bool() 函数的复杂度¶
bool() 函数通过真值判断（truthiness）将对象转换为布尔值。
复杂度分析¶
| 情况 | 时间 | 空间 | 备注 |
|---|---|---|---|
| 转换数字（int、float、complex） | O(1) | O(1) | 判断是否为零；不随数值大小增长 |
| 转换内置容器（str、list、dict、set、range 等） | O(1) | O(1) | 根据已存储的长度判断是否为空；不逐个计数元素 |
定义了 __bool__() 的类 |
O(k) | O(1) | k = __bool__() 的开销（通常为 O(1)）；不会调用 __len__() |
只定义了 __len__() 的类 |
O(k) | O(1) | k = __len__() 的开销 |
| 两者都未定义的类 | O(1) | O(1) | 总是 True |
bool() 至多调用一次 __bool__() 或 __len__()，自定义方法本身的时间和空间开销要另外加到对应行上。
结果是 True 和 False 这两个单例之一，因此不会为结果分配任何内存。
基本用法¶
布尔值¶
# O(1) - direct conversion
bool(True) # True
bool(False) # False
整数与浮点数¶
# O(1) - zero/non-zero check
bool(0) # False
bool(1) # True
bool(-1) # True
bool(0.0) # False
bool(3.14) # True
字符串¶
# O(1) - empty/non-empty check
bool("") # False (empty string)
bool("a") # True (non-empty)
bool("0") # True (non-empty, not zero!)
容器¶
# O(1) - check if empty (uses __len__)
bool([]) # False
bool([1, 2, 3]) # True
bool({}) # False
bool({"a": 1}) # True
bool(set()) # False
bool({1, 2}) # True
bool(()) # False
bool((1, 2)) # True
None¶
# O(1) - None is always falsy
bool(None) # False
复杂度细节¶
容器真值判断¶
# O(1) - uses __bool__() or __len__()
# Python checks for __bool__() first, then __len__()
class CustomClass:
def __init__(self, size):
self.size = size
def __len__(self):
# O(1) - just return stored size
return self.size
obj = CustomClass(0)
bool(obj) # False - calls __len__(), returns 0
obj = CustomClass(5)
bool(obj) # True - calls __len__(), returns 5
自定义 __bool__() 方法¶
# O(k) - depends on __bool__() implementation
class Expensive:
def __init__(self, data):
self.data = data
def __bool__(self):
# O(n) worst case - any() stops at the first truthy item
return any(self.data)
obj = Expensive([0, 0, 0])
bool(obj) # False - O(n), all three items checked
# Without __bool__, __len__() decides - O(1) for a list
# It answers a different question: non-empty, not "any item truthy"
class Efficient:
def __init__(self, data):
self.data = data
def __len__(self):
# O(1) - quick
return len(self.data)
obj = Efficient([0, 0, 0])
bool(obj) # True - O(1), calls __len__()
真值判断规则¶
# O(1) - these are always falsy:
bool(None) # False
bool(False) # False
bool(0) # False
bool(0.0) # False
bool(0j) # False (complex)
bool("") # False (empty string)
bool([]) # False (empty list)
bool({}) # False (empty dict)
bool(set()) # False (empty set)
bool(()) # False (empty tuple)
bool(range(0)) # False (empty range)
# O(1) - these values are truthy:
bool(True) # True
bool(1) # True
bool(-1) # True
bool(0.1) # True
bool("0") # True (non-empty!)
bool([0]) # True (non-empty!)
bool([None]) # True (non-empty!)
bool([False]) # True (non-empty!)
