列表语法：[expression for variable in list if conditions]
生成的是一个列表类型。
同理字典：{value_exp : key_exp for key,value in dict}
nums = [n*2 for n in numbers] #单层循环
words = [word for sent in document for word in sent] #嵌套循环，注意顺序
nums = [n*2 for n in numbers if n>0] #带条件判断的循环
#另外注意在括号表达式中是可以换行的。比如：
nums = [n*2
for n in numbers
if n>0
]
>>> str = "what"
>>> tmp = {index:value for index,value in enumerate(str)}
>>> tmp
{0: 'w', 1: 'h', 2: 'a', 3: 't'}
Built-in Functions for Iterables
# Apply func to every element in iterable; return an iterator (Python 3)
# Multiple iterables can be accepted, in which case
# the func must take elements from every iterabls in parallel.
map(func, iterable, ...)
# Construct an iterator from those elements of iterable
# for which func returns true.
# If func is None, the identity function is used.
# Equivalent to (item for item in iterable if func(item))
filter(function, iterable)