type-concepts.md

Understanding Typing

Getting started with typing in Python is easy, but it’s important to understand a few simple concepts.

Type Declarations

When you add a type annotation to a variable or a parameter in Python, you are declaring that the symbol will be assigned values that are compatible with that type. You can think of type annotations as a powerful way to comment your code. Unlike text-based comments, these comments are readable by both humans and enforceable by type checkers.

If a variable or parameter has no type annotation, the type checker must assume that any value can be assigned to it. This eliminates the ability for a type checker to identify type incompatibilities.

Type Assignability

When your code assigns a value to a symbol (in an assignment expression) or a parameter (in a call expression), the type checker first determines the type of the value being assigned. It then determines whether the target has a declared type. If so, it verifies that the type of the value is assignable to the declared type.

Let’s look at a few simple examples. In this first example, the declared type of a is float, and it is assigned a value that is an int. This is permitted because int is assignable to float.

a: float = 3

In this example, the declared type of b is int, and it is assigned a value that is a float. This is flagged as an error because float is not assignable to int.

b: int = 3.4  # Error

This example introduces the notion of a Union type, which specifies that a value can be one of several distinct types.

c: Union[int, float] = 3.4
c = 5
c = a
c = b
c = None  # Error
c = ""  # Error

This example introduces the Optional type, which is the same as a union with None.

d: Optional[int] = 4
d = b
d = None
d = ""  # Error

Those examples are straightforward. Let’s look at one that is less intuitive. In this example, the declared type of f is List[Optional[int]]. A value of type List[int] is being assigned to f. As we saw above, int is assignable to Optional[int]. You might therefore assume that List[int] is assignable to List[Optional[int]], but this is an incorrect assumption. To understand why, we need to understand generic types and type arguments.

e: List[int] = [3, 4]
f: List[Optional[int]] = e  # Error

Generic Types

A generic type is a class that is able to handle different types of inputs. For example, the List class is generic because it is able to operate on different types of elements. The type List by itself does not specify what is contained within the list. Its element type must be specified as a type argument using the indexing (square bracket) syntax in Python. For example, List[int] denotes a list that contains only int elements whereas List[Union[int, float]] denotes a list that contains a mixture of int and float elements.

We noted above that List[int] is not assignable to List[Optional[int]]. Why is this the case? Consider the following example.

my_list_1: List[int] = [1, 2, 3]
my_list_2: List[Optional[int]] = my_list_1  # Error
my_list_2.append(None)

for elem in my_list_1:
    print(elem + 1)  # Runtime exception

The code is appending the value None to the list my_list_2, but my_list_2 refers to the same object as my_list_1, which has a declared type of List[int]. The code has violated the type of my_list_1 because it no longer contains only int elements. This broken assumption results in a runtime exception. The type checker detects this broken assumption when the code attempts to assign my_list_1 to my_list_2.

List is an example of a mutable container type. It is mutable in that code is allowed to modify its contents — for example, add or remove items. The type parameters for mutable container types are typically marked as invariant, which means that an exact type match is enforced. This is why the type checker reports an error when attempting to assign a List[int] to a variable of type List[Optional[int]].

Most mutable container types also have immutable counterparts.

Mutable Type	Immutable Type
List	Sequence
Dict	Mapping
Set	AbstractSet
n/a	Tuple

Switching from a mutable container type to a corresponding immutable container type is often an effective way to resolve type errors relating to assignability. Let’s modify the example above by changing the type annotation for my_list_2.

my_list_1: List[int] = [1, 2, 3]
my_list_2: Sequence[Optional[int]] = my_list_1  # No longer an error
my_list_2.append(None)  # Error

The type error on the second line has now gone away, but a new error is reported on the third line because the append operation is not allowed on an immutable Sequence.

For more details about generic types, type parameters, and invariance, refer to PEP 483 — The Theory of Type Hints.

