>>> value = some_dict['foo'] KeyError: 'foo' 1. 2. 同样,如果您尝试弹出不存在的项目 >>> value = some_dict.pop('foo') KeyError: 'foo' 1. 2. 您可以使用通常设置为None的默认值来抑制 value = some_dict.pop('foo', None) if value is None: # booom! 1. 2. 3. None用作标志和...
Empty collections (such as [], "", ()) are falsey, but non-empty collections are truthy. Numbers that represent zero (0, 0.0, decimal.Decimal("0")) are falsey, but all other numbers are truthy. The value None is also falsey. See truthiness in Python for more on truthiness. ...
>>> empty is None False 情况2:使用None作为布尔值 以下两个测试 if value: # do something if not value: # do something 实际上被评估为 if bool(value): # do something if not bool(value): # do something None是 “falsey”,意味着如果转换为布尔值,它将返回False,如果应用not运算符,它将返回Tr...
"Python" >>> some_dict[5] "Python" >>> complex_five = 5 + 0j >>> type(complex_five) complex >>> some_dict[complex_five] "Python"So, why is Python all over the place?💡 ExplanationUniqueness of keys in a Python dictionary is by equivalence, not identity. So even though 5, ...
return False >>> f = MyFalsey() >>> bool(f) False >>> not f True 因此,在以下方式测试变量时,请特别注意测试中包含或排除的内容:def some_function(value=None): if not value: value = init_value() 在上文中,你是指当值被特定设置为None时调用init_value(),还是指将值设置为0、空字符串...
Extra keyword arguments are passed when initializing the wait generator, so themax_valueparam above is passed as a keyword arg when initializing the fibo generator. When not specified, the predicate param defaults to the falsey test, so the above can more concisely be written: ...
5) # filled_dict["five"] is set to 5 filled_dict.setdefault("five", 6) # filled_dict["five"] is still 5 # Sets store ... well sets (which are like lists but can contain no duplicates) empty_set = set() # Initialize a "set()" with a bunch of values some_set ...
is now 4 # Dictionaries store mappings empty_dict = {} # Here is a prefilled dictionary filled_dict = {"one": 1, "two": 2, "three": 3} # Look up values with [] filled_dict["one"] # => 1 # Get all keys as a list with "keys()" filled_dict.keys() # => ["three",...
def set_reminders(self, event, reminders=None): reminders = reminders or [] # or, if there are valid falsey inputs # that we'd like to preserve: reminders = [] if reminders is None else reminders ... This way we’re guaranteed to start with a fresh instance each time the function...
# all numbers are treated as true, except 0 print(bool(0)) print(bool("asf")) # all strings are treated as true, except the empty string "" print(bool("")) # Generally empty sequences (strings, lists, and other types we've yet to see like lists and tuples) # are "falsey" ...