National Bank of Poland leads on helping Ukraine’s currency

Ukrainian refugees see their currency hardly convertible at fair rates or even accepted. The lack of liquidity will also hit Ukraine’s government urgent expenses (medical supplies or weapons).

The National Bank of Poland (NBP) offered a comprehensive financial package to address these problems. Firstly, it enabled the refugees to exchange the currency at a nearly official rate, through the agreement with the National Bank of Ukraine (NBU) signed on March 18.

Secondly, in a follow-up agreement the NBP offered a FX Swap for USD 1 billion which will provide the Ukraine central bank with more liquidity.

As Reuters announced on March 24, EU countries are close to follow and agree on a scheme enabling exchanging Ukraine’s cash. The rules disclosed in the draft are close to the NBP scheme (exchange capped at around 10,000 UAH per individual)

Evidence-based quick wins in reducing oil demand

10-point action plan on cutting the oil demand by IEA.

Following the Russia invasion on Ukraine an oil supply shock is expected and what is worse there may be no supply increase from OPEC+ . A way out is to cut the demand as advised by IEA. The proposed steps are quick-wins: easy to implement and reverse while having measurable and significant impact. The overall impact should balance the supply gap.

Quantitative estimates can be backed with empirical or even statistical evidence. A good example is the speed reduction, as recent research advances demonstrate quantitatively what drivers learn by experience: fuel consumption considerably increases with speed beyond a turning point. Turning this statement around: you can likely save lots of petrol driving at a lower speed. The study of He at al. finds cubic approximations fit well and enables several optimisation considerations for various groups of car users.

Source: „Study on a Prediction Model of Superhighway Fuel Consumption Based on the Test of Easy Car Platform”, He at al.

Robust Azure ETLs with Python

Microsoft Azure cloud computing platform faces criticism due to its Python API being little customisable and poorly documented. Still it is a popular choice for many companies, thus data scientists need to squeeze maximum of features and performance out of it rather than complain. Below I am sharing thoughts on creating robust Extract-Transform-Load processes in this setup.

Extract from database: consumer-producer!

The key trick is to enable the consumer-producer pattern: you want to process your data in batches, for the sake of robustness and efficiency. The popular pandas library dedicated for tabular data is not enough for this task, as it hinders useful features. Gain more control using dedicated database drivers, e.g. psycogp driver for the popular Postgres database.

import psycopg2

dsn = f'user={db_user} password={db_password} dbname={db_name} host={db_host}'
conn = psycopg2.connect(dsn)

query = """
    SELECT *
    FROM sales
    WHERE date > '2021'
    """


def process_rows(row_group):
    '''do your stuff, e.g. filter and append to a file'''
    pass


n_prefetch = 10000


# mind the server-side cursor! 
with conn.cursor(name='server_side_cursor') as cur:
    cur.itersize = n_prefetch
    cur.execute(query)
    while True:
        row_group = cur.fetchmany(n_prefetch)
        if len(row_group) > 0:
            process_rows(row_group)
        else:
            break

Cast datatypes

Cast datatypes early and explicitly having these point in mind

Best to do this casting upstream, adapting at the database driver level, like in this example:

import psycopg2

# cast some data types upon receiving from database
datetype_casted = psycopg2.extensions.new_type(
    psycopg2.extensions.DATE.values, "date", psycopg2.DATETIME
)
psycopg2.extensions.register_type(datetype_casted)
decimal_casted = psycopg2.extensions.new_type(
    psycopg2.extensions.DECIMAL.values, "decimal", psycopg2.extensions.FLOAT
)
psycopg2.extensions.register_type(decimal_casted)

Use Parquet to store tabular data

The Apache Parquet is invaluable to efficient work with large data in tabular format. There are two major drivers for Pyhon: pyarrow and fastparquet. The first one can be integrated with Azure data flows (e.g. you can stream and filter data), although it has been limited in supporting more sophisticated data such as timedelta. Remember that writing Parquet incurs memory overhead so better to do this in batches. The relevant code may look as below:

def sql_to_parquet(
    conn, query, column_names, target_dir, n_prefetch=1000000, **parquet_kwargs
):
    """Writes the result of a SQL query to a Parquet file (in chunks).

    Args:
        conn: Psycopg connection object (must be open)
        query: SQL query of "select" type
        column_names: column names given to the resulting SQL table
        target_dir: local directory where Parquet is written to; must exist, data is overwritten
        n_prefetch: chunk of SQL data processed (read from SQL and dumped to Parquet) at a time. Defaults to 1000000.
    """
    with conn.cursor(name="server_side_cursor") as cur:
        # start query
        cur.itersize = n_prefetch
        cur.execute(query)
        # set up consumer
        chunk = 0
        # consume until stream is empty
        while True:
            # get and process one batch
            row_group = cur.fetchmany(n_prefetch)
            chunk += 1
            if len(row_group) > 0:
                out = pd.DataFrame(data=row_group, columns=column_names)
                fname = os.path.join(target_dir, f"part_{chunk:04d}.parquet")
                out.to_parquet(fname, engine="pyarrow", **parquet_kwargs)
            else:
                break


def df_to_parquet(df, target_dir, chunk_size=100000, **parquet_kwargs):
    """Writes pandas DataFrame to parquet format with pyarrow.

