TypeError: '_SpecialGenericAlias' object does not support item assignment

Question

I am coding Constraint Satisfaction Problem in Artificial Intelligence using Python:

from typing import Generic, TypeVar, Dict, List, Optional
from abc import ABC, abstractmethod
V = TypeVar('V')
D = TypeVar('D')

class Constraint(Generic[V,D], ABC):
    def __init__(self, variables: List[V]) -> None:
        self.variables = variables

    @abstractmethod
    def satisfied(self, assignment: Dict[V, D]) -> bool:
        ...


class CSP(Generic[V, D]):
    def __init__(self, variables: List[V], domains: Dict[V, List[D]]) -> None:
        self.variables: List[V] = variables
        self.domain: Dict[V, List[D]] = domains
        self.constraints: Dict[V, List[Constraint[V, D]]] = {}
        for variable in self.variables:
            self.constraints[variable] = []

    def add_constraint(self, constraint: Constraint[V, D]) -> None:
        for variable in constraint.variables:
            if variable not in constraint.variables:
                print("Variable in Constraint, not in CSP")
            else:
                self.constraints[variable].append(constraint)

    def consistent(self, variable: V, assignment: Dict[V, D]) -> bool:
        for constraint in self.constraints[variable]:
            if not constraint.satisfied(assignment):
                return False
        return True

    def backtracking_search(self, assignment: Dict[V, D] = {}) -> Optional[Dict[V, D]]:
        if len(assignment) == len(self.variables):
            return assignment
        unassigned: List[V] = [v for v in self.variables if v not in assignment]
        first: V = unassigned[0]
        for value in self.domains[first]:
            local_assignment = assignment.copy()
            local_assignment[first] = value
            if self.consistent(first, local_assignment):
                result: Optional[Dict[V, D]] = self.backtracking_search(local_assignment)
                if result is not None:
                    return result
        return None


class MapColoringConstraint(Constraint[str, str]):
    def __init__(self, place1: str, place2: str) -> None:
        super().__init__([place1, place2])
        self.place1: str = place1
        self.place2: str = place2

    def satisfied(self, assignment: Dict[V, D]) -> bool:
        if self.place1 not in assignment or self.place2 not in assignment:
            return True
        return assignment[self.place1] != assignment[self.place2]


variables: List[str] = ["DJ", "SO", "ET", "KE", "UG", "TA", "RW", "BU"]
domains = Dict[str, List[str]] = {}
for variable in variables:
    domains[variable].append("Red")
    domains[variable].append("Green")
    domains[variable].append("Blue")
csp: CSP[str, str] = CSP(variables, domains)
edges = [("DJ", "SO"), ("DJ", "ET"),
         ("SO", "KE"), ("SO", "ET"),
         ("ET", "KE"), ("KE", "UG"),
         ("KE", "TA"), ("UG", "TA"),
         ("UG", "RW"), ("TA", "BU"),
         ("TA", "RW"), ("RW", "BU")]
for i, j in edges:
    print(i, j)

The error occurs as:

line 64, in domains = Dict[str, List[str]] = {} TypeError: '_SpecialGenericAlias' object does not support item assignment

I don't exactly know the problem that is occurring, i do have to declare the dictionary 'domains' as it is necessary for the question, can anyone help me out?

Answer 1

The problem was assigning a DataType to a variable:

domains = Dict[str, List[str]] = {}

The variable 'domains' could not be assigned a DataType and an assignment of type dictionary. That is why the error was occurring. However, I fixed it using:

domains: Dict[str, List[str]] = {}

Now, it works fine. Sorry for the inconvenience, I really hope it'll help someone one day.