sys – system specific functions

This module implements a subset of the corresponding CPython module, as described below. For more information, refer to the original CPython documentation: sys.


sys.exit(retval=0, /)

Terminate current program with a given exit code. Underlyingly, this function raise as SystemExit exception. If an argument is given, its value given as an argument to SystemExit.



A mutable list of arguments the current program was started with.


The byte order of the system ("little" or "big").


Object with information about the current Python implementation. For CircuitPython, it has following attributes:

  • name - string “circuitpython”

  • version - tuple (major, minor, micro), e.g. (1, 7, 0)

  • _machine - string describing the underlying machine

  • _mpy - supported mpy file-format version (optional attribute)

This object is the recommended way to distinguish CircuitPython from other Python implementations (note that it still may not exist in the very minimal ports).

Difference to CPython

CPython mandates more attributes for this object, but the actual useful bare minimum is implemented in CircuitPython.


Maximum value which a native integer type can hold on the current platform, or maximum value representable by CircuitPython integer type, if it’s smaller than platform max value (that is the case for CircuitPython ports without long int support).

This attribute is useful for detecting “bitness” of a platform (32-bit vs 64-bit, etc.). It’s recommended to not compare this attribute to some value directly, but instead count number of bits in it:

bits = 0
v = sys.maxsize
while v:
    bits += 1
    v >>= 1
if bits > 32:
    # 64-bit (or more) platform
    # 32-bit (or less) platform
    # Note that on 32-bit platform, value of bits may be less than 32
    # (e.g. 31) due to peculiarities described above, so use "> 16",
    # "> 32", "> 64" style of comparisons.

Dictionary of loaded modules. On some ports, it may not include builtin modules.


A mutable list of directories to search for imported modules.

Difference to CPython

On MicroPython, an entry with the value ".frozen" will indicate that import should search frozen modules at that point in the search. If no frozen module is found then search will not look for a directory called .frozen, instead it will continue with the next entry in sys.path.


The platform that CircuitPython is running on. For OS/RTOS ports, this is usually an identifier of the OS, e.g. "linux". For baremetal ports it is an identifier of the chip on a board, e.g. "MicroChip SAMD51". It thus can be used to distinguish one board from another. If you need to check whether your program runs on CircuitPython (vs other Python implementation), use sys.implementation instead.


Mutable attributes holding strings, which are used for the REPL prompt. The defaults give the standard Python prompt of >>> and ....


Standard error stream.


Standard input stream.


Standard output stream.


A mutable attribute holding an integer value which is the maximum number of traceback entries to store in an exception. Set to 0 to disable adding tracebacks. Defaults to 1000.

Note: this is not available on all ports.


Python language version that this implementation conforms to, as a string.


Python language version that this implementation conforms to, as a tuple of ints.

Difference to CPython

Only the first three version numbers (major, minor, micro) are supported and they can be referenced only by index, not by name.