python typing: typed dictionary or defaultdict extending classes

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  • Techknowledgy :

Now, the following is working for me:

from collections
import defaultdict
from typing
import Tuple, Dict, DefaultDict, Set, List, NewType

Key = NewType('Key', str)
Lang = NewType('Lang', str)
Translation = NewType('Translation', str)
PLIndex = NewType('PLIndex', int)

FormsDict = DefaultDict[PLIndex, Translation]
TranslationsDict = DefaultDict[Lang, FormsDict]
TermsDict = DefaultDict[Key, TranslationsDict]

terms: TermsDict = defaultdict(# TermsDict lambda: defaultdict(# TranslationsDict lambda: defaultdict(# FormsDict lambda: Translation("") # Default value ""(as Translation))))

I have tested this with mypy --strict and it passes validation. Using this with defaultdict and still passing validation, it seems that you will need cast

from typing
import cast

   cast(TranslationsDict, {
      Lang("en_GB.UTF-8"): cast(FormsDict, {
         PLIndex(100): Translation("key1")



defaultdict(<function <lambda> at 0x107d31cb0>, {
    'key1': defaultdict(<function <lambda>.<locals>.<lambda> at 0x107d31d40>, {
        'en_GB.UTF-8': {100: 'key1'}})})

Suggestion : 2

This module defines several types that are subclasses of pre-existing standard library classes which also extend Generic to support type variables inside []. These types became redundant in Python 3.9 when the corresponding pre-existing classes were enhanced to support [].,Introducing types.GenericAlias and the ability to use standard library classes as generic types,Generic[T] as a base class defines that the class LoggedVar takes a single type parameter T . This also makes T valid as a type within the class body.,A user-defined generic class can have ABCs as base classes without a metaclass conflict. Generic metaclasses are not supported. The outcome of parameterizing generics is cached, and most types in the typing module are hashable and comparable for equality.

def greeting(name: str) - > str:
   return 'Hello ' + name
Vector = list[float]

def scale(scalar: float, vector: Vector) - > Vector:
   return [scalar * num
      for num in vector

# typechecks;
a list of floats qualifies as a Vector.
new_vector = scale(2.0, [1.0, -4.2, 5.4])
import Sequence

ConnectionOptions = dict[str, str]
Address = tuple[str, int]
Server = tuple[Address, ConnectionOptions]

def broadcast_message(message: str, servers: Sequence[Server]) - > None:

   # The static type checker will treat the previous type signature as
# being exactly equivalent to this one.
def broadcast_message(
      message: str,
      servers: Sequence[tuple[tuple[str, int], dict[str, str]]]) - > None:
from typing
import NewType

UserId = NewType('UserId', int)
some_id = UserId(524313)
def get_user_name(user_id: UserId) - > str:

   # typechecks
user_a = get_user_name(UserId(42351))

# does not typecheck;
an int is not a UserId
user_b = get_user_name(-1)
# 'output'
is of type 'int', not 'UserId'
output = UserId(23413) + UserId(54341)

Suggestion : 3

Last Updated : 08 Jul, 2022


Dictionary: {
   1: 'Geeks',
   2: 'For',
   3: 'Geeks'
{1: 'Geeks', 2: 'For', 3: 'Geeks'}
Traceback(most recent call last):
   File "/home/", line 11, in
KeyError: 4

Suggestion : 4

December 13, 2021February 22, 2022

Let’s take a look at how to access a key’s value from a Python dictionary:

# Accessing a Key 's Value from a Python Dictionary
data = {
   'Name': 'Nik',
   'Location': 'Toronto',
   'Age': 33

# Returns: Nik

Now, let’s take a look at what happens when we try to access a key that doesn’t exist:

# Accessing a Missing Key 's Value from a Python Dictionary
data = {
   'Name': 'Nik',
   'Location': 'Toronto',
   'Age': 33

# Raises: KeyError: 'Hobbies'

We can use the dictionary .get() method to prevent a KeyError from being raised when a dictionary key doesn’t exist. If we try to access a key’s value that doesn’t exist using the .get() method, the method simply returns the None value. Let’s see what this looks like:

# Using.get() to Prevent a KeyError
data = {
   'Name': 'Nik',
   'Location': 'Toronto',
   'Age': 33

# Returns: None

Python dictionaries also provide a method, .defaultvalue(), which allows us to set, well, a default value for a key. This method sets the default value when a key doesn’t exist and returns that value. Let’s see how we can use that:

# Using.setdefault() to Set a Default Value
for a Missing Key
data = {
   'Name': 'Nik',
   'Location': 'Toronto',
   'Age': 33
data.setdefault('Hobbies', None)


We can make use of the class by importing it in our program:

# Importing the defaultdict Class
from collections
import defaultdict

Suggestion : 5

January 3, 2022 19 min read 5429

When declaring a variable in statically-typed languages like C and Java, you are mandated to declare the data type of the variable. As a result, you cannot assign a value that does not conform to the data type you specified for the variable. For example, if you declare a variable to be an integer, you can’t assign a string value to it at any point in time.

int x = 4;
x = "hello"; // this would trigger a type error

In Python, you can define a variable with a type hint using the following syntax:

variable_name: type = value

Let’s look at the following variable:

name = "rocket”

In Python, you can read the type hints defined on variables using the __annotations__ dictionary:

>>> name: str = "rocket"
>>> __annotations__
{'name': <class 'str'>}

As mentioned earlier, the Python interpreter does not enforce types, so defining a variable with a wrong type won’t trigger an error:

>>> name: int = "rocket" >>>