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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.

Functions

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.

sys.atexit(func)

Register func to be called upon termination. func must be a callable that takes no arguments, or None to disable the call. The atexit function will return the previous value set by this function, which is initially None.

Difference to CPython

This function is a MicroPython extension intended to provide similar functionality to the atexit module in CPython.

sys.print_exception(exc, file=sys.stdout, /)

Print exception with a traceback to a file-like object file (or sys.stdout by default).

Difference to CPython

This is simplified version of a function which appears in the traceback module in CPython. Unlike traceback.print_exception(), this function takes just exception value instead of exception type, exception value, and traceback object; file argument should be positional; further arguments are not supported. CPython-compatible traceback module can be found in micropython-lib.

sys.settrace(tracefunc)

Enable tracing of bytecode execution. For details see the CPython documentaion.

This function requires a custom MicroPython build as it is typically not present in pre-built firmware (due to it affecting performance). The relevant configuration option is MICROPY_PY_SYS_SETTRACE.

Constants

sys.argv

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

sys.byteorder

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

sys.implementation

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

  • name - string “micropython”

  • 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 MicroPython 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 MicroPython.

sys.maxsize

Maximum value which a native integer type can hold on the current platform, or maximum value representable by MicroPython integer type, if it’s smaller than platform max value (that is the case for MicroPython 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
    ...
else:
    # 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.
sys.modules

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

sys.path

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.

sys.platform

The platform that MicroPython 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 a board, e.g. "pyboard" for the original MicroPython reference board. It thus can be used to distinguish one board from another. If you need to check whether your program runs on MicroPython (vs other Python implementation), use sys.implementation instead.

sys.ps1
sys.ps2

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

sys.stderr

Standard error stream.

sys.stdin

Standard input stream.

sys.stdout

Standard output stream.

sys.tracebacklimit

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.

sys.version

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

sys.version_info

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.