python学习之路（1）

列表解析表达式:

List comprehensions derive from set notation; they are a way to build a new list by

running an expression on each item in a sequence, one at a time, from left to right。

List

comprehensions are coded in square brackets (to tip you off to the fact that they make

a list) and are composed of an expression and a looping construct that share a variable

name (row, here).

If the last section’s for loop looks like the list comprehension expression introduced

earlier, it should: both are really general iteration tools. In fact, both will work on any

object that follows the iteration protocol—a pervasive idea in Python that essentially

means a physically stored sequence in memory, or an object that generates one item at

a time in the context of an iteration operation. An object falls into the latter category

if it responds to the iter built-in with an object that advances in response to next. The

generator comprehension expression we saw earlier is such an object.