutableList:
"""
An unmodifiable List class which uses a closure to wrap the original list.
Since nothing is truly private in python, even closures can be accessed and
modified using the __closure__ member of a function. As, long as this is
not done by the client, this can be considered as an unmodifiable list.
This is a wrapper around the python list class
which is passed in the constructor while creating an instance of this class.
The second optional argument to the constructor 'copy_input_list' specifies
whether to make a copy of the input list and use it to create the immutable
list. To make the list truly immutable, this has to be set to True. The
default value is False, which makes this a mere wrapper around the input
list. In scenarios where the input list handle is not available to other
pieces of code, for modification, this approach is fine. (E.g., scenarios
where the input list is created as a local variable within a function OR
it is a part of a library for which there is no public API to get a handle
to the list).
The instance of this class can be used in almost all scenarios where a
normal python list can be used. For eg:
01. It can be used in a for loop
02. It can be used to access elements by index i.e. immList[i]
03. It can be clubbed with other python lists and immutable lists. If
lst is a python list and imm is an immutable list, the following can be
performed to get a clubbed list:
ret_list = lst + imm
ret_list = imm + lst
ret_list = imm + imm
04. It can be multiplied by an integer to increase the size
(imm * 4 or 4 * imm)
05. It can be used in the slicing operator to extract sub lists (imm[3:4] or
imm[:3] or imm[4:])
06. The len method can be used to get the length of the immutable list.
07. It can be compared with other immutable and python lists using the
>, <, ==, <=, >= and != operators.
08. Existence of an element can be checked with 'in' clause as in the case
of normal python lists. (e.g. '2' in imm)
09. The copy, count and index methods behave in the same manner as python
lists.
10. The str() method can be used to print a string representation of the
list similar to the python list.
"""
@staticmethod
def _list_append(lst, val):
"""
Private utility method used to append a value to an existing list and
return the list itself (so that it can be used in funcutils.reduce
method for chained invocations.
@param lst: List to which value is to be appended
@param val: The value to append to the list
@return: The input list with an extra element added at the end.
"""
lst.append(val)
return lst
@staticmethod
def _methods_impl(lst, func_id, *args):
"""
This static private method is where all the delegate methods are
implemented. This function should be invoked with reference to the
input list, the function id and other arguments required to
invoke the function
@param list: The list that the Immutable list wraps.
@param func_id: should be the key of one of the functions listed in the
'functions' dictionary, within the method.
@param args: Arguments required to execute the function. Can be empty
@return: The execution result of the function specified by the func_id
"""
# returns iterator of the wrapped list, so that for loop and other
# functions relying on the iterable interface can work.
_il_iter = lambda: lst.__iter__()
_il_get_item = lambda: lst[args[0]] # index access method.
_il_len = lambda: len(lst) # length of the list
_il_str = lambda: lst.__str__() # string function
# Following represent the >, < , >=, <=, ==, != operators.
_il_gt = lambda: lst.__gt__(args[0])
_il_lt = lambda: lst.__lt__(args[0])
_il_ge = lambda: lst.__ge__(args[0])
_il_le = lambda: lst.__le__(args[0])
_il_eq = lambda: lst.__eq__(args[0])
_il_ne = lambda: lst.__ne__(args[0])
# The following is to check for existence of an element with the
# in clause.
_il_contains = lambda: lst.__contains__(args[0])
# * operator with an integer to multiply the list size.
_il_mul = lambda: lst.__mul__(args[0])
# + operator to merge with another list and return a new merged
# python list.
_il_add = lambda: reduce(
lambda x, y: ImmutableList._list_append(x, y), args[0], list(lst))
# Reverse + operator, to have python list as the first operand of the
# + operator.
_il_radd = lambda: reduce(
lambda x, y: ImmutableList._list_append(x, y), lst, list(args[0]))
# Reverse * operator. (same as the * operator)
_il_rmul = lambda: lst.__mul__(args[0])
# Copy, count and index methods.
_il_copy = lambda: lst.copy()
_il_count = lambda: lst.count(args[0])
_il_index = lambda: lst.index(
args[0], args[1], args[2] if args[2] else len(lst))
functions = {0: _il_iter, 1: _il_get_item, 2: _il_len, 3: _il_str,
4: _il_gt, 5: _il_lt, 6: _il_ge, 7: _il_le, 8: _il_eq,
9: _il_ne, 10: _il_contains, 11: _il_add, 12: _il_mul,
13: _il_radd, 14: _il_rmul, 15: _il_copy, 16: _il_count,
17: _il_index}
return functions[func_id]()
def __init__(self, input_lst, copy_input_list=False):
"""
Constructor of the Immutable list. Creates a dynamic function/closure
that wraps the input list, which can be later passed to the
_methods_impl static method defined above. This is
required to avoid maintaining the input list as a data member, to
prevent the caller from accessing and modifying it.
@param input_lst: The input list to be wrapped by the Immutable list.
