[harshitgupta31415]

write_mo builds two byte strings as it adds each message. Each append copies the content already in that string, so larger catalogs spend more time copying earlier messages.

This changes those two buffers to separate bytearrays. The rest of the function stays the same: it filters and sorts the same messages, calculates the same offsets, and makes one write with a bytes object.

For similarly sized messages, building the string tables goes from quadratic copying to linear work. Sorting is unchanged. This does not remove all allocation costs, and peak memory is slightly higher in the measured cases.

Tests

• 389 existing messaging tests and writer doctests passed.
• 414 before/after output and error comparisons matched: 336 produced identical output bytes and 78 produced the same errors, including encoding failures and failed output writes.
• The full-suite run on this PC had 7,832 passes, two failures, nine skips and two expected failures. Both failures were the timezone-name test parametrizations comparing GMT+00:00 with GMT+05:30; they also failed with the original writer.
• Ruff checks passed for the changed code and tests.

Benchmarks

I measured the complete write_mo call on Python 3.15.0 and Windows 11, using the median of eight timing runs and eight separate peak-allocation runs per version. The base was e2a1791.

Catalog construction, PO parsing, downloads and output checks were outside the timer. Each call wrote to a fresh BytesIO. One process was limited to two physical cores at a time; core pairs rotated between repeats, paired versions used the same pair, and their order alternated. Memory below is newly traced Python allocations during the write, not total process RAM.

There are 100 generated catalog examples, two empty-catalog controls, and six cases using unchanged Django and Wagtail catalogs. The real-project cases measured 1.09×–1.48× faster. Tiny cases sometimes measured slower, and small differences can be noise. All 108 cases, including slower results and memory increases, are included below.

The runner, CPU helper, raw samples and rerun instructions are on a separate branch in my fork, so they do not add files to this PR.