Type Narrowing

Pyright uses a technique called “type narrowing” to track the type of an expression based on code flow. Consider the following code:

val_str: str = "hi"
val_int: int = 3

def func(val: Union[float, str, complex], test: bool):
    reveal_type(val) # Union[int, str, complex]

    val = val_int # Type is narrowed to int
    reveal_type(val) # int

    if test:
        val = val_str # Type is narrowed to str
        reveal_type(val) # str
    
    reveal_type(val) # Union[int, str]

    if isinstance(val, int):
        reveal_type(val) # int
        print(val)
    else:
        reveal_type(val) # str
        print(val)

At the start of this function, the type checker knows nothing about val other than that its declared type is Union[float, str, complex]. Then it is assigned a value that has a known type of int. This is a legal assignment because int is considered a subclass of float. At the point in the code immediately after the assignment, the type checker knows that the type of val is an int. This is a “narrower” (more specific) type than Union[float, str, complex]. Type narrowing is applied when ever a symbol is assigned a new value.

Another assignment occurs several lines further down, this time within a conditional block. The symbol val is assigned a value known to be of type str, so the narrowed type of val is now str. Once the code flow of the conditional block merges with the main body of the function, the narrowed type of val becomes Union[int, str] because the type checker cannot statically predict whether the conditional block will be executed at runtime.

Another way that types can be narrowed is through the use of conditional code flow statements like if, while, and assert. Type narrowing applies to the block of code that is “guarded” by that condition, so type narrowing in this context is sometimes referred to as a “type guard”. For example, if you see the conditional statement if x is None:, the code within that if statement can assume that x contains None. Within the code sample above, we see an example of a type guard involving a call to isinstance. The type checker knows that isinstance(val, int) will return True only in the case where val contains a value of type int, not type str. So the code within the if block can assume that val contains a value of type int, and the code within the else block can assume that val contains a value of type str. This demonstrates how a type (in this case Union[int, str]) can be narrowed in both a positive (if) and negative (else) test.

The following expression forms support type narrowing:

• <ident> (where <ident> is an identifier)
• <expr>.<member> (member access expression where <expr> is a supported expression form)
• <expr>[<int>] (subscript expression where <int> is a non-negative integer)
• <expr>[<str>] (subscript expression where <str> is a string literal)

Examples of expressions that support type narrowing:

• my_var
• employee.name
• a.foo.next
• args[3]
• kwargs["bar"]
• a.b.c[3]["x"].d

Type Guards

In addition to assignment-based type narrowing, Pyright supports the following type guards.

• x is None and x is not None
• x == None and x != None
• type(x) is T and type(x) is not T
• x is E and x is not E (where E is an enum value or True or False)
• x == L and x != L (where L is a literal expression)
• x.y == L and x.y != L (where L is a literal expression and x is a type that is distinguished by a field with a literal type)
• x[K] == V and x[K] != V (where K and V are literal expressions and x is a type that is distinguished by a TypedDict field with a literal type)
• x[I] == V and x[I] != V (where I and V are literal expressions and x is a known-length tuple that is distinguished by the index indicated by I)
• x[I] is None and x[I] is not None (where I is a literal expression and x is a known-length tuple that is distinguished by the index indicated by I)
• x in y (where y is instance of list, set, frozenset, or deque)
• S in D and S not in D (where S is a string literal and D is a TypedDict)
• isinstance(x, T) (where T is a type or a tuple of types)
• issubclass(x, T) (where T is a type or a tuple of types)
• callable(x)
• f(x) (where f is a user-defined type guard as defined in PEP 647)
• x (where x is any expression that is statically verifiable to be truthy or falsy in all cases)

Expressions supported for type guards include simple names, member access chains (e.g. a.b.c.d), the unary not operator, the binary and and or operators, subscripts that are constant numbers (e.g. a[2]), and call expressions. Other operators (such as arithmetic operators or other subscripts) are not supported.

Some type guards are able to narrow in both the positive and negative cases. Positive cases are used in if statements, and negative cases are used in else statements. (Positive and negative cases are flipped if the type guard expression is preceded by a not operator.) In some cases, the type can be narrowed only in the positive or negative case but not both. Consider the following examples:

class Foo: pass
class Bar: pass

def func1(val: Union[Foo, Bar]):
    if isinstance(Bar):
        reveal_type(val) # Bar
    else:
        reveal_type(val) # Foo

def func2(val: Optional[int]):
    if val:
        reveal_type(val) # int
    else:
        reveal_type(val) # Optional[int]

In the example of func1, the type was narrowed in both the positive and negative cases. In the example of func2, the type was narrowed only the positive case because the type of val might be either int (specifically, a value of 0) or None in the negative case.

