列表 Lists
列表就是 Python 中的数组，但是列表长度可变，且能包含不同类型元素。
xs = [3, 1, 2] # Create a list
print xs, xs[2] # Prints "[3, 1, 2] 2"
print xs[-1] # Negative indices count from the end of the list ; prints "2"
xs[2] = 'foo ' # Lists can contain elements of different types
print xs # Prints "[3, 1, 'foo ']"
xs.append('bar ') # Add a new element to the end of the list
print xs # Prints
x = xs.pop() # Remove and return the last element of the list
print x, xs # Prints "bar [3, 1, 'foo ']"
列表的细节，同样可以查阅文档。
切片 Slicing：为了一次性地获取列表中的元素，Python 提供了一种简洁的语法，这就是切片。
nums = range (5) # range is a built -in function that creates a list of integers
print nums # Prints "[0, 1, 2, 3, 4]"
print nums[2:4] # Get a slice from index 2 to 4 ( exclusive ); prints "[2, 3]"
print nums[2:] # Get a slice from index 2 to the end; prints "[2, 3, 4]"
print nums[:2] # Get a slice from the start to index 2 ( exclusive ); prints "[0, 1]"
print nums[:] # Get a slice of the whole list; prints ["0, 1, 2, 3, 4]"
print nums[:-1] # Slice indices can be negative ; prints ["0 , 1, 2, 3]"
nums[2:4] = [8, 9] # Assign a new sublist to a slice
print nums # Prints "[0, 1, 8, 8, 4]"
在 Numpy 数组的内容中，我们会再次看到切片语法。
循环 Loops：我们可以这样遍历列表中的每一个元素：
animals = ['cat ', 'dog', 'monkey ']
for animal in animals:
print animal
# Prints " cat", "dog", "monkey", each on its own line .
如果想要在循环体内访问每个元素的指针，可以使用内置的 enumerate 函数
animals = ['cat ', 'dog', 'monkey ']
for idx , animal in enumerate(animals):
print '#%d: %s' % (idx + 1, animal)
# Prints "#1: cat", "#2: dog", "#3: monkey", each on its own line
列表推导 List comprehensions：在编程的时候，我们常常想要将一种数据类型转换为另一种。下面
是一个简单例子，将列表中的每个元素变成它的平方。
nums = [0, 1, 2, 3, 4]
squares = []
for x in nums:
squares.append(x ** 2)
print squares # Prints [0, 1, 4, 9, 16]
使用列表推导，你就可以让代码简化很多：
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