Difference between TypeVar('T', A, B) and TypeVar('T', bound=Union[A, B])

Question

I\'m struggling to get my head around the difference between the following two TypeVars

from typing impor         


        

Answer 1

After a bunch of reading, I believe mypy correctly raises the type-var error in the OP's question:

generics.py:31: error: Value of type variable "T" of "X" cannot be "AA"

See the below explanation.

---

Second Case: TypeVar("T", bound=Union[A, B])

I think @Michael0x2a's answer does a great job of describing what's happening. See that answer.

---

First Case: TypeVar("T", A, B)

The reason boils down to Liskov Substitution Principle (LSP), also known as behavioral subtyping. Explaining this is outside the scope of this answer, you will need to read up on + understanding the meaning of invariance vs covariance.

From python's typing docs for TypeVar:

By default type variables are invariant.

Based on this information, T = TypeVar("T", A, B) means type variable T has value restrictions of classes A and B, but because it's invariant... it only accepts those two (and not any child classes of A or B).

Thus, when passed AA, mypy correctly raises a type-var error.

---

You might then say: well, doesn't AA properly match behavioral subtyping of A? And in my opinion, you would be correct.

Why? Because one can properly substitute out and A with AA, and the behavior of the program would be unchanged.

However, because mypy is a static type checker, mypy can't figure this out (it can't check runtime behavior). One has to state the covariance explicitly, via the syntax covariant=True.

Also note: when specifying a covariant TypeVar, one should use the suffix _co in type variable names. This is documented in PEP 484 here.

from typing import TypeVar, Generic

class A: pass
class AA(A): pass

T_co = TypeVar("T_co", AA, A, covariant=True)

class X(Generic[T_co]): pass

class XA(X[A]): pass
class XAA(X[AA]): pass

Output: Success: no issues found in 1 source file

---

So, what should you do?

I would use TypeVar("T", bound=Union[A, B]), since:

• A and B aren't related
• You want their subclasses to be allowed

---

Further reading on LSP-related issues in mypy:

• python/mypy #2984: List[subclass] is incompatible with List[superclass]
• python/mypy #7049: [Question] why covariant type variable isn't allowed in instance method parameter?
  • Contains a good example from @Michael0x2a