使用 matplotlib 从 SQL (MariaDB) 绘制表值

Question

我想提取两列数据并将它们用作值在 matplotlib 上绘制直方图。我尽可能地密切关注Matplotlib: Plot the result of an SQL query 中的这个答案，只是在几个方面有所不同（即使用的 SQL 模块）。但是，我的脚本运行时出错：

Traceback (most recent call last):
  File "plot_script.py", line 22, in <module>
    for row in result:
TypeError: 'NoneType' object is not iterable

我的python脚本是：

import mysql.connector as mariadb 
import matplotlib as mpl
mpl.use('Agg')
import numpy as np
import matplotlib.mlab as mlab
import matplotlib.pyplot as plt

fig = plt.figure()
ax = fig.add_subplot(111)


mariadb_connection = mariadb.connect(user='user1', password='pw', database='sensordb')
cursor = mariadb_connection.cursor()
result=cursor.execute("SELECT time, voltage FROM sensortable")

#the data

data = []
xTickMarks = []

for row in result:
    data.append(int(row[1]))
    xTickMarks.append(str(row[0]))

mariadb_connection.close()

## necessary variables
ind = np.arange(len(data))                # the x locations for the groups
width = 0.35                      # the width of the bars

## the bars
rects1 = ax.bar(ind, data, width,
                color='black',
                error_kw=dict(elinewidth=2,ecolor='red'))


# axes and labels
ax.set_xlim(-width,len(ind)+width)
ax.set_ylim(0,45)


ax.set_ylabel('Voltage')
ax.set_xlabel('Time')
ax.set_title('Sensor measurements')

ax.set_xticks(ind+width)
xtickNames = ax.set_xticklabels(xTickMarks)
plt.setp(xtickNames, rotation=45, fontsize=10)

plt.show()
fig.savefig('plot1.jpg')

更新： 我还打印了使用另一个 python 脚本获得的数据，以确定我从 SQL 查询中获得的数据。终端打印：Decimal('2479.00')), (u'13:15', Decimal('3182.00')), (u'13:20', Decimal('3076.00')), (u'13:25', Decimal('2795.00')), (u'13:30', Decimal('3457.00')), (u'13:35', Decimal('2515.00')), (u'13:40', Decimal('3006.00')), (u'13:45', Decimal('3618.00')), (u'13:50', Decimal('3857.00'))] (and more similar data).......)

鉴于这种“十进制”数据类型可能会导致问题，我该如何解决？

我已将我的 python 脚本的一行修改为data.append(int(float(row[1])))，但它仍然返回错误Traceback (most recent call last): File "test_select.py", line 19, in <module> data.append(int(float(row[1]))) TypeError: float() argument must be a string or a number

Answer 1

不要将对象分配给execute() 调用，而只是迭代分配的游标对象：

cur = mariadb_connection.cursor()
cur.execute("SELECT time, voltage FROM sensortable")

data = []
xTickMarks = []

for row in cur.fetchall():
    data.append(int(row[1]))
    xTickMarks.append(str(row[0]))