Content-Type: text/x-zim-wiki
Wiki-Format: zim 0.4
Creation-Date: 2012-10-04T13:47:32+08:00

====== unpack ======
Created Thursday 04 October 2012

>>> for k,v in ["fdf",23,"dfdf",33]:
...   print k,v
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: __too many__ values to unpack

顺序容器类型如str, list, tuple__每次迭代时只能返回其中的一个元素__。
所以第一次返回循环返回**"fdf"**，但是它有三个元素最多只能赋值给两个
变量。

>>> for k,v in "dfdf":
...   print k,v
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: __need more than 1 value__ to unpack

字符串迭代时，每次返回其中的一个字符。所以最多只能unpack给一个变量。

>>> k,v="dfdf"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: __too many values to unpack__

unpack一个顺序容器类型时，左边变量的数目必须要与容器中元素的个数相同。

>>> k,v="df"
>>> print k,v
d f
>>> 

In [9]: for k,v in 'dfdf':  //对于字符串的迭代，迭代其每次只返回__一个字符__，所以赋值给k,v时出错
   ...:     print k,v
   ...:     
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-9-5ba17a6181e2> in <module>()
----> 1 for k,v in 'dfdf':
      2     print k,v
      3 

ValueError: need more than 1 value to unpack

In [10]: k,v='ff'   __//对于可迭代对象，展开后的元素个数必须与等式左边的相等。__

In [11]: print k,v
**f f**
In [13]: k,v='fff'
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-13-0aecd2a10b05> in <module>()
----> 1 k,v='fff'

ValueError: too many values to unpack

In [14]: 
In [12]: 

In [12]: for k,v in ['df',[1,2],(3,4)]: __//每次迭代器返回列表中的一个元素，每个元素都可以展开为2个变量。__
    print k,v
   ....:     
d f
1 2
3 4

In [13]: