Python中不爲人知的特性（一）

問題

Python中不爲人知的特性（一）

討論

函數參數unpack

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

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

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

鏈式比較操做符

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

注意函數的默認參數

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

更安全的作法:

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

帶關鍵字的格式化

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

for...else 語法

>>> 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__

當查找不到 key 的時候，會執行這個方法。

>>> 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]

切片操做的步長參數

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

Python解釋器中的」_」

_ 即Python解釋器上一次返回的值

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

Python之禪

>>> 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!

來源

PyZh

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