Narrowing for Implied Else

When an “if” or “elif” clause is used without a corresponding “else”, Pyright will generally assume that the code can “fall through” without executing the “if” or “elif” block. However, there are cases where the analyzer can determine that a fall-through is not possible because the “if” or “elif” is guaranteed to be executed based on type analysis.

def func1(x: int):
    if x == 1 or x == 2:
        y = True
    
    print(y) # Error: "y" is possibly unbound

def func2(x: Literal[1, 2]):
    if x == 1 or x == 2:
        y = True
    
    print(y) # No error

This can be especially useful when exhausting all members in an enum or types in a union.

from enum import Enum

class Color(Enum):
    RED = 1
    BLUE = 2
    GREEN = 3

def func3(color: Color) -> str:
    if color == Color.RED or color == Color.BLUE:
        return "yes"
    elif color == Color.GREEN:
        return "no"

def func4(value: str | int) -> str:
    if isinstance(value, str):
        return "received a str"
    elif isinstance(value, int):
        return "received an int"

If you later added another color to the Color enumeration above (e.g. YELLOW = 4), Pyright would detect that func3 no longer exhausts all members of the enumeration and possibly returns None, which violates the declared return type. Likewise, if you modify the type of the value parameter in func4 to expand the union, a similar error will be produced.

This “narrowing for implied else” technique works for all narrowing expressions listed above with the exception of simple falsy/truthy statements and type guards. These are excluded because they are not generally used for exhaustive checks, and their inclusion would have a significant impact on analysis performance.

Narrowing Any

In general, the type Any is not narrowed. The only exceptions to this rule are the built-in isinstance and issubclass type guards plus user-defined type guards. In all other cases, Any is left as is, even for assignments.

a: Any = 3
reveal_type(a) # Any

a = "hi"
reveal_type(a) # Any

The same applies to Any when it is used as a type argument.

b: Iterable[Any] = [1, 2, 3]
reveal_type(b) # List[Any]

c: Iterable[str] = [""]
b = c
reveal_type(b) # List[Any]

Constrained Type Variables and Conditional Types

When a TypeVar is defined, it can be constrained to two or more types.

# Example of unconstrained type variable
_T = TypeVar("_T")

# Example of constrained type variables
_StrOrFloat = TypeVar("_StrOrFloat", str, float)

When a constrained TypeVar appears more than once within a function signature, the type provided for all instances of the TypeVar must be consistent.

def add(a: _StrOrFloat, b: _StrOrFloat) -> _StrOrFloat:
    return a + b

# The arguments for `a` and `b` are both `str`
v1 = add("hi", "there")
reveal_type(v1) # str

# The arguments for `a` and `b` are both `float`
v2 = add(1.3, 2.4)
reveal_type(v2) # float

# The arguments for `a` and `b` are inconsistent types
v3 = add(1.3, "hi") # Error

When checking the implementation of a function that uses constrained type variables in its signature, the type checker must verify that type consistency is guaranteed. Consider the following example, where the input parameter and return type are both annotated with a constrained type variable. The type checker must verify that if a caller passes an argument of type str, then all code paths must return a str. Likewise, if a caller passes an argument of type float, all code paths must return a float.

def add_one(value: _StrOrFloat) -> _StrOrFloat:
    if isinstance(value, str):
        sum = value + "1"
    else:
        sum = value + 1

    reveal_type(sum)  # str* | float*
    return sum

Notice that the type of variable sum is reported with asterisks (*). This indicates that internally the type checker is tracking the type as conditional. In this particular example, it indicates that sum is a str type if the parameter value is a str but is a float if value is a float. By tracking these conditional types, the type checker can verify that the return type is consistent with the return type _StrOrFloat.

Inferred type of self and cls parameters

When a type annotation for a method’s self or cls parameter is omitted, pyright will infer its type based on the class that contains the method. The inferred type is internally represented as a type variable that is bound to the class.

The type of self is represented as Self@ClassName where ClassName is the class that contains the method. Likewise, the cls parameter in a class method will have the type Type[Self@ClassName].

class Parent:
    def method1(self):
        reveal_type(self)  # Self@Parent
        return self
    
    @classmethod
    def method2(cls):
        reveal_type(cls)  # Type[Self@Parent]
        return cls

class Child(Parent):
     ...
    
reveal_type(Child().method1())  # Child
reveal_type(Child.method2())  # Type[Child]