Generic types (classes)

A generic type in Python is a class, that inherits the typing.Generic class. It can be used to define arbitrary type parameters, which can be given a value using a subscription operator [].

Arbitrary types / type variables can be created using the typing.TypeVar class.

Note

Since Python 3.12, the typing.TypeVar no longer requires manual definition and can be omitted.

from typing import Generic, TypeVar


T = TypeVar('T')  # From Python 3.12 forward this is optional

class MyClass(Generic[T]):
    def __init__(self, a: int, b: T):
        ...


my_instance1: MyClass[float] = MyClass(5, 5.5)
my_instance2: MyClass[str] = MyClass(5, "Some text")
my_instance3: MyClass[int] = MyClass(5, 0)

In the above example, the T is a type variable. It is also a type parameter inside MyClass. When instances are created, the variables are annotated with a type subscription (e. g., MyClass[int]).

While Python itself doesn’t care about the types given in the subscription, and just ignores incorrect types, TkClassWizard resolves these subscripted types into the __init__ function of a class, allowing the definition of arbitrary types with the use of type generics.

If we take the above example and pop it into TkClassWizard with MyClass having a float annotation, we would get the following types in the New <type> options:

Listing 1 Generic type, given float as type parameter

from typing import Generic, TypeVar

import tkclasswiz as wiz
import tkinter as tk
import tkinter.ttk as ttk

T = TypeVar('T')  # From Python 3.12 forward this is optional

class MyClass(Generic[T]):
    def __init__(self, a: int, b: T):
        ...

root = tk.Tk("Test")

combo = wiz.ComboBoxObjects(root)
combo.pack(fill=tk.X, padx=5)

def open():
    window = wiz.ObjectEditWindow()  # The object definition window / wizard
    window.open_object_edit_frame(
        MyClass[float],
        combo
    )  # Open the actual frame

ttk.Button(text="Define", command=open).pack()
root.mainloop()