Partial application and piping with ... and @

In newer versions of Python we have two not much used features: ellipsis:

>>> print(...)

And matrix multiplication operator:

class Dummy(str):
    def __matmul__(self, other):
        print('{}@{}'.format(self, other))

>>> Dummy('ok') @ 'there'

So let’s start with ..., in Scala we can partially apply (or curry) function with _:

def add(x: Int, y: Int) = x + y
val addOne = add(1, _: Int)
addOne(5)  6: Int

Wouldn’t it be nice to have similar option in Python, like:

def add(x, y):
    return x + y
addFive = add(..., 5)

And we can easily implement it with some decorator:

from functool import wraps

class Partial:
    def __init__(self, fn, args, kwargs):
        self._fn = fn
        self._args = args
        self._kwargs = kwargs

    def __call__(self, replacement):
        args = [replacement if arg is ... else arg
                for arg in self._args]
        kwargs = {key: replacement if val is ... else val
                  for key, val in self._kwargs.items()}
        return self._fn(*args, **kwargs)

    def __repr__(self):
        return '<Partial: {}(*{}, **{})>'.format(
            self._fn.__name__, repr(self._args), repr(self._kwargs))

def ellipsis_partial(fn):
    def wrapper(*args, **kwargs):
        ellipsises = (list(args) + list(kwargs.values())).count(...)
        if ellipsises > 1:
            raise TypeError('Only one ... allowed as an argument.')
        elif ellipsises:
            return Partial(fn, args, kwargs)
            return fn(*args, **kwargs)

    return wrapper

So here if we find ... in arguments, we return Partial object. And when the object called – we replace ... with passed value. In action:

def add(x, y):
    return x + y

addFive = add(5, ...)
>>> addFive(10)

And it works! So back to matrix multiplication operator. In F# there’s nice piping operators:

> [1..10] |> List.filter (fun x -> x % 3 = 0)
val it : int list = [3; 6; 9]

So with our operator in Python it should look like:

range(1, 10) @ filter(lambda x: x % 3 == 0, ...)

And we can easily implement it just by adding __rmatmul__ to Partial:

class Partial:
    def __init__(self, fn, args, kwargs):
        self._fn = fn
        self._args = args
        self._kwargs = kwargs

    def __call__(self, replacement):
        args = [replacement if arg is ... else arg
                for arg in self._args]
        kwargs = {key: replacement if val is ... else val
                  for key, val in self._kwargs.items()}
        return self._fn(*args, **kwargs)

    def __rmatmul__(self, replacement):
        return self(replacement)

    def __repr__(self):
        return '<Partial: {}(*{}, **{})>'.format(
            self._fn.__name__, repr(self._args), repr(self._kwargs))

And in action:

filter = ellipsis_partial(filter)
to_list = ellipsis_partial(list)
>>> range(1, 10) @ filter(lambda x: x % 3 == 0, ...) @ to_list(...)
[3, 6, 9]

And we can use it in even more complex cases:

map = ellipsis_partial(map)
join = ellipsis_partial(str.join)
>>> range(1, 10) @ map(lambda x: x + 4, ...) \
                 @ filter(lambda x: x % 3 == 0, ...) \
                 @ map(str, ...) \
                 @ join(', ', ...)
6, 9, 12

But it’s a bit not nice to wrap all callables in ellipsis_partial, we can use some hacks with inspect or module loaders to doing it automatically, but it’s too magic for me. So we can add little function that wrap and call:

def _(fn, *args, **kwargs):
    return ellipsis_partial(fn)(*args, **kwargs)


from functools import reduce
>>> range(1, 10) @ map(lambda x: x + 4, ...) \
                 @ filter(lambda x: x % 3 == 0, ...) \
                 @ _(reduce, lambda x, y: x * y, ...) \
                 @ _('value: {}'.format, ...)
value: 648

However it may look strange and unpythonic, but I guess it would be nice to see something like this in future Python releases.

Gist with sources, next part.

Abusing annotations with dependency injection

Python 3 has a nice feature – type annotations:

def add(x: int, y: int) -> int:
    return x + y

That can be used by IDEs and stuff like mypy for type checking. However we can easily access it:

>>> add.__annotations__
{'return': int, 'x': int, 'y': int}

And use it for things like dependency injection. For example we have a web app:

def get_db_connection() -> abc.DBConnection:

def get_routes() -> abc.Router:

def get_cache(db: abc.DBConnection) -> abc.CacheManager:

def init_app():
    db = get_db_connection()
    routes = get_routes()
    cache = get_cache(db)
    app = Application(routes=routes,

if __name__ == '__main__':

Looks a bit Java-like with interfaces (abstract classes, abc), but it’s useful in huge apps. However components are tightly coupled, and we need to use monkey patching for testing it.

Let’s examine annotations:

>>> get_cache.__annotations__
{'db': abc.DBConnection, 'return': abc.CacheManager}

We can see that the function requires abc.DBConnection and provides abc.CacheManager. We need to track all functions like this, it’ll be easy with some decorator:

from weakref import WeakValueDictionary

_provides = WeakValueDictionary()

def provides(fn):
    """Register function that provides something."""
        _provides[fn.__annotations__['return']] = fn
    except KeyError:
        raise ValueError('Function not annotated.')

    return fn

We use WeakValueDictionary in case function somehow can be deleted.

Let’s apply this decorator:

def get_db_connection() -> abc.DBConnection:

def get_routes() -> abc.Router:

def get_cache(*, db: abc.DBConnection) -> abc.CacheManager:

And move dependencies of main function to arguments:

def init_app(*, routes: abc.Router,
             db: abc.DBConnection,
             cache: abc.CacheManager):
    app = Application(routes=routes,

So we can think about our functions as a graph:

graph TB A[init_app]---B[get_routes] A---C[get_db_connection] A---D[get_cache] D---C

And we can easily write injector that resolve and inject dependencies:

class Injector:
    """Resolve and inject dependencies."""

    def __init__(self):
        self._resolved = {}

    def _get_value(self, name):
        """Get dependency by name (type)."""
        if name not in _provides:
            raise ValueError("Dependency {} not registered.".format(
        if name not in self._resolved:           
            fn = _provides[name]
            kwargs = self._get_dependencies(fn)
            return fn(**kwargs)
        return self._resolved[name]

    def _get_dependencies(self, fn):
        """Get dependencies for function."""
        return {key: self._get_value(value)
                for key, value in fn.__annotations__.items()
                if key != 'return'}

    def run(self, fn):
        """Resolve dependencies and run function."""
        kwargs = self._get_dependencies(fn)
        return fn(**kwargs)

So we can make our app work by adding:

if __name__ == '__main__':

Although this approach is simple and straightforward, it’s overkill for most of apps.

Package on github.

Rerenderer rendering performance

Rerenderer is a React-like library for cross platform drawing on canvas. And we experimenting a lot with ways to improve performance of rendering. Not so while ago I wrote about intermediate language and interpreter, this approach was interesting, but not so much efficient. Performing optimizations on that language wasn’t enough fast and interpreter on Android was based on reflection and was a bit slow because of that.

So now we decided to sacrifice flexibility and move component implementation to host platforms (we use Kotlin for Android and ClojureScript for browsers). And use simpler approach with canvases tree (one canvas for each component), where we rerender only changed canvases and ancestors. So for example we have a simple app:

(ns rerenderer.example.core
  (:require [rerenderer.core :refer [init!]]
            [rerenderer.primitives :refer [rectangle text]]
            [rerenderer.debug :refer [swap-state!]]))

(defn labeled
  [{:keys [label width] :as options} & children]
  (rectangle options
    (text {:x (- width 50) :y 0
           :width 40 :height 40
           :font-size 36
           :color "#3E454C"
           :value label})))

(defn root
  [{:keys [background-color first-color second-color third-color]}]
  (labeled {:x 0 :y 0
            :width 800 :height 400
            :color background-color
            :label "A"}
    (labeled {:x 30 :y 30
              :width 200 :height 200
              :color first-color
              :label "B"})
    (labeled {:x 100 :y 100
              :width 500 :height 250
              :color second-color
              :label "C"}
      (labeled {:x 400 :y 150
                :width 100 :height 100
                :color third-color
                :label "D"}))))

(def initial-state
  {:background-color "#FFF6E5"
   :first-color "#7ECEFD"
   :second-color "#FF7F66"
   :third-color "#2185C5"})

(defonce app (init! :root-view #'root
                    :state initial-state
                    :canvas (.getElementById js/document "canvas")
                    :width 800
                    :height 600))

Whole scene can be represented as a tree:

graph TB A[A]-->B[B] A-->C[C] C-->D[D]

So when we change D:

(swap-state! app assoc :third-color :red)

We rerender D, C and A, but don’t touch B:

graph TB A[A]-->B[B] A-->C[C] C-->D[D] style D fill:red; style C stroke:red; style A stroke:red;

But when we change C:

(swap-state! app assoc :second-color :white)

We rerender only C, A and don’t touch B and D. Because changes doesn’t affect it canvases:

graph TB A[A]-->B[B] A-->C[C] C-->D[D] style C fill:red; style A stroke:red;

You can easily notice performance boost in browser on more complex example – Conway’s Game of Life:

And on Android:

It’s faster then before, but you can notice that it’s not smooth, now we have a little problem with GC.

Configure Jest for React Native

Jest is a very popular unit testing tool for React, but it doesn’t work with React Native out of the box. And even instruction in React Native docs is wrong. If you use it you’ll end up without ability to disable mocks, without working es6 imports and with ton of ReferenceError.

So, I accumulated information from Jest issues and got working config.

First of all we need to install jest-cli and babel-jest:

npm install --save-dev jest-cli babel-jest babel-polyfill babel-preset-react-native

Then fill .babelrc with:

  "presets": ["react-native"],
  "retainLines": true

Without retainLines you’ll get wrong line numbers in tests traces.

And update package.json:

  "scripts": {
    "test": "jest"
  "jest": {
    "haste": {
      "defaultPlatform": "android",
      "platforms": [
      "providesModuleNodeModules": [
    "unmockedModulePathPatterns": [

That’s all, now you can run tests with:

npm test

Also a lot of outdated docs uses jest.autoMockOff, it’s deprecated and doesn’t work with es6 imports. You should use jest.disableAutomock().

If you want to use use enzyme for testing React Native components, you should install:

npm install --save-dev enzyme react-addons-test-utils react-dom react-native-mock

Then add react and enzyme to unmockedModulePathPatterns. And create __mocks__/react-native.js with:

import ReactNative from 'react-native-mock';

__DEV__ = true;

module.exports = ReactNative;

Now you can mock React Native and use enzyme.

Parse shell one-liners with pyparsing

For one of my projects I needed some one-liners parser to AST. I’ve tried PLY, pyPEG and a few more. And stopped on pyparsing. It’s actively maintained, works without magic and easy to use.

Ideally I wanted to parse something like:

LANG=en_US.utf-8 git diff | wc -l >> diffs

To something like:

(= LANG en_US.utf-8)
(>> (| (git diff) (wc -l))

So let’s start with simple shell command, it’s just space-separated tokens:

import pyparsing as pp

token = pp.Word(pp.alphanums + '_-.')
command = pp.OneOrMore(token)

command.parseString('git branch --help')
>>> ['git', 'branch', '--help']

It’s simple, another simple part is parsing environment variables. One environment variable is token=token, and list of them separated by spaces:

env = pp.Group(token + '=' + token)

>>>[['A', '=', 'B']]

env_list = pp.OneOrMore(env)

env_list.parseString('VAR=test X=1')
>>> [['VAR', '=', 'test'], ['X', '=', '1']]

And now we can easily merge command and environment variables, mind that environment variables are optional:

command_with_env = pp.Optional(pp.Group(env_list)) + pp.Group(command)

command_with_env.parseString('LOCALE=en_US.utf-8 git diff')
>>> [[['LOCALE', '=', 'en_US.utf-8']], ['git', 'diff']]

Now we need to add support of pipes, redirects and logical operators. Here we don’t need to know what they’re doing, so we’ll treat them just like separators between commands:

separators = ['1>>', '2>>', '>>', '1>', '2>', '>', '<', '||', '|', '&&', '&', ';']
separator = pp.oneOf(separators)
command_with_separator = pp.OneOrMore(pp.Group(command) + pp.Optional(separator))

command_with_separator.parseString('git diff | wc -l >> out.txt')
>>> [['git', 'diff'], '|', ['wc', '-l'], '>>', ['out.txt']]

And now we can merge environment variables, commands and separators:

one_liner = pp.Optional(pp.Group(env_list)) + pp.Group(command_with_separator)
one_liner.parseString('LANG=C DEBUG=true git branch | wc -l >> out.txt')
>>> [[['LANG', '=', 'C'], ['DEBUG', '=', 'true']], [['git', 'branch'], '|', ['wc', '-l'], '>>', ['out.txt']]]

Result is hard to process, so we need to structure it:

one_liner = pp.Optional(env_list).setResultsName('env') + \
result = one_liner.parseString('LANG=C DEBUG=true git branch | wc -l >> out.txt')

print('env:', result.env, '\ncommand:', result.command)
>>> env: [['LANG', '=', 'C'], ['DEBUG', '=', 'true']] 
>>> command: [['git', 'branch'], '|', ['wc', '-l'], '>>', ['out.txt']]

Although we didn’t get AST, but just a bunch of grouped tokens. So now we need to transform it to proper AST:

def prepare_command(command):
    """We don't need to work with pyparsing internal data structures,
    so we just convert them to list.
    for part in command:
        if isinstance(part, str):
            yield part
            yield list(part)

def separator_position(command):
    """Find last separator position."""
    for n, part in enumerate(command[::-1]):
        if part in separators:
            return len(command) - n - 1

def command_to_ast(command):
    """Recursively transform command to AST.""" 
    n = separator_position(command)
    if n is None:
        return tuple(command[0])
        return (command[n],
                command_to_ast(command[n + 1:]))

def to_ast(parsed):
    if parsed.env:
        for env in parsed.env:
            yield ('=', env[0], env[2])
    command = list(prepare_command(parsed.command))
    yield command_to_ast(command)
>>> [('=', 'LANG', 'C'),
>>>  ('=', 'DEBUG', 'true'),
>>>  ('>>', ('|', ('git', 'branch'),
>>>               ('wc', '-l')),
>>>         ('out.txt',))]

It’s working. The last part, glue that make it easier to use:

def parse(command):
    result = one_liner.parseString(command)
    ast = to_ast(result)
    return list(ast)
parse('LANG=en_US.utf-8 git diff | wc -l >> diffs')
>>> [('=', 'LANG', 'en_US.utf-8'),
     ('>>', ('|', ('git', 'diff'),
                  ('wc', '-l')),

Although it can’t parse all one-liners, it doesn’t support nested commands like:

echo $(git branch)
echo `git branch`

But it’s enough for my task and support of not implemented features can be added easily.

Gist with source code.

Three months in Cyprus

Nicosia aqueduct

From March to May I was in Cyprus. Most of time I was in Nicosia, but visited most of Cyprus’s cities. Nicosia located in the middle of the island, 30-50 minutes by bus from the sea (Larnaca). The city split between two countries and has some sort of wall.

For entering country I was using Pro Visa, it’s very easy to obtain, you just need to fill some form and send to consulate by email. It allows to stay 90 days in Cyprus.

In Nicosia I was living in city center, and was renting apartment with shared kitchen for €300 including water, electricity and internet. Internet was like 20 MBit/s.

Mobile internet is a bit pricey, I was using MTN and paid €15 for 1.5GB for month. LTE available in almost all cities.

Food is relatively cheap, for everyday meat/fish/seafood I spend around €40 per week. Prices is supermarket near my apartment was €4-5 for pork, €5-6 for chicken, €5-10 for fish and €8-10 for octopus and other seafood, €0.3-1 for vegetables.

Alcohol is also cheap, €1-1.5 for can of beer, €4-5 for six pack. Local wine was around €4-5. Local strong drinks like Ouzo and Zivania was €5-10.

Main problem with shopping in Cyprus, is that shops close very early on Wednesday and Saturday, and they doesn’t work on Sundays.

Although, Nicosia is very comfortable place to live.

In this trip I also visited Israel for two weeks, lived in Tel-Aviv. And Georgia, for two weeks too, lived in Tbilisi. They’re both very nice and interesting places.

The Performance of Open Source Applications

book cover I already read all other books from this series (The Architecture of Open Source Applications volumes I and II) so I decided to read latest book – The Performance of Open Source Applications. And it’s quite a nice book. Not all chapters were interesting for me. But it was lovely to read about network stack of Chrome and about performance problems in it and about Mozilla’s stack problems and solutions.

Also the book contains great explanation why pings on mobile devices can be big, and what problems can it cause.

Lightweight events in Kotlin

In one of my applications I needed some minimalistic event system and all I can found was require creating a ton of classes or use a lot of annotations, or was magick-scala-like with overloaded -=, += and () just because author can do it.

