pySerial API


Native ports

class serial.Serial
__init__(port=None, baudrate=9600, bytesize=EIGHTBITS, parity=PARITY_NONE, stopbits=STOPBITS_ONE, timeout=None, xonxoff=False, rtscts=False, writeTimeout=None, dsrdtr=False, interCharTimeout=None)
  • port – Device name or port number number or None.
  • baudrate – Baud rate such as 9600 or 115200 etc.
  • bytesize – Number of data bits. Possible values: FIVEBITS, SIXBITS, SEVENBITS, EIGHTBITS
  • parity – Enable parity checking. Possible values: PARITY_NONE, PARITY_EVEN, PARITY_ODD PARITY_MARK, PARITY_SPACE
  • stopbits – Number of stop bits. Possible values: STOPBITS_ONE, STOPBITS_ONE_POINT_FIVE, STOPBITS_TWO
  • timeout – Set a read timeout value.
  • xonxoff – Enable software flow control.
  • rtscts – Enable hardware (RTS/CTS) flow control.
  • dsrdtr – Enable hardware (DSR/DTR) flow control.
  • writeTimeout – Set a write timeout value.
  • interCharTimeout – Inter-character timeout, None to disable (default).
  • ValueError – Will be raised when parameter are out of range, e.g. baud rate, data bits.
  • SerialException – In case the device can not be found or can not be configured.

The port is immediately opened on object creation, when a port is given. It is not opened when port is None and a successive call to open() will be needed.

Possible values for the parameter port:

  • Number: number of device, numbering starts at zero.
  • Device name: depending on operating system. e.g. /dev/ttyUSB0 on GNU/Linux or COM3 on Windows.

The parameter baudrate can be one of the standard values: 50, 75, 110, 134, 150, 200, 300, 600, 1200, 1800, 2400, 4800, 9600, 19200, 38400, 57600, 115200. These are well supported on all platforms.

Standard values above 115200, such as: 230400, 460800, 500000, 576000, 921600, 1000000, 1152000, 1500000, 2000000, 2500000, 3000000, 3500000, 4000000 also work on many platforms and devices.

Non-standard values are also supported on some platforms (GNU/Linux, MAC OSX >= Tiger, Windows). Though, even on these platforms some serial ports may reject non-standard values.

Possible values for the parameter timeout:

  • timeout = None: wait forever
  • timeout = 0: non-blocking mode (return immediately on read)
  • timeout = x: set timeout to x seconds (float allowed)

Writes are blocking by default, unless writeTimeout is set. For possible values refer to the list for timeout above.

Note that enabling both flow control methods (xonxoff and rtscts) together may not be supported. It is common to use one of the methods at once, not both.

dsrdtr is not supported by all platforms (silently ignored). Setting it to None has the effect that its state follows rtscts.

Also consider using the function serial_for_url() instead of creating Serial instances directly.

Changed in version 2.5: dsrdtr now defaults to False (instead of None)


Open port.


Close port immediately.


Destructor, close port when serial port instance is freed.

The following methods may raise ValueError when applied to a closed port.

Parameters:size – Number of bytes to read.
Returns:Bytes read from the port.

Read size bytes from the serial port. If a timeout is set it may return less characters as requested. With no timeout it will block until the requested number of bytes is read.

Changed in version 2.5: Returns an instance of bytes when available (Python 2.6 and newer) and str otherwise.

Parameters:data – Data to send.
Returns:Number of bytes written.
Raises SerialTimeoutException:
 In case a write timeout is configured for the port and the time is exceeded.

Write the string data to the port.

Changed in version 2.5: Accepts instances of bytes and bytearray when available (Python 2.6 and newer) and str otherwise.

Changed in version 2.5: Write returned None in previous versions.


Return the number of chars in the receive buffer.


Flush of file like objects. In this case, wait until all data is written.


Flush input buffer, discarding all it’s contents.


Clear output buffer, aborting the current output and discarding all that is in the buffer.

Parameters:duration – Time (float) to activate the BREAK condition.

Send break condition. Timed, returns to idle state after given duration.

Parameters:level – when true activate BREAK condition, else disable.

Set break: Controls TXD. When active, no transmitting is possible.

Parameters:level – Set control line to logic level.

Set RTS line to specified logic level.

Parameters:level – Set control line to logic level.

Set DTR line to specified logic level.

Returns:Current state (boolean)

Return the state of the CTS line.

Returns:Current state (boolean)

Return the state of the DSR line.

Returns:Current state (boolean)

Return the state of the RI line.

Returns:Current state (boolean)

Return the state of the CD line

Read-only attributes:


Device name. This is always the device name even if the port was opened by a number. (Read Only).

New in version 2.5.

Deprecated :use name instead

New values can be assigned to the following attributes (properties), the port will be reconfigured, even if it’s opened at that time:


Read or write port. When the port is already open, it will be closed and reopened with the new setting.


Read or write current baud rate setting.


Read or write current data byte size setting.


Read or write current parity setting.


Read or write current stop bit width setting.


Read or write current read timeout setting.


Read or write current write timeout setting.


Read or write current software flow control rate setting.


Read or write current hardware flow control setting.


Read or write current hardware flow control setting.


Read or write current inter character timeout setting.

The following constants are also provided:


A list of valid baud rates. The list may be incomplete such that higher baud rates may be supported by the device and that values in between the standard baud rates are supported. (Read Only).


A list of valid byte sizes for the device (Read Only).


A list of valid parities for the device (Read Only).


A list of valid stop bit widths for the device (Read Only).

The following methods are for compatibility with the io library.


New in version 2.5.


New in version 2.5.


New in version 2.5.

Parameters:b – bytearray or array instance
Returns:Number of byte read

Read up to len(b) bytes into bytearray b and return the number of bytes read.

New in version 2.5.

The port settings can be read and written as dictionary.

Returns:a dictionary with current port settings.

Get a dictionary with port settings. This is useful to backup the current settings so that a later point in time they can be restored using applySettingsDict().

Note that control lines (RTS/DTR) are part of the settings.

New in version 2.5.

Parameters:d – a dictionary with port settings.

Applies a dictionary that was created by getSettingsDict(). Only changes are applied and when a key is missing it means that the setting stays unchanged.

Note that control lines (RTS/DTR) are not changed.

New in version 2.5.

Platform specific methods.


Programs using the following methods and attributes are not portable to other platforms!

Platform :Unix
Platform :Windows

Return the number of bytes in the output buffer.

Changed in version 2.7: (Posix support added)

Platform :Unix

Configure the device for nonblocking operation. This can be useful if the port is used with select.

Platform :Unix
Returns:File descriptor.

Return file descriptor number for the port that is opened by this object. It is useful when serial ports are used with select.

Platform :Windows
Platform :Posix
Parameters:level – Set flow control state.

Manually control flow - when software flow control is enabled.

This will send XON (true) and XOFF (false) to the other device.

Changed in version 2.7: (renamed on Posix, function was called flowControl)

Platform :Posix
Parameters:enable – Set flow control state.

Manually control flow of outgoing data - when hardware or software flow control is enabled.

Sending will be suspended when called with False and enabled when called with True.

New in version 2.7: (Posix support added)

Platform :Windows

Attribute to configure RTS toggle control setting. When enabled and supported by OS, RTS will be active when data is available and inactive if no data is available.

New in version 2.6.


For systems that provide the io library (Python 2.6 and newer), the class Serial will derive from io.RawIOBase. For all others from FileLike.

Implementation detail: some attributes and functions are provided by the class SerialBase and some by the platform specific class and others by the base class mentioned above.

class serial.FileLike

An abstract file like class. It is used as base class for Serial when no io module is available.

This class implements readline() and readlines() based on read() and writelines() based on write().

Note that when the serial port was opened with no timeout, that readline() blocks until it sees a newline (or the specified size is reached) and that readlines() would never return and therefore refuses to work (it raises an exception in this case)!


Write a list of strings to the port.

The following three methods are overridden in Serial.


Flush of file like objects. It’s a no-op in this class, may be overridden.


Raises NotImplementedError, needs to be overridden by subclass.


Raises NotImplementedError, needs to be overridden by subclass.

The following functions are implemented for compatibility with other file-like objects, however serial ports are not seekable.

seek(pos, whence=0)
Raises IOError:always, as method is not supported on serial port

New in version 2.5.

Raises IOError:always, as method is not supported on serial port

New in version 2.5.

truncate(self, n=None)
Raises IOError:always, as method is not supported on serial port

New in version 2.5.

Raises IOError:always, as method is not supported on serial port

New in version 2.5.

To be able to use the file like object as iterator for e.g. for line in Serial(0): ... usage:


Return the next line by calling readline().


Returns self.

Other high level access functions.

readline(size=None, eol='\n')
  • size – Max number of bytes to read, None -> no limit.
  • eol – The end of line character.

Read a line which is terminated with end-of-line (eol) character (\n by default) or until timeout.

readlines(sizehint=None, eol='\n')
  • sizehint – Ignored parameter.
  • eol – The end of line character.

Read a list of lines, until timeout. sizehint is ignored and only present for API compatibility with built-in File objects.

Note that this function only returns on a timeout.


Read lines, implemented as generator. Unlike readlines (that only returns on a timeout) is this function yielding lines as they are received.

Deprecated since version 2.5: Use for line in Serial(...): ... instead. This method is not available in Python 2.6 and newer where the io library is available and pySerial bases on it.

Changed in version 2.5: Implement as generator.

RFC 2217 Network ports


This implementation is currently in an experimental state. Use at your own risk.

class rfc2217.Serial

This implements a RFC 2217 compatible client. Port names are URLs in the form: rfc2217://<host>:<port>[/<option>[/<option>]]

This class API is compatible to Serial with a few exceptions:

  • numbers as port name are not allowed, only URLs in the form described above.
  • writeTimeout is not implemented
  • The current implementation starts a thread that keeps reading from the (internal) socket. The thread is managed automatically by the rfc2217.Serial port object on open()/close(). However it may be a problem for user applications that like to use select instead of threads.

Due to the nature of the network and protocol involved there are a few extra points to keep in mind:

  • All operations have an additional latency time.
  • Setting control lines (RTS/CTS) needs more time.
  • Reading the status lines (DSR/DTR etc.) returns a cached value. When that cache is updated depends entirely on the server. The server itself may implement a polling at a certain rate and quick changes may be invisible.
  • The network layer also has buffers. This means that flush(), flushInput() and flushOutput() may work with additional delay. Likewise inWaiting() returns the size of the data arrived at the object internal buffer and excludes any bytes in the network buffers or any server side buffer.
  • Closing and immediately reopening the same port may fail due to time needed by the server to get ready again.

Not implemented yet / Possible problems with the implementation:

  • RFC 2217 flow control between client and server (objects internal buffer may eat all your memory when never read).
  • No authentication support (servers may not prompt for a password etc.)
  • No encryption.

Due to lack of authentication and encryption it is not suitable to use this client for connections across the internet and should only be used in controlled environments.

New in version 2.5.

class rfc2217.PortManager

This class provides helper functions for implementing RFC 2217 compatible servers.

Basically, it implements every thing needed for the RFC 2217 protocol. It just does not open sockets and read/write to serial ports (though it changes other port settings). The user of this class must take care of the data transmission itself. The reason for that is, that this way, this class supports all programming models such as threads and select.

Usage examples can be found in the examples where two TCP/IP - serial converters are shown, one using threads (the single port server) and an other using select (the multi port server).


Each new client connection must create a new instance as this object (and the RFC 2217 protocol) has internal state.

__init__(serial_port, connection, debug_output=False)
  • serial_port – a Serial instance that is managed.
  • connection – an object implementing write(), used to write to the network.
  • debug_output – enables debug messages: a logging.Logger instance or None.

Initializes the Manager and starts negotiating with client in Telnet and RFC 2217 protocol. The negotiation starts immediately so that the class should be instantiated in the moment the client connects.

The serial_port can be controlled by RFC 2217 commands. This object will modify the port settings (baud rate etc.) and control lines (RTS/DTR) send BREAK etc. when the corresponding commands are found by the filter() method.

The connection object must implement a write(data)() function. This function must ensure that data is written at once (no user data mixed in, i.e. it must be thread-safe). All data must be sent in its raw form (escape() must not be used) as it is used to send Telnet and RFC 2217 control commands.

For diagnostics of the connection or the implementation, debug_output can be set to an instance of a logging.Logger (e.g. logging.getLogger('rfc2217.server')). The caller should configure the logger using setLevel for the desired detail level of the logs.

Parameters:data – data to be sent over the network.
Returns:data, escaped for Telnet/RFC 2217

A generator that escapes all data to be compatible with RFC 2217. Implementors of servers should use this function to process all data sent over the network.

The function returns a generator which can be used in for loops. It can be converted to bytes using serial.to_bytes().

Parameters:data – data read from the network, including Telnet and RFC 2217 controls.
Returns:data, free from Telnet and RFC 2217 controls.

A generator that filters and processes all data related to RFC 2217. Implementors of servers should use this function to process all data received from the network.

The function returns a generator which can be used in for loops. It can be converted to bytes using serial.to_bytes().

Parameters:force_notification – Set to false. Parameter is for internal use.

This function needs to be called periodically (e.g. every second) when the server wants to send NOTIFY_MODEMSTATE messages. This is required to support the client for reading CTS/DSR/RI/CD status lines.

The function reads the status line and issues the notifications automatically.

New in version 2.5.

See also

RFC 2217 - Telnet Com Port Control Option


exception serial.SerialException

Base class for serial port exceptions.

Changed in version 2.5: Now derives from IOError instead of Exception

exception serial.SerialTimeoutException

Exception that is raised on write timeouts.




Stop bits


Note that 1.5 stop bits are not supported on POSIX. It will fall back to 2 stop bits.

Byte size



Default control characters (instances of bytes for Python 3.0+) for software flow control:


Module version:


A string indicating the pySerial version, such as 2.5.

New in version 2.3.

Module functions and attributes

Parameters:number – Port number.
Returns:String containing device name.
Deprecated :Use device names directly.

Convert a port number to a platform dependent device name. Unfortunately this does not work well for all platforms; e.g. some may miss USB-Serial converters and enumerate only internal serial ports.

The conversion may be made off-line, that is, there is no guarantee that the returned device name really exists on the system.

serial.serial_for_url(url, *args, **kwargs)
  • url – Device name, number or URL
  • do_not_open – When set to true, the serial port is not opened.

an instance of Serial or a compatible object.

Get a native or a RFC 2217 implementation of the Serial class, depending on port/url. This factory function is useful when an application wants to support both, local ports and remote ports. There is also support for other types, see URL section below.

The port is not opened when a keyword parameter called do_not_open is given and true, by default it is opened.

New in version 2.5.


This attribute is a list of package names (strings) that is searched for protocol handlers.

e.g. we want to support a URL foobar://. A module my_handlers.protocol_foobar is provided by the user:

s = serial.serial_for_url("foobar://")

For an URL starting with XY:// is the function serial_for_url() attempts to import PACKAGE.protocol_XY with each candidate for PACKAGE from this list.

New in version 2.6.

Parameters:sequence – String or list of integers
Returns:an instance of bytes

Convert a sequence to a bytes type. This is used to write code that is compatible to Python 2.x and 3.x.

In Python versions prior 3.x, bytes is a subclass of str. They convert str([17]) to '[17]' instead of '\x11' so a simple bytes(sequence) doesn’t work for all versions of Python.

This function is used internally and in the unit tests.

New in version 2.5.


The function serial_for_url() accepts the following types of URLs:

  • rfc2217://<host>:<port>[/<option>[/<option>]]
  • socket://<host>:<port>[/<option>[/<option>]]
  • loop://[<option>[/<option>]]

Device names are also supported, e.g.:

  • /dev/ttyUSB0 (Linux)
  • COM3 (Windows)

Future releases of pySerial might add more types. Since pySerial 2.6 it is also possible for the user to add protocol handlers using protocol_handler_packages.


