python一些有用但是很少人知道的技巧

今天在stack overflow上看到了一个帖子，上面汇集了一些python的隐藏属性，感觉有一些还是挺不错，记录了一些下来：

1. 比较符号链接

>>> x = 5
>>> 1 < x < 10
True
>>> 10 < x < 20 
False
>>> x < 10 < x*10 < 100
True
>>> 10 > x <= 9
True
>>> 5 == x > 4
True

In case you're thinking it's doing 1 < x, which comes out as True, and then comparing True < 10, which is also True, then no, that's really not what happens (see the last example.) It's really translating into 1 < x and x < 10, and x < 10 and 10 < x * 10 and x*10 < 100, but with less typing and each term is only evaluated once.

2. 列举(enumerate)

Wrap an iterable with enumerate and it will yield the item along with its index.

For example:

>>> a = ['a', 'b', 'c', 'd', 'e']
>>> for index, item in enumerate(a): print index, item
...
0 a
1 b
2 c
3 d
4 e
>>>

References:

• Python tutorial—looping techniques
• Python docs—built-in functions—enumerate
• PEP 279
  这个方法对我们习惯在c/java中写i的同学很实用。
  
  3. 生成器(Creating generators objects)
  

  If you write

  x=(n for n in foo if bar(n))

  you can get out the generator and assign it to x. Now it means you can do

  for n in x:

  The advantage of this is that you don't need intermediate storage, which you would need if you did

  x = [n for n in foo if bar(n)]

  In some cases this can lead to significant speed up.

  You can append many if statements to the end of the generator, basically replicating nested for loops:

  >>> n = ((a,b) for a in range(0,2) for b in range(4,6))
  >>> for i in n:
  ...   print i 
  
  (0, 4)
  (0, 5)
  (1, 4)
  (1, 5)

  4. iter()能使用callable的参数

  For instance:

  def seek_next_line(f):
      for c in iter(lambda: f.read(1),'\n'):
          pass

  The iter(callable, until_value) function repeatedly calls callable and yields its result untiluntil_value is returned.

  5.注意那些不可变的缺省参数

  Be careful with mutable default arguments

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

  Instead, you should use a sentinel value denoting "not given" and replace with the mutable you'd like as default:

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

  6. 列表的一些切片的trick

  The step argument in slice operators. For example:

  a = [1,2,3,4,5]
  >>> a[::2]  # iterate over the whole list in 2-increments
  [1,3,5]

  The special case x[::-1] is a useful idiom for 'x reversed'.

  >>> a[::-1]
  [5,4,3,2,1]
  
  6. 原地交换两个变量的值：In-place value swapping
  >>> a = 10
  >>> b = 5
  >>> a, b
  (10, 5)
  
  >>> a, b = b, a
  >>> a, b
  (5, 10)
  The right-hand side of the assignment is an expression that creates a new tuple. The left-hand side of the assignment immediately unpacks that (unreferenced) tuple to the names a and b.
  After the assignment, the new tuple is unreferenced and marked for garbage collection, and the values bound to a and b have been swapped.
  As noted in the Python tutorial section on data structures,
     
  
      
  Note that multiple assignment is really just a combination of tuple packing and sequence unpacking.
     
  http://stackoverflow.com/questions/101268/hidden-features-of-python#101447