So I decided to write something very simple, with interface like:

ClickEvent on {
    println("CLICK: ${it.x} ${it.y}")

ClickEvent(10, 20).emit()

And it was implemented easily:

open class Event<T> {
    var handlers = listOf<(T) -> Unit>()

    infix fun on(handler: (T) -> Unit) {
        handlers += handler

    fun emit(event: T) {
        for (subscriber in handlers) {

data class ClickEvent(val x: Int, val y: Int) {
    companion object : Event<ClickEvent>()

    fun emit() = Companion.emit(this)

ClickEvent on {
    println("CLICK: ${it.x} ${it.y}")

ClickEvent(10, 20).emit()

>>> CLICK: 10 20

You can notice a bit of boilerplate, main reason to have it is because in Kotlin we don’t have static methods, we can’t use inheritance with data classes and also we can’t use companion object of interface (or even parent class) in class that implements it. So in each event we have to duplicate some redundant code:

data class MyEvent(val data: String) {
    companion object : Event<MyEvent>()

    fun emit() = Companion.emit(this)

Back to positive side, let’s implement a few other events and try it in action. Start from event without arguments, it can’t be a data class:

class ConnectionLostEvent {
    companion object : Event<ConnectionLostEvent>()

    fun emit() = Companion.emit(this)

ConnectionLostEvent on {
    println("Oh, snap!")


>>> Oh, snap!

What if we want events hierarchy, we can use sealed class for event type and few classes for events. They also can’t be data classes:

sealed class MouseEvent {
    companion object : Event<MouseEvent>()

    class Click(val x: Int, val y: Int) : MouseEvent() {
        fun emit() = Companion.emit(this)

    class Move(val fromX: Int, val fromY: Int, val toX: Int, val toY: Int) : MouseEvent() {
        fun emit() = Companion.emit(this)

MouseEvent on {
    when (it) {
        is MouseEvent.Click -> println("Click x:${it.x} y:${it.y}")
        is MouseEvent.Move -> println(
                "Move from ${it.fromX}:${it.fromY} to ${it.toX}:${it.toY}")

MouseEvent.Click(50, 40).emit()
MouseEvent.Move(0, 0, 9, 12).emit()

>>> Click x:50 y:40
>>> Move from 0:0 to 9:12

As you can see it isn’t that nice that it can be implemented with Scala or Clojure. Kotlin isn’t rich language, but it’s simple and works pretty well on Android.

Also you notice that code highlighting in the article looks wrong, it’s because rouge, highlighting engine used by github pages, doesn’t support Kotlin right now, and I just used one for Scala.

Daniel Holden: Build Your Own Lisp

book cover On Reddit I stumbled on Build Your Own Lisp by Daniel Holden and decided to read it. And it’s one of the most interesting technical book I’m read. Reading this book I refreshed my knowledge of C, got few ideas for my own projects and even almost built my own Lisp.

Although in this book explains building process of not actual Lisp, it doesn’t have quotation '(f x), it uses Q-Expressions {f x}. And also it doesn’t have macros. But the book suggested readers to write all of that in the last chapter.

Freeze time in tests even with GAE datastore

It’s not so rare thing to freeze time in tests and for that task I’m using freezegun and it works nice almost every time:

from freezegun import freeze_time

def test_get_date():
    with freeze_time("2016.1.1"):
        assert get_date() == datetime(2016, 1, 1)

But not with GAE datastore. Assume that we have a model Document with created_at = db.DateTimeProperty(auto_now_add=True), so test like:

from freezegun import freeze_time

def test_created_at():
    with freeze_time('2016.1.1'):
        doc = Document()
        assert doc.created_at == datetime(2016, 1, 1)

Will fail with:

BadValueError: Unsupported type for property created_at: <class 'freezegun.api.FakeDatetime'>

But it can be easily fixed if we monkey patch GAE internals:

from contextlib import contextmanager
from google.appengine.api import datastore_types
from mock import patch
from freezegun import freeze_time as _freeze_time
from freezegun.api import FakeDatetime

def freeze_time(*args, **kwargs):
    with patch('google.appengine.ext.db.DateTimeProperty.data_type',
        datastore_types._VALIDATE_PROPERTY_VALUES[FakeDatetime] = \
        datastore_types._PACK_PROPERTY_VALUES[FakeDatetime] = \
        datastore_types._PROPERTY_MEANINGS[FakeDatetime] = \
        with _freeze_time(*args, **kwargs):

And now it works!