Short introduction

Opening serial ports

Open port at “9600,8,N,1”, no timeout:

>>> import serial
>>> ser = serial.Serial('/dev/ttyUSB0')  # open serial port
>>> print(         # check which port was really used
>>> ser.write(b'hello')     # write a string
>>> ser.close()             # close port

Open named port at “19200,8,N,1”, 1s timeout:

>>> with serial.Serial('/dev/ttyS1', 19200, timeout=1) as ser:
...     x =          # read one byte
...     s =        # read up to ten bytes (timeout)
...     line = ser.readline()   # read a '\n' terminated line

Open port at “38400,8,E,1”, non blocking HW handshaking:

>>> ser = serial.Serial('COM3', 38400, timeout=0,
...                     parity=serial.PARITY_EVEN, rtscts=1)
>>> s =       # read up to one hundred bytes
...                         # or as much is in the buffer

Configuring ports later

Get a Serial instance and configure/open it later:

>>> ser = serial.Serial()
>>> ser.baudrate = 19200
>>> ser.port = 'COM1'
>>> ser
Serial<id=0xa81c10, open=False>(port='COM1', baudrate=19200, bytesize=8, parity='N', stopbits=1, timeout=None, xonxoff=0, rtscts=0)
>>> ser.is_open
>>> ser.close()
>>> ser.is_open

Also supported with context manager:

with serial.Serial() as ser:
    ser.baudrate = 19200
    ser.port = 'COM1'


readline() reads up to one line, including the \n at the end. Be careful when using readline(). Do specify a timeout when opening the serial port otherwise it could block forever if no newline character is received. If the \n is missing in the return value, it returned on timeout.

readlines() tries to read “all” lines which is not well defined for a serial port that is still open. Therefore readlines() depends on having a timeout on the port and interprets that as EOF (end of file). It raises an exception if the port is not opened correctly. The returned list of lines do not include the \n.

Both functions call read() to get their data and the serial port timeout is acting on this function. Therefore the effective timeout, especially for readlines(), can be much larger.

Do also have a look at the example files in the examples directory in the source distribution or online.


The eol parameter for readline() is no longer supported when pySerial is run with newer Python versions (V2.6+) where the module io is available.


To specify the EOL character for readline() or to use universal newline mode, it is advised to use io.TextIOWrapper:

import serial
import io
ser = serial.serial_for_url('loop://', timeout=1)
sio = io.TextIOWrapper(io.BufferedRWPair(ser, ser))

sio.flush() # it is buffering. required to get the data out *now*
hello = sio.readline()
print(hello == unicode("hello\n"))

Testing ports

Listing ports

python -m will print a list of available ports. It is also possible to add a regexp as first argument and the list will only include entries that matched.


The enumeration may not work on all operating systems. It may be incomplete, list unavailable ports or may lack detailed descriptions of the ports.

Accessing ports

pySerial includes a small console based terminal program called It can be started with python -m <port_name> (use option -h to get a listing of all options).