Type hint in Python (3)

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*Memo:

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

Setting multiple types to a type hint narrows(limits) the usage of object attributes as shown below:

<str | list[str]>:

*Memo:

• Both str and list have index() with one required and two optional parameters so index() with one, two or three arguments works.
• Both str and list have count() but str.count() has one required and two optional parameters while list.count() has one required parameter so count() with one argument works.
• Only str has upper() so upper() doesn't work.

v: str | list[str] = 'hello world'

print(v.index('rl'))
print(v.index('rl', 6))
print(v.index('rl', 6, 11))
print(v.count('ll'))
# No error

print(v.count('l', 0))
print(v.count('l', 0, 5))
print(v.upper())
# Error

v = ['A', 'B', 'C', 'D', 'E']
# No error

<Cls1 | Cls2>:

*Memo:

• Both Cls1 and Cls2 have add() but str.add() has x and y required and z optional float parameters while list.add() has a and y required int parameters so add() with two positional or one positional and y keyword int arguments works.
• Only Cls1 has PI and sub() so PI and sub() don't work.

class Cls1:
    PI: float = 3.14

    def add(self, x:float, y:float, z:float = 4) -> float:
        return x+y+z

    def sub(self, x:float, y:float) -> float:
        return x-y

class Cls2:
    def add(self, a:int, y:int) -> int:
        return a+y

cls1: Cls1 | Cls2 = Cls1()

print(cls1.add(5, 2))
print(cls1.add(5, y=2))
# No error

print(cls1.add(x=5, y=2))
print(cls1.add(5, 2, 6))
print(cls1.add(5.3, 2.8))
print(cls1.PI)
print(cls1.sub(5, 2))
# Error

The typed variable with no value is possible as an annotated assignment statement(annasgstmt) even though it's not defined yet while the untyped variable with no value is impossible as an assignment statement(asgstmt) as shown below:

v: str

v = 'Hello'

v: str

print(v)
# NameError: name 'v' is not defined

v
# NameError: name 'v' is not defined

Multiple variables cannot be typed at once so they need to be typed first one by one 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 typed at once so variables need to be typed first one by one as shown below:

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

v: str

v, = 'Hello',

print(v) # Hello

v1: str
v2: int

v1, v2 = 'Hello', 23

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

for statement unpacking cannot be typed at once so variables need to be typed first one by one as shown below:

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

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