    Args:
        df: pandas DataFrame
        target_dir: local directory where parquet files are written to
        chunk_size: number of rows stored in one chunk of parquet file. Defaults to 100000.
    """
    for i in range(0, len(df), chunk_size):
        slc = df.iloc[i : i + chunk_size]
        chunk = int(i / chunk_size)
        fname = os.path.join(target_dir, f"part_{chunk:04d}.parquet")
        slc.to_parquet(fname, engine="pyarrow", **parquet_kwargs)

Leverage Azure API properly!

The Azure API is both cryptic in documentation and under-explained in data science blogs. Below I am sharing a comprehensive receipt on how to upload and register Parquet data with minimum effort. While uploading makes the data persist, registering enables further Azure features (such as pipe-lining). Best to dump your Parquet file to a directory (large Parquet files should be chunked) with tempfile then use the Azure Dataset class to both upload to the storage (use universally unique identifier for reproducibility/avoiding path collisions) and register to the workspace (use Tabular subclass). Use classes Workspace and Datastore to facilitate interaction with Azure.

from azureml.core import Workspace, Dataset
import tempfile
from uuid import uuid4
import psycopg2

ws = Workspace.from_config()
dstore = ws.datastores.get("my_datastore")

with tempfile.TemporaryDirectory() as tempdir:
    sql_to_parquet(conn, query, query_cols, tempdir)
    target = (dstore, f"raw/{tab}/{str(uuid4())}")
    Dataset.File.upload_directory(tempdir, target)
    ds = Dataset.Tabular.from_parquet_files(target)
    ds.register(
        ws,
        name=f"dataset_{tab}",
        description="created from Sales table",
        tags={"JIRA": "ticket-01"},
        create_new_version=True,
    )

Quantitative overview of Nord Stream 2 issues

The briefing document of the European Parliament offers a quantitative and comprehensive view on the issues with Nord Stream 2. This excellent figure alone explains a lot:

European Parliament Briefings: Russian pipelines to Europe and Turkey.

It may be interesting to realise that the points this document makes are underrepresented in popular press such as BBC. These concerns are discussed in measurable terms:

  • The existing capacity is enough
  • Nord Stream 2 harms the diversification of routes (Polish/Ukrainian paths reduced, Germany becoming the main hub)
  • There is a documented history of using the gas infrastructure for political purposes
  • EU bodies warned on political and security risks

Iron logic defeats strawman

The straw man rhetorical technique is widely used: from scientific reviews (academia), through job interviews (industry) to the political discourse (international relations). The idea is simple: you distort arguments or opposing views to easier refute them. Not only is this trick popular, but proven successful statistically.

The best weapon against it is logically strict reasoning. Do not rush in quick answers and judgments, but rather use the playback and evaluate/falsify the claim in measurable terms as much as possible. This note aims to spread awareness of this logical fallacy and give illustrating examples, which appears particularly noteworthy in the context of the rise of misinformation.

  • Academia: a popular „straw man” variation is when reviewers exaggerate on English mistakes in scientific papers, concluding that the math content may be equally incorrect. While scientists should strive for perfection at the communication level, in life and computer sciences the technical content is more important. Fix English but don’t let your „this paper presents merit findings” claim get distorted by this. Remember that a) researchers compete with each other and b) it’s easier to complain on misspelled words than to evaluate complex math.
  • Industrial career: you believe you are a good fit, but the interviewers exaggerate on the necessity of some skill X which you don’t have at a moment. We speak of a fallacy when X is not essential for the role, or trivial to learn. Don’t let your „I am a good fit” get distorted!
  • International relations: instead of my own examples, I will refer to the excellent article written by the UK Defence Secretary. He attempts to falsify several claims coming from the government circles of the Russian Federation against the NATO alliance. One is „Russia being encircled by NATO”, which is replied by asking to measure the overlapping border.

Can Russia defend its economy from sanctions?

While western leaders are proud of imposing largest sanctions ever, it seems that Russia can withstand the crisis longer than some expect. The following two points, underrepresented in popular media, support this claim:

Russia seems both experienced and equipped to contain this crisis in short-term. After the crisis in 2014 it has used oil-generated income to collect more reserves and keep founding military.

