¿Python tiene una lista inmutable?

Question

¿Tiene Python listas inmutables?

Supongamos que deseo tener la funcionalidad de una colección ordenada de elementos, pero que quiero garantizar que no cambiará, ¿cómo se puede implementar esto? Las listas están ordenadas pero pueden mutarse.

Answer 1

Sí. Se llama tuple.

Así, en lugar de [1,2] los cuales es un list y que puede ser mutado, (1,2) es un tuple y no puede.

---

información:

Un elemento de una sola tuple no se pueden crear instancias escribiendo (1), en cambio, tiene que escribir (1,). Esto se debe a que el intérprete tiene otros usos para paréntesis.

También puede acabar con los paréntesis: 1,2 es lo mismo que (1,2)

Tenga en cuenta que una tupla no es exactamente una lista inmutable. Haga clic aquí para leer más sobre el differences between lists and tuples

Answer 2

Aquí hay una implementación de ImmutableList. La lista subyacente no está expuesta en ningún miembro directo de datos. Aún así, se puede acceder utilizando la propiedad de cierre de la función miembro. Si seguimos la convención de no modificar el contenido del cierre utilizando la propiedad anterior, esta implementación servirá para el propósito. La instancia de esta clase ImmutableList se puede usar en cualquier lugar donde se espere una lista python normal.

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() 

Answer 3

Pero si hay una tupla de tuplas matrices y, a continuación, la matriz dentro de una tupla puede ser modificado.

>>> a 
([1, 2, 3], (4, 5, 6)) 

>>> a[0][0] = 'one' 

>>> a 
(['one', 2, 3], (4, 5, 6)) 

Answer 4

En lugar de tuple, puede usar frozenset. frozenset crea un conjunto inmutable. puede usar list como miembro de frozenset y acceder a cada elemento de la lista dentro de frozenset usando single for loop.