[Python-ideas] Use lazy loading with hashtable in python gettext module

Serge Ballesta s-ball at laposte.net
Sun Dec 23 12:06:48 EST 2018

Hi all,

The feed back on my initial mail convinced me that it was important to 
allow the current behaviour of eagerly loading the whole catalog, and 
that keeping the files opened should also be optional.

All that lead to this proposal:

The gettext module should be allowed to load lazily the catalogs from mo 
file. This lazy load should be optional and make use of the hash tables 
from mo files when they are present or revert to a binary search. The 
translation strings should be cached for better performances.

API changes:
3 functions from the gettext module will have 2 new optional parameter 
named caching, and keepopen:

gettext.bindtextdomain(domain, localedir=None) would become
gettext.bindtextdomain(domain, localedir=None, caching=None, keepopen=False)

gettext.translation(domain, localedir=None, languages=None, class_=None, 
fallback=False, codeset=None) would become
gettext.translation(domain, localedir=None, languages=None, class_=None, 
fallback=False, codeset=None, caching=None, keepopen=False)

gettext.install(domain, localedir=None, codeset=None, names=None) would 
gettext.install(domain, localedir=None, codeset=None, names=None, 
caching=None, keepopen=False)

The new caching parameter could receive the following values:
caching=None: revert to the previour eager loading of the full catalog. 
It will be the default to allow previous application to see no change
caching=1: lazy loading with unlimited cache
caching=n where n is a positive (>=0) integer value: lazy loading with a 
LRU cache limited to n strings

The keepopen parameter would be a boolean:
keepopen=False (default): the mo file is only opened before loading a 
translation string and closed immediately after - it is also opened once 
when the GNUTranslation class is initialized to load the file description
keepopen=True: the mo file is kept open during the lifetime of the 
GNUTranslation object.
This parameter is ignored and not used if caching is None

The current GNUTranslation class loads the content of the mo file to 
build a dictionnary where the original strings are the keys and the 
translated keys the values. Plural forms use a special processing: the 
key is a 2 tuple (singular original string, order), and the value is the 
corresponding translated string - order=0 is normally for the singular 
translated string.

The proposed implementation would simply replace this dictionary with a 
special mapping subclass when caching is not None. That subclass would 
use same keys as the original directory and would:
- first search in its cache
- if not found in cache and if the hashtable has not a zero size search 
the original string by hash
- if not found in cache and if the hashtable has a zero size, search the 
original string with a binary search algorithm.
- if a string is found, it should feed the LRU cache, eventually 
throwing away the oldest entry (entries)

That should allow to implement the new feature with minimal refactoring 
for the gettext module.

Le 18/12/2018 à 10:10, Serge Ballesta via Python-ideas a écrit :
> In a project of mine, I have used the gettext module from Python 
> Standard Library. I have found that several tools could be used to 
> generate the Machine Object (mo) file from the source Portable Object 
> (one): pybabel (http://babel.pocoo.org/en/latest/), msgfmt.py from 
> Python tools or the original msgfmt from GNU gettext.
> I could find that only the original msgfmt was able to generate a 
> hashtable, and that anyway the Python gettext module loaded everything 
> in memory and did not use it. But I also find a TODO note saying
> # TODO:
> # - Lazy loading of .mo files.  Currently the entire catalog is loaded into
> #   memory, but that's probably bad for large translated programs.  Instead,
> #   the lexical sort of original strings in GNU .mo files should be 
> exploited
> #   to do binary searches and lazy initializations.  Or you might want 
> to use
> #   the undocumented double-hash algorithm for .mo files with hash 
> tables, but
> #   you'll need to study the GNU gettext code to do this.
> I have studied GNU gettext code and found that implemententing the 
> hashing algorithm in Python would not be that hard.
> The undocumented features required for implementation are:
> - the version number can safely stay to 0 when processing Python code
> - the size of the hash table is the first odd prime greater than or 
> equal to 4 * n / 3 where n is the number of strings
> - the first hashing function uses a variant of PJW hash function 
> described in https://en.wikipedia.org/wiki/PJW_hash_function, where the 
> line h = h & ~high is replaced with h = h ^ high, and using 32 bits 
> integers. The index in the table in the result of the function modulus 
> the size of the hash table
> - when there is a conflict (the slot given by the first hashing function 
> is already used by another string) the following is used:
>    - let h be the result of the PJW variant hash function and size be 
> the size of the hash table, an increment value is set to 1 +( h % (size -2))
>    - that increment is repeatedly added to the index in the hash table 
> (modulus the table size) until an empty slot is found (or the correct 
> original string is found)
> For now, my (alpha) code is able to generate in pure Python the same mo 
> file that GNU msgfmt generates, and use the hashtable to access the strings.
> Remaining problems:
> - I had to read GPL copyrighted code to find the undocumented features. 
> I have of course wrote my own code from scratch, but may I use an Apache 
> Free License 2.1 on it?
> - the current code for gettext loads everything from the mo file and 
> immediately closes it. My own code keeps the file opened to be able to 
> access it with the mmap module. There could be use case where first 
> option is better
> - I should either rely on the current way (load everything in memory) or 
> implement a binary search algo for the case where the hash table is not 
> present (it is of course optional)
> - it would be an important change, and I think that options should be 
> allow to choose between an eager or lazy access
> Before going further, I would like to know whether implementing lazy 
> access through the hash table that way seems to be a interesting 
> improvement or a dead end.
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