Type hints in Python (3)

#python #typehint #statictype #annotation

*Memo:

My post explains type hints (1).
My post explains type hints (2).
My post explains type hints (4).
My post explains type hints (5).
My post explains type hints (6).
My post explains type hints (7).

Type checkers infer the type of the non-type-hinted variable outside a function from the assigned value, and infer the types of the non-type-hinted parameter(including *args, **kwargs or self), variable and return value within a function as Any as shown below:

*Memo:

reveal_type() can ask a type checker to print the static type of a value:
- The 1st argument is obj(Required-Type:Any):
  - It's a value.
  - Don't use obj=.
- At runtime, it prints the runtime type of a value.
The doc explains type inference.
mypy test.py is used.

from typing import reveal_type

# `v` is `int`
v = 100
reveal_type(v) # builtins.int (Runtime type is 'int')

v = 200  # No error
v = 3.14 # Error

from typing import reveal_type

# The type of `x`, `y` and return value are `Any`.
def func(x=100, *args, **kwargs):
    reveal_type(x) # Any (Runtime type is 'int')

    x = 200  # No error
    x = 3.14 # No error

    reveal_type(args) # Any (Runtime type is 'tuple')

    x = 200  # No error
    x = 3.14 # No error

    reveal_type(kwargs) # Any (Runtime type is 'dict')

    x = 200  # No error
    x = 3.14 # No error

    y = 100
    reveal_type(y) # Any (Runtime type is 'int')

    y = 200  # No error
    y = 3.14 # No error

    return 100

v = func()

reveal_type(v) # Any (Runtime type is 'int')

v = 200  # No error
v = 3.14 # No error

from typing import reveal_type

# The type of `v`, `self`, `x` and return value are `Any`.
class Cls:
    v = 100
    reveal_type(v) # builtins.int (Runtime type is 'int')

    v = 200  # No error
    v = 3.14 # Error

    def func(self, x):
        reveal_type(self) # Any (Runtime type is 'Cls')

        self = 200  # No error
        self = 3.14 # No error

        reveal_type(x) # Any (Runtime type is 'int')

        x = 200  # No error
        x = 3.14 # No error

cls = Cls().func(100)

reveal_type(cls) # Any (Runtime type is 'NoneType')

cls = 200  # No error
cls = 3.14 # No error

assert_type() can ask a type checker to check if the static type of val is typ as shown below:

*Memo:

The 1st argument is val(Required-Type:Any):
- It's a value.
- Don't use val=.
The 2nd argument is typ(Required-Type:Type):
- It's a type.
- Don't use typ=.
Error occurs if the static type of val isn't typ while nothing happens if the static type of val is typ.
At runtime, it returns the 1st argument.

from typing import assert_type

v: int = 100
assert_type(v, int)   # No error(No message)
assert_type(v, float) # Error

cast can cast the static type of a value to other (or same) static type as shown below:

*Memo:

The 1st argument is typ(Required-Type:Any):
- It's a type.
- Don't use typ=.
The 2nd argument is val(Required-Type:Type):
- It's a value.
- Don't use val=.
At runtime, it does nothing.

from typing import reveal_type, cast

v1: int = 100

v1 = 200 # No error

# `int` is casted to `float`.
v2 = cast(float, v1)
reveal_type(v2) # builtins.float (Runtime type is 'int')

v2 = 3.14 # No error

DEV Community

Type hints in Python (3)

Top comments (0)