Core language

Generated Tue 05 Mar 2024 02:12:06 UTC

f-strings don’t support concatenation with adjacent literals if the adjacent literals contain braces or are f-strings

Cause: MicroPython is optimised for code space.

Workaround: Use the + operator between literal strings when either or both are f-strings

Sample code:

x, y = 1, 2
print("aa" f"{x}")  # works
print(f"{x}" "ab")  # works
print("a{}a" f"{x}")  # fails
print(f"{x}" "a{}b")  # fails
print(f"{x}" f"{y}")  # fails

CPy output:

uPy output:

 
aa1
1ab
a{}a1
1a{}b
12

/bin/sh: 1: ../ports/unix/build-standard/micropython: not found

f-strings cannot support expressions that require parsing to resolve unbalanced nested braces and brackets

Cause: MicroPython is optimised for code space.

Workaround: Always use balanced braces and brackets in expressions inside f-strings

Sample code:

print(f'{"hello { world"}')
print(f'{"hello ] world"}')

CPy output:

uPy output:

 
hello { world
hello ] world

/bin/sh: 1: ../ports/unix/build-standard/micropython: not found

Raw f-strings are not supported

Cause: MicroPython is optimised for code space.

Sample code:

rf"hello"

CPy output:

uPy output:

 
/bin/sh: 1: ../ports/unix/build-standard/micropython: not found

f-strings don’t support !a conversions

Cause: MicropPython does not implement ascii()

Workaround: None

Sample code:

f"{'unicode text'!a}"

CPy output:

uPy output:

 
/bin/sh: 1: ../ports/unix/build-standard/micropython: not found

Classes

Special method __del__ not implemented for user-defined classes

Sample code:

import gc


class Foo:
    def __del__(self):
        print("__del__")


f = Foo()
del f

gc.collect()

CPy output:

uPy output:

 
__del__

/bin/sh: 1: ../ports/unix/build-standard/micropython: not found

Method Resolution Order (MRO) is not compliant with CPython

Cause: Depth first non-exhaustive method resolution order

Workaround: Avoid complex class hierarchies with multiple inheritance and complex method overrides. Keep in mind that many languages don’t support multiple inheritance at all.

Sample code:

class Foo:
    def __str__(self):
        return "Foo"


class C(tuple, Foo):
    pass


t = C((1, 2, 3))
print(t)

CPy output:

uPy output:

 
Foo

/bin/sh: 1: ../ports/unix/build-standard/micropython: not found

When inheriting from multiple classes super() only calls one class

Cause: See Method Resolution Order (MRO) is not compliant with CPython

Workaround: See Method Resolution Order (MRO) is not compliant with CPython

Sample code:

class A:
    def __init__(self):
        print("A.__init__")


class B(A):
    def __init__(self):
        print("B.__init__")
        super().__init__()


class C(A):
    def __init__(self):
        print("C.__init__")
        super().__init__()


class D(B, C):
    def __init__(self):
        print("D.__init__")
        super().__init__()


D()

CPy output:

uPy output:

 
D.__init__
B.__init__
C.__init__
A.__init__

/bin/sh: 1: ../ports/unix/build-standard/micropython: not found

Calling super() getter property in subclass will return a property object, not the value

Sample code:

class A:
    @property
    def p(self):
        return {"a": 10}


class AA(A):
    @property
    def p(self):
        return super().p


a = AA()
print(a.p)

CPy output:

uPy output:

 
{'a': 10}

/bin/sh: 1: ../ports/unix/build-standard/micropython: not found

Functions

Error messages for methods may display unexpected argument counts

Cause: MicroPython counts “self” as an argument.

Workaround: Interpret error messages with the information above in mind.

Sample code:

try:
    [].append()
except Exception as e:
    print(e)

CPy output:

uPy output:

 
append() takes exactly one argument (0 given)

/bin/sh: 1: ../ports/unix/build-standard/micropython: not found

Function objects do not have the __module__ attribute

Cause: MicroPython is optimized for reduced code size and RAM usage.

Workaround: Use sys.modules[function.__globals__['__name__']] for non-builtin modules.

Sample code:

def f():
    pass


print(f.__module__)

CPy output:

uPy output:

 
__main__

/bin/sh: 1: ../ports/unix/build-standard/micropython: not found

User-defined attributes for functions are not supported

Cause: MicroPython is highly optimized for memory usage.

Workaround: Use external dictionary, e.g. FUNC_X[f] = 0.

Sample code:

def f():
    pass


f.x = 0
print(f.x)

CPy output:

uPy output:

 
0

/bin/sh: 1: ../ports/unix/build-standard/micropython: not found

Generator

Context manager __exit__() not called in a generator which does not run to completion

Sample code:

class foo(object):
    def __enter__(self):
        print("Enter")

    def __exit__(self, *args):
        print("Exit")


def bar(x):
    with foo():
        while True:
            x += 1
            yield x


def func():
    g = bar(0)
    for _ in range(3):
        print(next(g))


func()

CPy output:

uPy output:

 
Enter
1
2
3
Exit

/bin/sh: 1: ../ports/unix/build-standard/micropython: not found

Runtime

Local variables aren’t included in locals() result

Cause: MicroPython doesn’t maintain symbolic local environment, it is optimized to an array of slots. Thus, local variables can’t be accessed by a name.

Sample code:

def test():
    val = 2
    print(locals())


test()

CPy output:

uPy output:

 
{'val': 2}

/bin/sh: 1: ../ports/unix/build-standard/micropython: not found

Code running in eval() function doesn’t have access to local variables

Cause: MicroPython doesn’t maintain symbolic local environment, it is optimized to an array of slots. Thus, local variables can’t be accessed by a name. Effectively, eval(expr) in MicroPython is equivalent to eval(expr, globals(), globals()).

Sample code:

val = 1


def test():
    val = 2
    print(val)
    eval("print(val)")


test()

CPy output:

uPy output:

 
2
2

/bin/sh: 1: ../ports/unix/build-standard/micropython: not found

import

__all__ is unsupported in __init__.py in MicroPython.

Cause: Not implemented.

Workaround: Manually import the sub-modules directly in __init__.py using from . import foo, bar.

Sample code:

from modules3 import *

foo.hello()

CPy output:

uPy output:

 
hello

/bin/sh: 1: ../ports/unix/build-standard/micropython: not found

__path__ attribute of a package has a different type (single string instead of list of strings) in MicroPython

Cause: MicroPython doesn’t support namespace packages split across filesystem. Beyond that, MicroPython’s import system is highly optimized for minimal memory usage.

Workaround: Details of import handling is inherently implementation dependent. Don’t rely on such details in portable applications.

Sample code:

import modules

print(modules.__path__)

CPy output:

uPy output:

 
['/github/workspace/micropython/tests/cpydiff/modules']

/bin/sh: 1: ../ports/unix/build-standard/micropython: not found

MicroPython doesn’t support namespace packages split across filesystem.

Cause: MicroPython’s import system is highly optimized for simplicity, minimal memory usage, and minimal filesystem search overhead.

Workaround: Don’t install modules belonging to the same namespace package in different directories. For MicroPython, it’s recommended to have at most 3-component module search paths: for your current application, per-user (writable), system-wide (non-writable).

Sample code:

import sys

sys.path.append(sys.path[1] + "/modules")
sys.path.append(sys.path[1] + "/modules2")

import subpkg.foo
import subpkg.bar

print("Two modules of a split namespace package imported")

CPy output:

uPy output:

 
Two modules of a split namespace package imported

/bin/sh: 1: ../ports/unix/build-standard/micropython: not found