如果type为None或NoneType,则返回其他内容

Question

我有这个方法从SQL查询中检索一个值:

def get_base_price(self, date):
    sql = """
    SELECT TimeSeriesValue
    FROM dbo.TimeSeriesPosition
    WHERE  
    TimeSeriesTypeID = {0} AND
    FundID = {1} AND
    SecurityMasterID = 45889 AND
    EffectiveDate = '{2}'""".format(self.LAST_PRICE_ID, self.FUND_ID, date)

    with self.job.rap.connect() as conn:
        with conn.cursor() as cursor:
            price = cursor.execute(sql).fetchone()[0] 
            if price is NoneType:
               return 100
            else:
                return price # Returns base price value

我有一个测试,但如果price是的话,我无法返回100 None.我的运行测试只返回NoneType对象不可订阅'

我也尝试过:100 if price is None else price那没用.我错过了什么？

Answer 1

您None 在该测试之前订阅:

price = cursor.execute(sql).fetchone()[0]

.fetchone()返回None,它None[0]触发异常:

>>> None[0]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'NoneType' object is not subscriptable

该错误消息告诉您类型None,即NoneType不实现订阅支持.如果仔细观察Python打印出来的回溯,你会发现它price = cursor.execute(sql).fetchone()[0]是抛出异常的行,代码永远不会到达后面的if price is ...行.

打电话.fetchone()没有订阅,然后测试None:

with self.job.rap.connect() as conn:
    with conn.cursor() as cursor:
        cursor.execute(sql)
        price_row = cursor.fetchone()
        if price_row is None:
            return 100
        else:
            return price_row[0]

或者,您可以使用元组赋值并使用异常处理来返回默认值:

with self.job.rap.connect() as conn:
    with conn.cursor() as cursor:
        cursor.execute(sql)
        try:
            price, = cursor.fetchone()
            return price
        except TypeError:
            return 100

您也不应该使用字符串格式将数据与查询组合.使用SQL参数:

sql = """
SELECT TimeSeriesValue
FROM dbo.TimeSeriesPosition
WHERE  
TimeSeriesTypeID = %s AND
FundID = %s AND
SecurityMasterID = %s AND
EffectiveDate = %s"""

with self.job.rap.connect() as conn:
    with conn.cursor() as cursor:
        cursor.execute(sql, (self.LAST_PRICE_ID, self.FUND_ID, date))
        try:
            price, = cursor.fetchone()
            return price
        except TypeError:
            return 100

%s查询中的每个都是占位符,并且这些占位符的值cursor.execute()作为第二个参数传递给调用.然后,数据库驱动程序负责正确引用每列的值,确保数据不能作为SQL命令执行; 请注意,列的%s占位符周围没有引号EffectiveDate.这使您可以避免SQL注入攻击,并使数据库驱动程序可以重用给定查询的查询计划,这对数据库更有效.