Russian reserves in mln USD
Russian military spending in bln USD

But time will tell what the long-term financial consequences for the West and Russia would be.

Banks in Austria block financial help amid war on Ukraine

Amid the Russia’s invasion, some banks blocks money transfers to Ukraine. This effectively undermines the help efforts of both the international community and the expats. Here comes the wall of shame: a picture is worth thousand words:

Poland does the right thing: polish banks offered free transfers to Ukraine.

Demystifying TestDom

Recruiters nowadays use online timed tests when screening developers. I recently looked at Python & Algorithms Hard questions at TestDome. While the timing and hints seem to push towards implementing tricks from scratch, for the quality in long term it is better to structure the problem and use established solutions (divide & conquer). The battery of „hard” problems can be solved in few lines of code, using the standard Python libraries (Numpy, Pandas, Collections).

League Table (Hard)
The task is to essentially sort the football results by multiple criteria: first by the higher number of scores, next (breaking ties) by the lower number of games, finally (breaking ties) by the default order. While the hint suggests building a custom comparator, the effective way is by Pandas tables (5 lines).

import pandas as pd

def player_rank(self, rank):
        ts = []
        ts = map(lambda t:(t[1][0],t[1][1]['score'],t[1][1]['games_played'],t[0]),enumerate(self.standings.items()))
        df = pd.DataFrame(ts)
        df = df.sort_values(by=[1,2,3],ascending=[False,True,True]).reset_index()
        return df.loc[rank-1][0]

Sorted Search (Hard)
The task is to determine how many values in a sorted list are less than the given value. This is equivalent to finding the right position to insert (to maintain the sorted order). While the solution suggests playing with a variant of binary search on your own, the effective way is by the Python bisect algorithm (1 line).

from bisect import bisect_left

def count_numbers(sorted_list, less_than):
    return bisect_left(sorted_list,less_than)

Train Composition (Hard)
The task is to implement a data structure that mimics composing the train: one can quickly attach and detach to/from both ends. The effective way is to use Python double-ended queue (5 lines).

from collections import deque

class TrainComposition:
    
    def __init__(self):
        self.d = deque()
    
    def attach_wagon_from_left(self, wagonId):
        self.d.appendleft(wagonId)
    
    def attach_wagon_from_right(self, wagonId):
        self.d.append(wagonId)

    def detach_wagon_from_left(self):
        return self.d.popleft()
    
    def detach_wagon_from_right(self):
        return self.d.pop()

On Subgaussian Concentration of Missing Mass

The problem of missing mass goes back to the cryptographic work of Good and Turing during WWII, but has been also studied in the context of linguistic, ecology, and by probability theoreticians. The missing mass is defined as the weight of elements not observed in a sample, due to pure chance:

True and empirical frequency based on 100 samples, for Poiss(10).
The value of 12 is missing, and the corresponding bin is empty.

Such an event and the total mass of all unseen elements are exponentially small in the sample size.

Quite curiously, proving it rigorously has been quite hard. The first proof appeared at COLT’00, but has since then been reworked many times, in attempts to simplify and numerically improve. The arguments were based on a thermodynamic framework, logarithmic Sobolev inequalities and information theory. Formally, for some constant \(K>0\) we want

$$\Pr[\pm(M-\boldsymbol{E}M)>\epsilon]\leq \mathrm{e}^{-K\cdot n \epsilon^2}$$

where \(n\) is the sample size and \(M\) is the missing mass for the i.i.d. sample of size \(n\).

The problem of proving it „standard” concentration inequalities has been open so far. In reponse to this challenge, in my recent note I have proved it with Bernstein’s inequality, century old. So, no complicated approaches and refinements were necessary!

Approximating Tails of Beta Distribution

Beta distribution is ubiquitous in statistics, but particularly popular in real-world modeling. The beta-binomial model is perhaps the most known example, given the recent interest in Bayesian inference. But it was in use nearly 50 years ago, for example in toxicology.

Unfortunately, computing probabilities from the density depends on intractable incomplete beta integrals. This creates a demand for closed-form approximations, particularly for probability cfs/tails. The goal is to obtain an exponential concentration inequality in of Bernstein-type

$$\Pr[|X-\mathbf{E}[X]|>\epsilon]\leq \mathrm{e}^{-\frac{\epsilon^2}{2v^2+2c\epsilon}}.$$

Such bounds have been studied few times, the last one being the sub-gaussian approximation, that is when \(c=0\). Recently, I have further improved to optimal \(v\) (most important) and some good value of \(c\) (less important, but possibly worth further improvement). This gives a more accurate approximation when the distribution is very skewed (this happens when we model rare events, like conversion). For example with Beta(2,998) we get this:

The trick is to obtain a recursion scheme on central moments, and bound their growth by a geometric progression. The details are in my paper, and the code is shared in this notebook.