python list append方法

Question

问题：将数据库中查出的数据（列表中包含元组）转换为列表中字典。

原数据结构，从数据库查出：

cur = [("t1", "d1"), ("t2", "d2")]

转换后数据结构：

[{‘description‘: ‘d1‘, ‘title‘: ‘t1‘}, {‘description‘: ‘d2‘, ‘title‘: ‘t2‘}]

方法一，使用append, 出现错误结果

cur = [("t1", "d1"), ("t2", "d2")] 
post_dict = {}
posts = []
for row in cur:
    post_dict[‘title‘] = row[0]
    post_dict[‘description‘] = row[1]
    print "post_dict:",post_dict
    posts.append(post_dict)
    print "posts:",posts

方法一运行结果：

post_dict: {‘description‘: ‘d1‘, ‘title‘: ‘t1‘}
posts: [{‘description‘: ‘d1‘, ‘title‘: ‘t1‘}]
post_dict: {‘description‘: ‘d2‘, ‘title‘: ‘t2‘}
posts: [{‘description‘: ‘d2‘, ‘title‘: ‘t2‘}, {‘description‘: ‘d2‘, ‘title‘: ‘t2‘}]

方法二，使用列表解析,结果正常

cur = [("a", "a1"), ("b", "b1")] 
posts = []
posts = [dict(title=row[0], description=row[1]) for row in cur]
print "posts:",posts

方法二运行结果，正常

posts: [{‘description‘: ‘d1‘, ‘title‘: ‘t1‘}, {‘description‘: ‘d2‘, ‘title‘: ‘t2‘}]

Answer 1

方法一中，你的post_dict是一个字典对象，for循环的操作都是在更新这个对象的key和value，自始至终就这一个对象，append多少次都一样。

把字典对象放在循环内创建即可：

cur = [("t1", "d1"), ("t2", "d2")] 
posts = []
for row in cur:
    post_dict = {}
    post_dict[‘title‘] = row[0]
    post_dict[‘description‘] = row[1]
    print "post_dict:",post_dict
    posts.append(post_dict)
    print "posts:",posts

优先考虑列表解析，另，本例的tupel列表可以用循环解包，大致如下：

In [1]: cur = [("t1", "d1"), ("t2", "d2")]

In [2]: r = [{‘description‘: description, ‘title‘: title} for description, title in cur]

In [3]: r
Out[3]: [{‘description‘: ‘t1‘, ‘title‘: ‘d1‘}, {‘description‘: ‘t2‘, ‘title‘: ‘d2‘}

Answer 2

方法一的循环中，post_dict始终指向的是同一个对象。在for循环中，使用匿名对象就可以了：

for row in cur:
    posts.append({‘title‘:row[0],‘description‘:row[1]})