Python Pandas plot. Script does not see a numeric value in text file

Question

I have a script that queries data from a database. It then exports it to a CSV file. The goal is to create a plot with the numeric data in the column. But the script does not see any numerical values in the column specified.

import pandas.io.data
import matplotlib.pyplot as plt
import pandas as pd
from pandas import DataFrame
import datetime
import pandas.io.data
import psycopg2
import csv
import pandas.io.data

conn = psycopg2.connect("host='172.31.98.161' dbname='servers' user='seeker'")
cur = conn.cursor() #Create the cursor
cur.execute("select hostname, used_ram_raw from server_perf  where                 hostname='localhost.localdomain'")
rows = cur.fetchall()
conn.close()

df = pd.DataFrame(rows, columns=['One','Two'])
df.to_csv("file.csv")
df = df.convert_objects(convert_numeric=True)    
df[['Two']].plot()
plt.show()

The above script produces the following file called "file.csv"

,One,Two
0,localhost.localdomain,1521819648.00
1,localhost.localdomain,1632538624.00
2,localhost.localdomain,1633038336.00
3,localhost.localdomain,1632264192.00

The error produced is whn trying to plot is:

TypeError: Empty 'DataFrame': no numeric data to plot

Just for testing I did a

  print df[['Two']]

and it printed the following

             Two
0  1521819648.00
1  1632538624.00
2  1633038336.00
3  1632264192.00

What am I doing wrong? :(

Ami Tavory · Accepted Answer · 2015-07-12 22:45:22Z

3

It's very likely that if you would type

df.Two.dtype

It would output that its type is a string (It would say it's Object or something like that).

This is easily remedied. If you need to do something numeric with it, use astype:

df.Two.astype(float)

In fact, you can just reassign it back to itself:

df.Two = df.Two.astype(float)

answered Jul 12, 2015 at 22:45

Ami Tavory

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2 Comments

George Mastrokostas Over a year ago

Thank you Ami !!! It worked. Here is the corrected code. df = pd.DataFrame(rows, columns=['One','Two']) df.to_csv("file.csv") print df[['Two']] print df.Two.dtype df.Two = df.Two.astype(float) df[['Two']].plot() plt.show()

PM0087 Over a year ago

I had to do this for the x-axis and y-axis.

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