Pyserial problem with Arduino - works with the Python shell but not in a program

Question

All right, so I am positive my Arduino circuit is correct and the code for it. I know this because when I use the serial monitor built into the Arduino IDE and send 'H' an LED lights up, when I send 'L' that LED turns off.

Now I made a Python program

import serial
ser = serial.Serial("COM4",9600)
ser.write("H")

When I run the code the LED blinks on for a second then goes back off. However when I do each of these lines separately in the shell it works just like it is supposed to.

Any ideas?

Answer 1

When you open the serial port, this causes the Arduino to reset. Since the Arduino takes some time to bootup, all the input goes to the bitbucket (or probably to the bootloader which does god knows what with it). If you insert a sleep, you wait for the Arduino to come up so your serial code. This is why it works interactively; you were waiting the 1.5 seconds needed for the software to come up.

I confirmed that opening the serial port resets my Arduino Uno; I flashed a program which will blink the LED from the setup() routine -- calling open("/dev/ttyACM0") was sufficient to trigger the reset. This is IMHO a confusing and undocumented wrinkle in the serial support.