Modules

Generated Mon 05 Jan 2026 06:11:07 UTC

Positional-only Parameters

To save code size, many functions that accept keyword arguments in CPython only accept positional arguments in MicroPython.

MicroPython marks positional-only parameters in the same way as CPython, by inserting a / to mark the end of the positional parameters. Any function whose signature ends in / takes only positional arguments. For more details, see PEP 570.

Example

For example, in CPython 3.4 this is the signature of the constructor socket.socket:

socket.socket(family=AF_INET, type=SOCK_STREAM, proto=0, fileno=None)

However, the signature documented in MicroPython is:

socket(af=AF_INET, type=SOCK_STREAM, proto=IPPROTO_TCP, /)

The / at the end of the parameters indicates that they are all positional-only in MicroPython. The following code works in CPython but not in most MicroPython ports:

import socket
s = socket.socket(type=socket.SOCK_DGRAM)

MicroPython will raise an exception:

TypeError: function doesn't take keyword arguments

The following code will work in both CPython and MicroPython:

import socket
s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

array

Comparison between different typecodes not supported

Cause: Code size

Workaround: Compare individual elements

Sample code:

import array

array.array("b", [1, 2]) == array.array("i", [1, 2])

Overflow checking is not implemented

Cause: MicroPython implements implicit truncation in order to reduce code size and execution time

Workaround: If CPython compatibility is needed then mask the value explicitly

Sample code:

import array

a = array.array("b", [257])
print(a)

Looking for integer not implemented

Sample code:

import array

print(1 in array.array("B", b"12"))

Array deletion not implemented

Sample code:

import array

a = array.array("b", (1, 2, 3))
del a[1]
print(a)

Subscript with step != 1 is not yet implemented

Sample code:

import array

a = array.array("b", (1, 2, 3))
print(a[3:2:2])

builtins

Second argument to next() is not implemented

Cause: MicroPython is optimised for code space.

Workaround: Instead of val = next(it, deflt) use:

try:
    val = next(it)
except StopIteration:
    val = deflt

Sample code:

print(next(iter(range(0)), 42))

deque

Deque not implemented

Workaround: Use regular lists. micropython-lib has implementation of collections.deque.

Sample code:

import collections

D = collections.deque()
print(D)

json

JSON module does not throw exception when object is not serialisable

Sample code:

import json

a = bytes(x for x in range(256))
try:
    z = json.dumps(a)
    x = json.loads(z)
    print("Should not get here")
except TypeError:
    print("TypeError")

os

environ attribute is not implemented

Workaround: Use getenv, putenv and unsetenv

Sample code:

import os

try:
    print(os.environ.get("NEW_VARIABLE"))
    os.environ["NEW_VARIABLE"] = "VALUE"
    print(os.environ["NEW_VARIABLE"])
except AttributeError:
    print("should not get here")
    print(os.getenv("NEW_VARIABLE"))
    os.putenv("NEW_VARIABLE", "VALUE")
    print(os.getenv("NEW_VARIABLE"))

getenv returns actual value instead of cached value

Cause: The environ attribute is not implemented

Sample code:

import os

print(os.getenv("NEW_VARIABLE"))
os.putenv("NEW_VARIABLE", "VALUE")
print(os.getenv("NEW_VARIABLE"))

random

getrandbits method can only return a maximum of 32 bits at a time.

Cause: PRNG’s internal state is only 32bits so it can only return a maximum of 32 bits of data at a time.

Workaround: If you need a number that has more than 32 bits then utilize the random module from micropython-lib.

Sample code:

import random


x = random.getrandbits(64)
print("{}".format(x))

randint method can only return an integer that is at most the native word size.

Cause: PRNG is only able to generate 32 bits of state at a time. The result is then cast into a native sized int instead of a full int object.

Workaround: If you need integers larger than native wordsize use the random module from micropython-lib.

Sample code:

import random


x = random.randint(2**128 - 1, 2**128)
print("x={}".format(x))

struct

Struct pack with too few args, not checked by uPy

Sample code:

import struct

try:
    print(struct.pack("bb", 1))
    print("Should not get here")
except:
    print("struct.error")

Struct pack with too many args, not checked by uPy

Sample code:

import struct

try:
    print(struct.pack("bb", 1, 2, 3))
    print("Should not get here")
except:
    print("struct.error")

Struct pack with whitespace in format, whitespace ignored by CPython, error on uPy

Cause: MicroPython is optimised for code size.

Workaround: Don’t use spaces in format strings.

Sample code:

import struct

try:
    print(struct.pack("b b", 1, 2))
    print("Should have worked")
except:
    print("struct.error")

sys

Overriding sys.stdin, sys.stdout and sys.stderr not possible

Cause: They are stored in read-only memory.

Sample code:

import sys

sys.stdin = None
print(sys.stdin)