Python中鲜为人知的特性（一）

问题

Python中鲜为人知的特性（一）

讨论

函数参数unpack

def foo(x, y):
    print x, y

alist = [1, 2]
adict = {'x': 1, 'y': 2}

foo(*alist)  # 1, 2
foo(**adict)  # 1, 2

链式比较操作符

>>> x = 3
>>> 1 < x < 5
True
>>> 4 > x >=3
True

注意函数的默认参数

>>> def foo(x=[]):
...     x.append(1)
...     print x
...
>>> foo()
[1]
>>> foo()
[1, 1]

更安全的做法:

>>> def foo(x=None):
...     if x is None:
...         x = []
...     x.append(1)
...     print x
...
>>> foo()
[1]
>>> foo()
[1]
>>>

带关键字的格式化

>>> print "Hello {name} !".format(name="James")
Hello James !

for...else 语法

>>> for i in (1, 3, 5):
...     if i % 2 == 0:
...         break
... else:
...     print "var i is always an odd"
...
var i is always an odd
>>>

dict 的特殊方法__missing__

当查找不到 key 的时候，会执行这个方法。

>>> class Dict(dict):
...   def __missing__(self, key):
...     self[key] = []
...     return self[key]
...
>>> dct = Dict()
>>> dct["foo"].append(1)
>>> dct["foo"].append(2)
>>> dct["foo"]
[1, 2]

切片操作的步长参数

>>> a = [1, 2, 3, 4, 5]
>>> a[::2]
[1, 3, 5]
>>> a[::-1]
[5, 4, 3, 2, 1]

Python解释器中的”_”

_ 即Python解释器上一次返回的值

>>> range(4)
[0, 1, 2, 3]
>>> _
[0, 1, 2, 3]

Python之禅

>>> import this
The Zen of Python, by Tim Peters

Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Flat is better than nested.
Sparse is better than dense.
Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it.
Although that way may not be obvious at first unless you're Dutch.
Now is better than never.
Although never is often better than *right* now.
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!

来源

PyZh

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