Does Python have an immutable list?

Question

Does python have immutable lists?

Suppose I wish to have the functionality of an ordered collection of elements, but which I want to guarantee will not change, how can this be implemented? Lists are ordered but they can be mutated.

Answer 1

Yes. It's called a tuple.

So, instead of [1,2] which is a list and which can be mutated, (1,2) is a tuple and cannot.

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Further Information:

A one-element tuple cannot be instantiated by writing (1), instead, you need to write (1,). This is because the interpreter has various other uses for parentheses.

You can also do away with parentheses altogether: 1,2 is the same as (1,2)

Note that a tuple is not exactly an immutable list. Click here to read more about the differences between lists and tuples

Answer 2

Here is an ImmutableList implementation. The underlying list is not exposed in any direct data member. Still, it can be accessed using the closure property of the member function. If we follow the convention of not modifying the contents of closure using the above property, this implementation will serve the purpose. Instance of this ImmutableList class can be used anywhere a normal python list is expected.

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()