Core language

Generated Sun 17 Oct 2021 12:08:56 UTC

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

Cause: MicroPython is optimised for code space.

Workaround: Use the + operator between literal strings when either is an f-string

Sample code:

x = 1
print("aa" f"{x}")
print(f"{x}" "ab")
print("a{}a" f"{x}")
print(f"{x}" "a{}b")

CPy output:

uPy output:

  File "<stdin>", line 9
    print("aa" f"{x}")
               ^
SyntaxError: invalid syntax
aa1
1ab
Traceback (most recent call last):
  File "<stdin>", line 11, in <module>
IndexError: tuple index out of range

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

Cause: MicroPython is optimised for code space.

Workaround: Only use simple expressions inside f-strings

Sample code:

f'{"hello {} world"}'
f"{repr({})}"

CPy output:

uPy output:

  File "<stdin>", line 8
    f'{"hello {} world"}'
                        ^
SyntaxError: invalid syntax
Traceback (most recent call last):
  File "<stdin>", line 8
SyntaxError: invalid syntax

Raw f-strings are not supported

Cause: MicroPython is optimised for code space.

Sample code:

rf"hello"

CPy output:

uPy output:

  File "<stdin>", line 8
    rf"hello"
            ^
SyntaxError: invalid syntax
Traceback (most recent call last):
  File "<stdin>", line 8
SyntaxError: raw f-strings are not supported

f-strings don’t support the !r, !s, and !a conversions

Cause: MicroPython is optimised for code space.

Workaround: Use repr(), str(), and ascii() explictly.

Sample code:

class X:
    def __repr__(self):
        return "repr"

    def __str__(self):
        return "str"


print(f"{X()!r}")
print(f"{X()!s}")

CPy output:

uPy output:

  File "<stdin>", line 17
    print(f"{X()!r}")
                   ^
SyntaxError: invalid syntax
Traceback (most recent call last):
  File "<stdin>", line 17
SyntaxError: invalid syntax

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__

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
(1, 2, 3)

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__
D.__init__
B.__init__
A.__init__

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}
<property>

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)
function takes 2 positional arguments but 1 were given

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__
Traceback (most recent call last):
  File "<stdin>", line 13, in <module>
AttributeError: 'function' object has no attribute '__module__'

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
Traceback (most recent call last):
  File "<stdin>", line 13, in <module>
AttributeError: 'function' object has no attribute 'x'

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
Enter
1
2
3

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}
{'test': <function test at 0x7f18d6fbb100>, '__name__': '__main__', '__file__': '<stdin>'}

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
2
1

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
Traceback (most recent call last):
  File "<stdin>", line 9, in <module>
NameError: name 'foo' isn't defined

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

Cause: MicroPython does’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:

['/home/micropython/micropython-docs/tests/cpydiff/modules']
../tests/cpydiff//modules

Failed to load modules are still registered as loaded

Cause: To make module handling more efficient, it’s not wrapped with exception handling.

Workaround: Test modules before production use; during development, use del sys.modules["name"], or just soft or hard reset the board.

Sample code:

import sys

try:
    from modules import foo
except NameError as e:
    print(e)
try:
    from modules import foo

    print("Should not get here")
except NameError as e:
    print(e)

CPy output:

uPy output:

foo
name 'xxx' is not defined
foo
name 'xxx' is not defined
foo
name 'xxx' isn't defined
Should not get here

MicroPython does’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
Traceback (most recent call last):
  File "<stdin>", line 13, in <module>
ImportError: no module named 'subpkg.bar'