Type hints in Python (2)

*Memo:

• My post explains type hints (1).

The value None should be used as a type within a type hint because the type NoneType gets error as shown below:

v: None
# No error

from types import NoneType

v: NoneType
# error: NoneType should not be used as a type, please use None instead

The function which only returns None gets the error if defining and calling the function, and printing the return value as shown below:

*Memo:

• The error can be disabled using --disable-error-code with func-returns-value:
  • mypy --strict --disable-error-code func-returns-value test.py.
  • mypy --disable-error-code func-returns-value test.py.
• Only defining and calling the function doesn't get the error.
• Only defining the function doesn't get the error.
• I reported the strange behaviour as the issue.

def func() -> None:
    return None

print(func())
# error: "func" does not return a value (it only ever returns None)

from types import def func() -> None:
    return None

func()
# No error

def func() -> None:
    return None
# No error

Multiple variables cannot be defined with type-hints at once so they need to be type-hinted first as shown below:

v1: str
v2: str

v1 = v2 = 'Hello'

print(v1) # Hello
print(v2) # Hello

v1: str = v2: str = 'Hello'
# SyntaxError: invalid syntax

Assignment statement unpacking cannot be done with type-hints at once so variables need to be type-hinted first as shown below:

v: str

v, = 'Hello',

print(v) # Hello

v1: str
v2: int

v1, v2 = 'Hello', 23

print(v1) # Hello
print(v2) # 23

v: str, = 'Hello',
v1: str, v2: int = 'Hello', 23
# SyntaxError: invalid syntax

for statement unpacking cannot be done with type-hints at once so variables need to be type-hinted first as shown below:

v: str

for v, in [['A'], ['B'], ['C']]:
    print(v)
# A
# B
# C

v1: str
v2: int

for v1, v2 in [['A', 0], ['B', 1], ['C', 2]]:
    print(v1, v2)
# A 0
# B 1
# C 2

for v: str, in [['A'], ['B'], ['C']]: pass
for v1: str, v2: int in [['A', 0], ['B', 1], ['C', 2]]: pass
# SyntaxError: invalid syntax

complex accepts float, int and bool, float accepts int and bool and int accepts bool as shown below:

*Memo:

• PEP 484 explains the numeric tower of complex, float and int.
• The doc explains bool is the subclass of int.

<complex>:

v: complex = 2.3+4.5j # complex
v = 2.3               # float
v = 23                # int
v = True              # bool
# No error

<float>:

v: float = 2.3 # float
v = 23         # int
v = True       # bool
# No error

v = 2.3+4.5j   # complex
# Error

<int>:

v: int = 23  # int
v = True     # bool
# No error

v = 2.3+4.5j # complex
v = 2.3      # float
# Error

<bool>:

v: bool = True # bool
# No error

v = 2.3+4.5j   # complex
v = 2.3        # float
v = 23         # int
# Error