Core Language

Generated Thu 16 May 2019 04:46:41 UTC

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: ../unix/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: ../unix/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: ../unix/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: ../unix/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: ../unix/micropython: not found

Unpacking function arguments in non-last position isn’t detected as an error

Workaround: The syntax below is invalid, never use it in applications.

Sample code:

print(*(1, 2), 3)
CPy output: uPy output: 
1 2 3

/bin/sh: 1: ../unix/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: ../unix/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: ../unix/micropython: not found

import

__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/parallels/Documents/micropython-doc/micropython/tests/cpydiff/modules']

/bin/sh: 1: ../unix/micropython: not found

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

/bin/sh: 1: ../unix/micropython: not found

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

/bin/sh: 1: ../unix/micropython: not found