python-scala-generics-variance

name	python-scala-generics-variance
description	Translating Python generics, variance annotations, protocols, and type variables to Scala type parameters, variance annotations, and traits. Use when converting Python Generic[T], TypeVar with covariant/contravariant, Protocol classes, or higher-kinded type simulations to Scala.

Python Generics & Variance to Scala

Python Generic[T] → Scala trait/class with type parameter

Python:

T = TypeVar("T")
class BaseTokenizer(ABC, Generic[T]):
    @abstractmethod
    def tokenize(self, value: T) -> Token: ...

Scala:

trait BaseTokenizer[T] {
  def tokenize(value: T): Token
  def tokenizeBatch(values: Iterable[T]): Iterator[Token] =
    values.iterator.map(tokenize)
}

Key: Python's ABC + Generic[T] becomes a Scala trait[T]. Abstract methods need no abstract keyword in traits — just leave them unimplemented.

Variance Annotations

Python:

T_co = TypeVar("T_co", covariant=True)
T_contra = TypeVar("T_contra", contravariant=True)

Scala:

class TokenContainer[+A](items: Seq[A])     // covariant
class TokenSink[-A]                          // contravariant
class BivariantHandler[A](default: A)        // invariant

Python Protocol → Scala trait (structural typing)

Python protocols are structural types. In Scala, use regular traits:

trait Tokenizable {
  def toToken: String
}

Bounded TypeVars → Scala type bounds or overloading

Python TypeVar("NumericT", int, float, Decimal) constrains to specific types. In Scala, use overloaded methods, union-style sealed traits, or context bounds depending on context.

For TypeVar("StrOrBytes", str, bytes):

• If runtime dispatch is needed, use pattern matching on Any or overloaded methods

Higher-Kinded Type Simulation → Scala class with type param

Python's TokenFunctor simulation translates naturally:

class TokenFunctor[A](private val value: A) {
  def map[B](f: A => B): TokenFunctor[B] = new TokenFunctor(f(value))
  def flatMap[B](f: A => TokenFunctor[B]): TokenFunctor[B] = f(value)
  def get: A = value
}

Note: getOrElse with null-checking becomes Option-based in idiomatic Scala.