Question

我试图在表格中检索行数与

<span class="image">
<img src="http://via.placeholder.com/100x100">
<p>some text</p>
</span>
<span>
<img class="image" src="http://via.placeholder.com/100x100">
<p>a really really really really really really really really really really long bit of text</p>
</span>
<span>
<img class="image" src="http://via.placeholder.com/100x100">
<p>some text</p>
</span>

，结果是

import postgresql
db = postgresql.open(...)
res = db.query("select count(1) from testdata")
print (res)

如何打印 10 ？

提前致谢

Answer 1

db.query("select count(1) from testdata")返回一个元组，而不是单个值。这个元组的第一个参数是查询的结果：

import postgresql
db = postgresql.open(...)
res = db.query("select count(1) from testdata")
count_result = res.next()
print (count_result)

这将使用查询响应的next方法读取查询结果，以获取第一个结果。由于我们使用了count，因此我们应该只有一个带有计数的响应（参见Data Wrangling with Python p.212）。

正如评论所说，有其他方法可以实现这一目标：

import postgresql
db = postgresql.open(...)
res = db.query("select count(1) from testdata")
print (res[0]) #first argument of res is the count

import postgresql
db = postgresql.open(...)
res, = db.query("select count(1) from testdata") # first argument assigned to res
                                                 # all subsequent arguments unassigned
print (res)

Python将元组转换为值

1 个答案: