None
成員變數是None
成員變數是None，輸出也是None。
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class Cat:
name = None
age = None
color = None
cat1 = Cat()
print(f"cat1 name: {cat1.name}, age: {cat1.age}, color: {cat1.color}")
cat1 name: None, age: None, color: None
NoneType
物件設為None，輸出為None，類型為NoneType。
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cat1 = None
print(f"cat1 = {cat1}, type = {type(cat1)}")
cat1 = None, type = <class 'NoneType'>
對None取得屬性
以下會產生找不到屬性的Error。
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cat1 = None
print(f"cat1 name: {cat1.name}, age: {cat1.age}, color: {cat1.color}")
print(f"cat1 name: {cat1.name}, age: {cat1.age}, color: {cat1.color}")
^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'name'
空的物件 bool()是False
Python 所有型別都是物件，對空的物件進行bool(obj)強制轉型，結果都會是False。
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# bool False
print(f"bool(False) = {bool(False)}")
# 數字0
print(f"bool(0) = {bool(0)}")
# None
print(f"bool(None) = {bool(None)}")
# 空字串
print(f"bool(empty str) = " + str(bool("")))
# 空list
print(f"bool([]) = {bool([])}")
# 空tuple
print(f"bool(()) = {bool(())}")
# 空字典
print(f"bool(dict()) = {bool({})}")
# 空set
print(f"bool(set()) = {bool(set())}")
bool(False) = False
bool(0) = False
bool(None) = False
bool(empty str) = False
bool([]) = False
bool(()) = False
bool(dict()) = False
bool(set()) = False
對空的物件if判斷，都是False。
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num = 0
if num:
print(num)
else:
print("num = 0")
str1 = ""
if str1:
print(str1)
else:
print("empty string")
cat = None
if cat:
print(cat)
else:
print("empty cat")
list1 = []
if list1:
print(list1)
else:
print("empty list")
tuple1 = ()
if tuple1:
print(tuple1)
else:
print("empty tuple")
dict1 = {}
if dict1:
print(dict1)
else:
print("empty dict")
set1 = set()
if set1:
print(set1)
else:
print("empty set")
num = 0
empty string
empty cat
empty list
empty tuple
empty dict
empty set