let statement in python

Today, when i wrote another context manager, i came up with the idea – write let statement in python using context manager.

First i invented simple realisation, where we manually pass locals:

from contextlib import contextmanager

def let(locals_, **bindings):
    original = {var: locals_.get(var) for var in bindings.keys()}

And usage:

>>> a = 1
>>> b = 2
>>> with let(locals(), a=10, b=20):
...     print(a, b)  # inside `let` scope
(10, 20)
>>> print(a, b)  # outside of `let` scope
(1, 2)

Looks ugly. But we can use little piece of magic with inspect. We can get outer frame and get his locals

from contextlib import contextmanager
from inspect import currentframe, getouterframes

def let(**bindings):
    frame = getouterframes(currentframe(), 2)[-1][0] # 2 because first frame in `contextmanager` decorator  
    locals_ = frame.f_locals
    original = {var: locals_.get(var) for var in bindings.keys()}

Now we don’t need to pass locals explicitly:

>>> a = 3
>>> b = 4
>>> with let(a=33, b=44):
...     print(a, b)
(33, 44)
>>> print(a, b)
(3, 4)

Green threads on pyboard

Some weeks ago i received new fancy device — pyboard. It’s like arduino, but using python instead of wiring. I was little disappointed, because micropython wasn’t have multithreading module.

And i developed microasync — library with green threads for micropython.

Little example, code for toggling leds concurrently:

from microasync.async import loop, coroutine, Delay
import pyb

@coroutine  # decorator for making green thread from function
def toggle_led_on_interval(led, interval):
    while True:
        yield Delay(interval)  # like time.sleep, but non-blocking

toggle_led_on_interval(1, 1)
toggle_led_on_interval(2, 2)
toggle_led_on_interval(3, 1)
toggle_led_on_interval(4, 2)

# start main loop:

And result:

doto from clojure in python

When i writing code in clojure i can use good macro – doto.

With whom python code like:

window = QMainWindow()
window.setAttribute(Qt.WA_TransparentForMouseEvents, True)

can be translated in clojure code like:

(doto (QMainWindow.)
      (.setTitle title)
      (.setWindowFlags Qt/FramelessWindowHint)
      (.setAttribute Qt/WA_TransparentForMouseEvents True)

And i wrote hackish class for doing something similar in python.

from functools import partial

class DoTo(object):
    def __init__(self, obj):
        self._obj = obj

    def _do(self, name, *args, **kwargs):
        getattr(self._obj, name)(*args, **kwargs)
        return self

    def __getattr__(self, item):
        return partial(self._do, item)

With method chaining we can emulate behavior of clojure doto:

window = QMainWindow()
    .setAttribute(Qt.WA_TransparentForMouseEvents, True)\