Used to connect to RFC 2217 compatible servers. All serial port functions are supported. Implemented by rfc2217.Serial.

Supported options in the URL are:

  • ign_set_control does not wait for acknowledges to SET_CONTROL. This option can be used for non compliant servers (i.e. when getting an remote rejected value for option 'control' error when connecting).
  • poll_modem: The client issues NOTIFY_MODEMSTATE requests when status lines are read (CTS/DTR/RI/CD). Without this option it relies on the server sending the notifications automatically (that’s what the RFC suggests and most servers do). Enable this option when getCTS() does not work as expected, i.e. for servers that do not send notifications.
  • timeout=<value>: Change network timeout (default 3 seconds). This is useful when the server takes a little more time to send its answers. The timeout applies to the initial Telnet / RFC 2271 negotiation as well as changing port settings or control line change commands.
  • logging=[debug|info|warning|error]: Prints diagnostic messages (not useful for end users). It uses the logging module and a logger called pySerial.rfc2217 so that the application can setup up logging handlers etc. It will call logging.basicConfig() which initializes for output on sys.stderr (if no logging was set up already).

The purpose of this connection type is that applications using pySerial can connect to TCP/IP to serial port converters that do not support RFC 2217.

Uses a TCP/IP socket. All serial port settings, control and status lines are ignored. Only data is transmitted and received.

Supported options in the URL are:

  • logging=[debug|info|warning|error]: Prints diagnostic messages (not useful for end users). It uses the logging module and a logger called pySerial.socket so that the application can setup up logging handlers etc. It will call logging.basicConfig() which initializes for output on sys.stderr (if no logging was set up already).

The least useful type. It simulates a loop back connection (RX<->TX RTS<->CTS DTR<->DSR). It could be used to test applications or run the unit tests.

Supported options in the URL are:

  • logging=[debug|info|warning|error]: Prints diagnostic messages (not useful for end users). It uses the logging module and a logger called pySerial.loop so that the application can setup up logging handlers etc. It will call logging.basicConfig() which initializes for output on sys.stderr (if no logging was set up already).

This type uses to obtain a list of ports and searches the list for matches by a regexp (see re) that follows the slashes.

Depending on the capabilities of the list_ports module on the system, it is possible to search for the description or hardware ID of a device, e.g. USB VID:PID or texts.

Unfortunately, on some systems list_ports only lists a subset of the port names with no additional information. Currently, on Windows and Linux it should find additional information.


  • rfc2217://localhost:7000
  • rfc2217://localhost:7000/poll_modem
  • rfc2217://localhost:7000/ign_set_control/timeout=5.5
  • socket://localhost:7777
  • loop://logging=debug
  • hwgrep://0451:f432 (USB VID:PID)


New in version 2.6.

This module can be executed to get a list of ports (python -m It also contains the following functions.
Returns:an iterable.

The function returns an iterable that yields tuples of three strings:

  • port name as it can be passed to serial.Serial or serial.serial_for_url()
  • description in human readable form
  • sort of hardware ID. E.g. may contain VID:PID of USB-serial adapters.

Items are returned in no particular order. It may make sense to sort the items. Also note that the reported strings are different across platforms and operating systems, even for the same device.


Support is limited to a number of operating systems. On some systems description and hardware ID will not be available (None).

Platform :Posix (/dev files)
Platform :Linux (/dev files, sysfs and lsusb)
Platform :OSX (iokit)
Platform :Windows (setupapi, registry)
Parameters:regexp – regular expression (see stdlib re)
Returns:filtered sequence, see comports().

Search for ports using a regular expression. Port name, description and hardware ID are searched (case insensitive). The function returns an iterable that contains the same tuples that comport() generates but only those that match the regexp.

New in version 2.6.

Miniterm is now available as module instead of example. see Miniterm for details.

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