Save sensor data in CSV file by using Raspberry pi3

Question

Could you help me to check whether my python code below is correct for saving data in csv format via raspberry pi3?

print("Gyro: %f, %f, %f  [deg/s]" % (cgx, cgy, cgz))
print("Accel: %f, %f, %f  [Gs]" % (cax, cay, caz))
print("Mag: %f, %f, %f  [gauss]" % (cmx, cmy, cmz))

file = open("/home/pi/data_log.csv", "a")

if os.stat("/home/pi/data_log.csv").st_size == 0:
    file.write("Time,Gyro,Accel,Mag\n")

while True:
    now = datetime.now())
    print >>f,("%s,%f,%f,%f,%f,%f,%f,%f,%f"%(now.datetime("%Y-%m-%d %H:%M:%S"),cgx,cgy,cgz,cax,cay,caz,cmx,cmy,cgz))
            file.flush()
            time.sleep(5)
    file.close()

Think about whether you plan to hold the file open all the time, or open and close it on every iteration. — Mark Setchell
– Mark Setchell, Commented Nov 30, 2017 at 9:43
You are open()ing the file only once at the top, but close()ing it multiple times in the eternal loop. Also I don't understand the print >>f part. — Tuwuh S
– Tuwuh S, Commented Nov 30, 2017 at 11:41

Martin Evans · Accepted Answer · 2017-12-01 15:33:09Z

The following type of approach should get you started. You are though missing code to update your variables, so at the moment it would result in the same values being written every 5 seconds:

from datetime import datetime
import csv
import os

print("Gyro: {}, {}, {}  [deg/s]".format(cgx, cgy, cgz))
print("Accel: {}, {}, {}  [Gs]".format(cax, cay, caz))
print("Mag: {}, {}, {}  [gauss]".format(cmx, cmy, cmz))

filename = "/home/pi/data_log.csv"
write_header = not os.path.exists(filename) or os.stat(filename).st_size == 0

with open(filename, "a", newline="") as f_output:
    csv_output = csv.writer(f_output)

    if write_header:
        csv_output.writerow(["Time", "Gyro", "Accel", "Mag"])

    while True:
        row = [datetime.now().strftime("%Y-%m-%d %H:%M:%S"), cgx, cgy, cgz, cax, cay, caz, cmx, cmy, cgz]
        csv_output.writerow(row)
        time.sleep(5)

Python's CSV library can be used to convert a list of values into a row which is correctly delimited automatically. By default the delimiter is a comma.

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