Python MySQLdb：迭代游标 [英] Python MySQLdb: Iterating over a cursor

问题描述

在另一篇文章中，这段代码：

In another post, this code:

connection = MySQLdb.connect(...)
cursor = connection.cursor()
cursor.execute("SHOW TABLES")
for (table_name,) in cursor:
    print(table_name)

正确迭代光标中的表名，而此代码：

correctly iterates over the table names in the cursor whereas this code:

for table_name in cursor:
    print(table_name)

返回元素中的元素表格：

returns elements in the form:

('some_table',)

经过多次搜索，我一直无法理解这一点。有人可以解释这个区别吗？我无法弄清楚execute（）返回的确切内容。另外，我无法弄清楚为什么第一个迭代器的形式 - 使用括号和逗号 - 工作。

After much searching, I have been unable to make sense of this. Could someone explain the difference? I cannot figure out exactly what execute() is returning. Also, I cannot figure out why the form of the first iterator -- using parentheses and a comma -- works.

推荐答案

In本身， execute（）不返回任何内容。一旦执行了查询，当你迭代游标时，你会从查询中获取数据作为元组。

In itself, execute() doesn't return anything. Once you've executed a query, you get the data back from the query as tuples when you iterate over the cursor.

你的查询只返回一列，所以你得到了1元组。

Your query returns only one column, so you get 1-tuples.

Python中的1元组看起来有点奇怪。 （）是一个空元组，而（1,2）是一个2元组，但（1）只是括号中的数字 1 ，而不是元组。因此，（1，）等1元组必须具有尾随逗号才能被识别为元组。

1-tuples in Python look a bit odd. () is an empty tuple, and (1, 2) is a 2-tuple, but (1) is just the digit 1 in parentheses, not a tuple. 1-tuples such as (1,) must therefore have the trailing comma in order to be recognised as tuples.

如果您运行了一个选择了三列的查询，您可以使用以下内容读取每行中的三个值：

If you ran a query that selected three columns, you could read the three values out from each row using something like the following:

cursor.execute("SELECT a, b, c FROM some_table")
for (a_value, b_value, c_value) in cursor:
    # do stuff...

你的第一个代码也是这样做的，但它正在解包1元组而不是3元组。

Your first code is doing the same, but it's unpacking 1-tuples instead of 3-tuples.

另一方面，你的第二个代码只是迭代游标出来的内容，即1元组，而不进行任何解包。

On the other hand, your second code is simply iterating over what comes out of the cursor, i.e. the 1-tuples, without doing any unpacking.

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