Python有一个不可变的列表吗?

python是否有不可变的列表?

假设我希望有一个有序的元素集合的function,但是我想要保证的function不会改变,这怎么能实现? 列表是有序的,但它们可以被改变。

4 Solutions collect form web for “Python有一个不可变的列表吗?”

是。 这叫做tuple

所以,而不是[1,2]这是一个list ,可以被突变, (1,2)是一个tuple ,不能。


更多信息:

一个元素的tuple不能通过写(1)来实例化,而是需要写(1,) 。 这是因为口译员对括号有其他各种用途。

你也可以完全不用括号: 1,2(1,2)一样,

请注意,元组不完全是不可变的列表。 点击这里阅读更多关于列表和元组之间的区别

但是,如果有数组和元组的元组,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)) 

这是一个ImmutableList实现。 底层列表不会在任何直接数据成员中公开。 不过,它可以使用成员函数的闭包属性来访问。 如果我们遵循不使用上述属性修改封闭内容的约定,这个实现就是为了达到目的。 这个ImmutableList类的实例可以用在普通的Python列表的任何地方。

 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. (Eg, 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 ie 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. (eg '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() 

而不是元组,你可以使用frozenset。 frozenset创build一个不可变的集合。 您可以使用list作为frozenset的成员,并使用single for循环访问frozenset中的list的每个元素。

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