Modules¶
Generated Wed 11 Dec 2024 08:37:00 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])
CPy output: |
uPy output: |
Traceback (most recent call last):
File "<stdin>", line 9, in <module>
NotImplementedError:
|
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)
CPy output: |
uPy output: |
Traceback (most recent call last):
File "<stdin>", line 9, in <module>
OverflowError: signed char is greater than maximum
|
array('b', [1])
|
Looking for integer not implemented¶
Sample code:
import array
print(1 in array.array("B", b"12"))
CPy output: |
uPy output: |
False
|
Traceback (most recent call last):
File "<stdin>", line 9, in <module>
NotImplementedError:
|
Array deletion not implemented¶
Sample code:
import array
a = array.array("b", (1, 2, 3))
del a[1]
print(a)
CPy output: |
uPy output: |
array('b', [1, 3])
|
Traceback (most recent call last):
File "<stdin>", line 10, in <module>
TypeError: 'array' object doesn't support item deletion
|
Subscript with step != 1 is not yet implemented¶
Sample code:
import array
a = array.array("b", (1, 2, 3))
print(a[3:2:2])
CPy output: |
uPy output: |
array('b')
|
Traceback (most recent call last):
File "<stdin>", line 10, in <module>
NotImplementedError: only slices with step=1 (aka None) are supported
|
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))
CPy output: |
uPy output: |
42
|
Traceback (most recent call last):
File "<stdin>", line 12, in <module>
TypeError: function takes 1 positional arguments but 2 were given
|
json¶
JSON module does not throw exception when object is not serialisable¶
Sample code:
import json
try:
print(json.dumps(b"shouldn't be able to serialise bytes"))
except TypeError:
print("TypeError")
CPy output: |
uPy output: |
TypeError
|
"shouldn't be able to serialise bytes"
|
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"))
CPy output: |
uPy output: |
None
VALUE
|
should not get here
None
VALUE
|
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"))
CPy output: |
uPy output: |
None
None
|
None
VALUE
|
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))
CPy output: |
uPy output: |
16200208048889254063
|
Traceback (most recent call last):
File "<stdin>", line 11, in <module>
ValueError: bits must be 32 or less
|
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))
CPy output: |
uPy output: |
x=340282366920938463463374607431768211456
|
Traceback (most recent call last):
File "<stdin>", line 11, in <module>
OverflowError: overflow converting long int to machine word
|
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")
CPy output: |
uPy output: |
struct.error
|
b'\x01\x00'
Should not get here
|
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")
CPy output: |
uPy output: |
struct.error
|
b'\x01\x02'
Should not get here
|
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")
CPy output: |
uPy output: |
b'\x01\x02'
Should have worked
|
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)
CPy output: |
uPy output: |
None
|
Traceback (most recent call last):
File "<stdin>", line 9, in <module>
AttributeError: 'module' object has no attribute 'stdin'
|