常见模式¶
条件检查¶
# O(1) - implicit bool conversion
def process(items):
return len(items)
items = []
if items: # O(1) - checks truthiness
process(items)
# Explicit conversion
if bool(items): # O(1) - same thing
process(items)
# More Pythonic - just use implicit
if items: # Preferred
process(items)
取反¶
# O(1) - logical not
not True # False
not False # True
# With objects
items = []
if not items: # O(1) - equivalent to if len(items) == 0
print("empty")
按真值过滤¶
# O(n) - check each item
items = [0, 1, "", "hello", [], [1], None, False, True]
# Remove falsy values - O(n)
truthy_items = [x for x in items if x]
# [1, "hello", [1], True]
# Using filter - O(n)
truthy_items = list(filter(bool, items))
# Same result, O(n)
布尔列表¶
# O(n) - convert each element
values = [0, 1, 2, 3, 0, 5]
bools = [bool(x) for x in values]
# [False, True, True, True, False, True]
# Using map - O(n)
bools = list(map(bool, values))
# Same result
性能模式¶
检查容器是否为空¶
# All O(1) with __len__
items = [1, 2, 3]
if items: # O(1) - uses __len__()
pass
if len(items) > 0: # O(1) - explicit
pass
if len(items) != 0: # O(1) - also explicit
pass
# Idiomatic - let Python do implicit bool
if items: # Best
pass
与显式比较的对比¶
obj = [1, 2, 3]
# Truthiness - O(1) for built-in types, else __bool__() or __len__()
if obj:
pass
# None check - O(1) for any type, but a different question:
# an empty list is not None
if obj is not None:
pass
# len() - for a type with no __bool__(), the same __len__() call
# that `if obj:` makes
if len(obj) > 0:
pass
实用示例¶
默认参数处理¶
# O(1) - check if argument provided
default_value = 10
def process(value=None):
if value is None: # O(1)
value = default_value
return value
process(0) # 0 - `if not value:` would replace it with 10
# With optional list
def extend_list(items=None):
if items is None: # O(1)
items = []
return items
校验¶
# O(1) - check required fields
def validate_data(data):
if not data: # O(1) - check if dict is empty
raise ValueError("Data required")
required_fields = ["id", "name"]
for field in required_fields:
if not data.get(field): # O(1) - get and check
raise ValueError(f"{field} required")
布尔聚合¶
# O(n) - check multiple conditions
items = [1, 2, 3, 4, 5]
# Check if any item is truthy (short-circuits)
if any(items): # Stops at the first truthy item - here the first
pass
# Check if all items are truthy
if all(items): # O(n) - checks all
pass
# With conditions
if any(x > 10 for x in items): # O(n) or early exit
pass
if all(x > 0 for x in items): # O(n) or early exit
pass
边界情况¶
空值、None 与 False 的区别¶
# All falsy, but different
bool(None) # False - no value
bool([]) # False - empty container
bool(False) # False - explicit false
bool("") # False - empty string
# All truthy
bool([0]) # True - non-empty
bool([None]) # True - non-empty
bool([False])# True - non-empty
自定义假值对象¶
# O(1) - return False from __bool__
class AlwaysFalse:
def __bool__(self):
return False
obj = AlwaysFalse()
bool(obj) # False
# But object exists
if obj: # False
pass
if obj is not None: # True - object exists!
pass
除零检查¶
# O(1) - check for zero before division
divisor = 4
if divisor: # O(1) - checks if non-zero
result = 100 / divisor
else:
result = None
# vs explicit check
if divisor != 0: # O(1)
result = 100 / divisor
最佳实践¶
✅ 应该：
- 使用隐式真值判断：
if items:而非if len(items) > 0: - 显式检查
is None：if value is None:而非if not value: - 需要显式布尔值时使用
bool()转换 - 如果类的
__len__()代价高昂，为它定义__bool__()，这样真值判断就不必调用__len__()
❌ 避免：
- 混淆空值与 False：
[]和False都是假值，但二者不同 - 在条件中不必要地使用
bool() - 假设所有假值都是 False
- 复杂的
__bool__()实现（应保持 O(1)）
版本说明¶
- Python 2.x：使用
__nonzero__()而非__bool__() - Python 3.x：使用
__bool__()方法 - Python 3.14+：
bool(NotImplemented)会引发TypeError；3.11 至 3.13 返回True并发出DeprecationWarning - 所有版本：假值保持一致（None、False、0、""、[]、{} 等）