Baptiste Fontaine’s Blog  (back to the website)

Unique Strings and Unicode

For the past three years I’ve been running a Twitter bot called @lemondeadit, a French equivalent of @NYT_first_said which tweets new words as they appear in the French newspaper Le Monde.

The account was created in June 2019 but the first commit dates back to 2018. One day I might blog a bit about how it works under the hood, but today I’d like to talk about a small bug that made me crazy for a while.

The bot has been running stable for a couple of years without me touching the code at all. But recently, I noticed that sometimes it would tweet some very common words as if they were new, like “économie” or “opéra”. I deleted them when I saw them, but I didn’t understand why it happened.

I initially attributed the bug to an issue with the search engine: when the bot sees a word it never saw before, it uses Le Monde’s internal search engine to verify that this word indeed appears in a single article. Around the end of 2019,Le Monde ditched their previous search engine, which was slow but exact, in favor of a Qwant-based one which is fast but inexact: it doesn’t respect your query (it may or may not autocorrect your query and there’s no way to prevent it from doing so), it’s not accurate (no result may or may not mean that there is no match) and it’s not stable (do the same query twice and you can get different results; even by a large order of magnitude). In French, we would say this is “de la merde”.

Anyway, I thought it was an issue where maybe using the internal search engine would give a single result instead of multiple ones, but this doesn’t make sense: the bot doesn’t check every single word with the search engine, but only those it thinks are unique. Since it has already scrapped virtually all articles from the newspaper, there’s no way it would consider “économie” as a new word.

Today, I decided to tackle the issue.

First, I searched in the database for a recent example, and I got “vaccinées” (the feminin plural form of “vaccinated”). This is a very old word; it first appeared in Le Monde in 1946 and we saw it more than 1.7k times since then. The word even showed up twice in the database, despite a UNIQUE constraint on the field:

   word
-----------
 vaccinées
 vaccinées

I inspected the strings and noticed they were encoded differently, although they were both in UTF-8. The difference was on the “é” character: it turns out that both are different: one is represented as the Unicode character LATIN SMALL LETTER E WITH ACUTE while the other is a combination of LATIN SMALL LETTER E and COMBINING ACUTE ACCENT. They both render exactly the same on screen, which makes this issue so hard to detect by a human.

>>> "é" == "é"
False

The fix was to normalize the text before interpreting its words. I did it in Python using unicodedata.normalize:

text = unicodedata.normalize("NFKC", text)

There are multiple normalization forms, and I chose NFKC: normalize equivalent characters to their canonical one, and compose all combining characters such that the non-combined and the combined versions become the same.

After fixing the code, I needed to fix the database to fix already-parsed words. In PostgreSQL, we can use SIMILAR TO to search for strings matching a certain characters range: in my case, the “Combining Diacritical Marks”, aka 0300-036F:

SELECT word FROM word WHERE word SIMILAR TO '%[\u0300-\u036F]%' ;

I use Peewee to interact with Postgres from Python. While it doesn’t support SIMILAR TO out of the box, it’s simple to use a custom expression:

Word.select().where(Expression(Word.word, "SIMILAR TO", "%[\\u0300-\\u036F]%"))

Unfortunately this feature is not available in SQLite, the database I use to run my integration tests, and so I had to adapt the code a little bit: first use SqliteExtDatabase instead of SqliteDatabase to get support for REGEXP, and then use the .regexp operator:

# SQLite
Word.select().where(Word.word.regexp("[\u0300-\u036F]"))

I was then able to run a quick function to normalize the ~1k words affected by the issue.

To conclude, it seemed a very weird issue at first but in the end it allowed me to learn a few things about Unicode, Postgres, and Python’s fantastic standard library.

Typing Tuples in Python

Python added support for type hints in 3.5. These are not typing as you may be used to in other languages since they have no effect on the compilation to bytecode nor at runtime, but rather hints for the (tools of the) developer.

def print_int(n: int):
    print(n)

print_int(1)
print_int("foo")

The code above doesn’t fail when you call print_int("foo") even though n is “typed” as an int. This is because this n: int is just a hint.

While you can check for type issues by running mypy by hand, type hints become really powerful when your editor/IDE supports them.

Types for collections can specify the inner types: a list (List) that contains strings (str) would be List[str].

from typing import List

stuff: list = []  # a list of anything (equivalent to List[Any])
stuff.append(1)
stuff.append("foo")

offsets: List[int] = []  # a list of ints
offsets.append(1)
offsets.append("foo")

In the snippet above, the last line is highlighted as an error in any good IDE, and mypy would complain about it.

Other container types exist as well, and they can be nested:

from typing import List, Dict, Iterable

# a dictionary where keys are str and values are List[str], i.e. lists of strings
friends: Dict[str, List[str]] = {}
friends["Alice"] = ["Sam", "Maria"]

# a function that takes an iterable of ints. It can be a list, a tuple, a generator, a set, etc
def average(s: Iterable[int]):
    total = 0
    count = 0
    for element in s:
        total += element
        count += 1

    return total / count

Given List[x], Collection[x], Sequence[x] and other Set[x], one would expect Tuple[x] to be a hint for a tuple that contains x. Well… no.

This is confusing at first, but Tuple[str] types a tuple of a single element of type str. To add more elements, you need to type them as well: a pair of ints would be Tuple[int, int], while a triplet of a string, a float and a boolean would be Tuple[str, int, bool].

While tuples can be used as sequences (e.g. for immutable/hashable equivalents to lists), I’d argue that their primary use is for fixed-length representations, such as pairs of results:

def match_object(data: bytes):
    # example code
    distances = [0.99, 0.97, 0.96]
    indices = [432, 12, 3923]
    return distances, indices

In this snippet, match_object returns a tuple of a list of floats and a list of integers (aka Tuple[List[float], List[int]]).

If you still want to type arbitrary-length homogeneous tuples, there’s a syntax for that: Tuple[int, ...] types a tuple of any length, including 0, that contains only int elements (and yes, ... is valid in Python).

For this and other interrogations (how to type a generator?), Mypy has a useful type hints cheat sheet.

TL;DR: if you know the size of the tuple, use Tuple[x], Tuple[x, x], Tuple[x, x, x], etc. If you don’t, use Tuple[x, ...], but all elements must be of type x.

Fix bin/magento taking all the RAM

While working with Magento 2.3.6 on Bixoto I hit a weird issue: the bin/magento command-line tool was always eating all the RAM, even with a simple command:

$ ./bin/magento --help
PHP Fatal error:  Allowed memory size of 2147483648 bytes exhausted (tried to allocate 262144 bytes) in /home/baptiste/.../vendor/magento/module-store/Model/Config/Placeholder.php on line 146
Check https://getcomposer.org/doc/articles/troubleshooting.md#memory-limit-errors for more info on how to handle out of memory errors.

The issue, as weird as it sounds, is an empty configuration value that causes Magento to end up in an infinite loop.

When I installed Magento on my local machine, I deactivated HTTPS by setting web/secure/base_url to NULL in the table core_config_data. This alone is the cause of the issue.

Check in MySQL:

SELECT * FROM core_config_data WHERE path = 'web/secure/base_url' LIMIT 1;

If this shows a line with a NULL value, either delete it or replace it with some non-null value:

UPDATE core_config_data SET value='http://...' WHERE path = 'web/secure/base_url' limit 1;

This has been reported to Magento but was closed because “it’s not a bug”. I don’t think falling in an infinite loop on --help because some config value is NULL should really be a normal behavior, but at least now you know how to solve it.