@param copy_input_list: specifies whether to clone the input list and
use the clone in the instance. See class documentation for more
details.
@return:
"""
assert(isinstance(input_lst, list))
lst = list(input_lst) if copy_input_list else input_lst
self._delegate_fn = lambda func_id, *args: \
ImmutableList._methods_impl(lst, func_id, *args)
# All overridden methods.
def __iter__(self): return self._delegate_fn(0)
def __getitem__(self, index): return self._delegate_fn(1, index)
def __len__(self): return self._delegate_fn(2)
def __str__(self): return self._delegate_fn(3)
def __gt__(self, other): return self._delegate_fn(4, other)
def __lt__(self, other): return self._delegate_fn(5, other)
def __ge__(self, other): return self._delegate_fn(6, other)
def __le__(self, other): return self._delegate_fn(7, other)
def __eq__(self, other): return self._delegate_fn(8, other)
def __ne__(self, other): return self._delegate_fn(9, other)
def __contains__(self, item): return self._delegate_fn(10, item)
def __add__(self, other): return self._delegate_fn(11, other)
def __mul__(self, other): return self._delegate_fn(12, other)
def __radd__(self, other): return self._delegate_fn(13, other)
def __rmul__(self, other): return self._delegate_fn(14, other)
def copy(self): return self._delegate_fn(15)
def count(self, value): return self._delegate_fn(16, value)
def index(self, value, start=0, stop=0):
return self._delegate_fn(17, value, start, stop)
def main():
lst1 = ['a', 'b', 'c']
lst2 = ['p', 'q', 'r', 's']
imm1 = ImmutableList(lst1)
imm2 = ImmutableList(lst2)
print('Imm1 = ' + str(imm1))
print('Imm2 = ' + str(imm2))
add_lst1 = lst1 + imm1
print('Liist + Immutable List: ' + str(add_lst1))
add_lst2 = imm1 + lst2
print('Immutable List + List: ' + str(add_lst2))
add_lst3 = imm1 + imm2
print('Immutable Liist + Immutable List: ' + str(add_lst3))
is_in_list = 'a' in lst1
print("Is 'a' in lst1 ? " + str(is_in_list))
slice1 = imm1[2:]
slice2 = imm2[2:4]
slice3 = imm2[:3]
print('Slice 1: ' + str(slice1))
print('Slice 2: ' + str(slice2))
print('Slice 3: ' + str(slice3))
imm1_times_3 = imm1 * 3
print('Imm1 Times 3 = ' + str(imm1_times_3))
three_times_imm2 = 3 * imm2
print('3 Times Imm2 = ' + str(three_times_imm2))
# For loop
print('Imm1 in For Loop: ', end=' ')
for x in imm1:
print(x, end=' ')
print()
print("3rd Element in Imm1: '" + imm1[2] + "'")
# Compare lst1 and imm1
lst1_eq_imm1 = lst1 == imm1
print("Are lst1 and imm1 equal? " + str(lst1_eq_imm1))
imm2_eq_lst1 = imm2 == lst1
print("Are imm2 and lst1 equal? " + str(imm2_eq_lst1))
imm2_not_eq_lst1 = imm2 != lst1
print("Are imm2 and lst1 different? " + str(imm2_not_eq_lst1))
# Finally print the immutable lists again.
print("Imm1 = " + str(imm1))
print("Imm2 = " + str(imm2))
# The following statemetns will give errors.
# imm1[3] = 'h'
# print(imm1)
# imm1.append('d')
# print(imm1)
if __name__ == '__main__':
main()
答案 3 :(得分：2)
您可以使用frozenset代替元组。 frozenset创建一个不可变集。您可以使用list作为frozenset的成员，并使用单个for循环访问frozenset中列表的每个元素。
答案 4 :(得分：1)
您可以使用两个元素的元组模拟Lisp样式的不可变单链列表（注意：这与any-element tuple answer不同，后者创建的元组的灵活性要差得多）：
nil = ()
cons = lambda ele, l: (ele, l)
例如对于列表[1, 2, 3]，您将具有以下内容：
l = cons(1, cons(2, cons(3, nil))) # (1, (2, (3, ())))
您的标准car和cdr函数很简单：
car = lambda l: l[0]
cdr = lambda l: l[1]
由于此列表是单链链接，因此附加在最前面的是O（1）。由于此列表是不可变的，因此如果列表中的基础元素也是不可变的，则可以安全地共享任何子列表以在另一个列表中重用。
答案 5 :(得分：0)
List和Tuple在工作方式上有所不同。
在LIST中，我们可以在创建后进行更改，但是如果您希望有序的序列在将来无法应用更改，则可以使用TUPLE。
更多信息：：
1) the LIST is mutable that means you can make changes in it after its creation
2) In Tuple, we can not make changes once it created
3) the List syntax is
abcd=[1,'avn',3,2.0]
4) the syntax for Tuple is
abcd=(1,'avn',3,2.0)
or abcd= 1,'avn',3,2.0 it is also correct