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2016-06-24 09:25 393 查看

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NoneType

is
simply the type of the

None

singleton:

>>> type(None)
<type 'NoneType'>

From the latter link above:

None

The sole value of the type

NoneType

None

is
frequently used to represent the absence of a value, as when default arguments are not passed to a function. Assignments to

None

are
illegal and raise a

SyntaxError

任何类型的变量都可以使用None判断是否初始化

what about

Python
`if x is not None` or `if not x is None`?

There's no performance difference, as they compile to the same bytecode:

Python 2.6.2 (r262:71600, Apr 15 2009, 07:20:39)
>>> import dis
>>> def f(x):
...    return x is not None
...
>>> dis.dis(f)
2           0 LOAD_FAST                0 (x)
3 LOAD_CONST               0 (None)
6 COMPARE_OP               9 (is not)
9 RETURN_VALUE
>>> def g(x):
...   return not x is None
...
>>> dis.dis(g)
2           0 LOAD_FAST                0 (x)
3 LOAD_CONST               0 (None)
6 COMPARE_OP               9 (is not)
9 RETURN_VALUE

Stylistically, I try to avoid

not
x is y

. Although the compiler will always treat it as

not
(x is y)

, a human reader might misunderstand the construct as

(not
x) is y

. If I write

x
is not y

then there is no ambiguity.

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python learning

Python`if x is not None` or `if not x is None`?

Python
`if x is not None` or `if not x is None`?