Show all 108 measured cases

Messages	Catalog	Include fuzzy	Base time	PR time	Speed	Base peak	PR peak	Memory
0	empty-catalog control	no	0.0794 ms	0.0735 ms	1.080× faster	4.95 KiB	5.11 KiB	1.032× more
0	empty-catalog control	yes	0.0727 ms	0.0730 ms	1.004× slower	4.95 KiB	5.11 KiB	1.032× more
1	generated: UI labels	no	0.0862 ms	0.0910 ms	1.056× slower	4.92 KiB	5.10 KiB	1.037× more
1	generated: UI labels	yes	0.1086 ms	0.0784 ms	1.385× faster	4.92 KiB	5.10 KiB	1.037× more
1	generated: context labels	no	0.0906 ms	0.0979 ms	1.081× slower	4.85 KiB	5.03 KiB	1.037× more
1	generated: context labels	yes	0.0974 ms	0.0908 ms	1.073× faster	4.85 KiB	5.03 KiB	1.037× more
1	generated: help paragraphs	no	0.0818 ms	0.0843 ms	1.031× slower	7.64 KiB	7.76 KiB	1.016× more
1	generated: help paragraphs	yes	0.0947 ms	0.0888 ms	1.067× faster	7.64 KiB	7.76 KiB	1.016× more
1	generated: mixed catalog	no	0.0922 ms	0.0984 ms	1.067× slower	4.93 KiB	5.09 KiB	1.032× more
1	generated: mixed catalog	yes	0.0840 ms	0.0807 ms	1.040× faster	4.93 KiB	5.09 KiB	1.032× more
1	generated: plural status	no	0.0829 ms	0.0880 ms	1.062× slower	5.06 KiB	5.25 KiB	1.036× more
1	generated: plural status	yes	0.0924 ms	0.0884 ms	1.045× faster	5.06 KiB	5.25 KiB	1.036× more
2	generated: UI labels	no	0.0867 ms	0.0988 ms	1.140× slower	5.22 KiB	5.38 KiB	1.031× more
2	generated: UI labels	yes	0.0939 ms	0.0900 ms	1.044× faster	5.22 KiB	5.38 KiB	1.031× more
2	generated: context labels	no	0.0847 ms	0.0917 ms	1.082× slower	5.10 KiB	5.27 KiB	1.034× more
2	generated: context labels	yes	0.0960 ms	0.0870 ms	1.103× faster	5.10 KiB	5.27 KiB	1.034× more
2	generated: help paragraphs	no	0.0939 ms	0.1026 ms	1.093× slower	10.04 KiB	10.17 KiB	1.012× more
2	generated: help paragraphs	yes	0.1086 ms	0.0969 ms	1.122× faster	10.04 KiB	10.17 KiB	1.012× more
2	generated: mixed catalog	no	0.0824 ms	0.0897 ms	1.088× slower	7.68 KiB	7.81 KiB	1.016× more
2	generated: mixed catalog	yes	0.0878 ms	0.0863 ms	1.017× faster	7.68 KiB	7.81 KiB	1.016× more
2	generated: plural status	no	0.1088 ms	0.1095 ms	1.007× slower	5.41 KiB	5.56 KiB	1.028× more
2	generated: plural status	yes	0.1118 ms	0.1050 ms	1.064× faster	5.41 KiB	5.56 KiB	1.028× more
10	generated: UI labels	no	0.1098 ms	0.1296 ms	1.180× slower	7.31 KiB	7.49 KiB	1.024× more
10	generated: UI labels	yes	0.1174 ms	0.1182 ms	1.007× slower	7.57 KiB	7.71 KiB	1.018× more
10	generated: context labels	no	0.1253 ms	0.1140 ms	1.099× faster	6.85 KiB	7.06 KiB	1.031× more
10	generated: context labels	yes	0.1240 ms	0.1220 ms	1.016× faster	7.05 KiB	7.24 KiB	1.027× more
10	generated: help paragraphs	no	0.1259 ms	0.1378 ms	1.095× slower	27.25 KiB	28.24 KiB	1.036× more
10	generated: help paragraphs	yes	0.1341 ms	0.1311 ms	1.023× faster	29.67 KiB	29.94 KiB	1.009× more
10	generated: mixed catalog	no	0.1767 ms	0.1646 ms	1.073× faster	11.63 KiB	11.83 KiB	1.017× more
10	generated: mixed catalog	yes	0.1505 ms	0.1684 ms	1.119× slower	14.24 KiB	14.53 KiB	1.020× more
10	generated: plural status	no	0.1391 ms	0.1364 ms	1.020× faster	7.87 KiB	8.11 KiB	1.031× more
10	generated: plural status	yes	0.1540 ms	0.1428 ms	1.079× faster	8.26 KiB	8.46 KiB	1.023× more
50	generated: UI labels	no	0.3981 ms	0.3506 ms	1.135× faster	21.14 KiB	21.29 KiB	1.007× more
50	generated: UI labels	yes	0.4514 ms	0.3917 ms	1.153× faster	23.37 KiB	23.62 KiB	1.011× more
50	generated: context labels	no	0.3862 ms	0.3979 ms	1.030× slower	18.18 KiB	18.38 KiB	1.011× more
50	generated: context labels	yes	0.4008 ms	0.3964 ms	1.011× faster	20.00 KiB	20.18 KiB	1.009× more
50	generated: help paragraphs	no	0.5686 ms	0.4991 ms	1.139× faster	116.24 KiB	118.39 KiB	1.018× more
50	generated: help paragraphs	yes	0.6699 ms	0.5115 ms	1.310× faster	128.68 KiB	132.09 KiB	1.026× more
50	generated: mixed catalog	no	0.4229 ms	0.4041 ms	1.047× faster	40.24 KiB	41.00 KiB	1.019× more
50	generated: mixed catalog	yes	0.4575 ms	0.4229 ms	1.082× faster	48.67 KiB	49.51 KiB	1.017× more
50	generated: plural status	no	0.4987 ms	0.4645 ms	1.074× faster	24.09 KiB	24.50 KiB	1.017× more
50	generated: plural status	yes	0.6297 ms	0.5483 ms	1.148× faster	26.68 KiB	26.84 KiB	1.006× more
100	generated: UI labels	no	0.5550 ms	0.5139 ms	1.080× faster	40.34 KiB	40.76 KiB	1.010× more
100	generated: UI labels	yes	0.7697 ms	0.5991 ms	1.285× faster	44.47 KiB	44.93 KiB	1.010× more
100	generated: context labels	no	0.6396 ms	0.5805 ms	1.102× faster	33.82 KiB	34.03 KiB	1.006× more
100	generated: context labels	yes	0.6035 ms	0.5970 ms	1.011× faster	37.65 KiB	37.97 KiB	1.009× more
100	generated: help paragraphs	no	1.0481 ms	0.6878 ms	1.524× faster	225.21 KiB	227.35 KiB	1.010× more
100	generated: help paragraphs	yes	1.1132 ms	0.7800 ms	1.427× faster	249.35 KiB	253.21 KiB	1.015× more
100	generated: mixed catalog	no	1.0351 ms	0.9084 ms	1.139× faster	75.13 KiB	76.76 KiB	1.022× more
100	generated: mixed catalog	yes	1.3102 ms	0.9102 ms	1.440× faster	88.81 KiB	89.64 KiB	1.009× more
100	generated: plural status	no	0.8139 ms	0.7807 ms	1.043× faster	45.54 KiB	45.88 KiB	1.007× more
100	generated: plural status	yes	0.9719 ms	0.8374 ms	1.161× faster	49.79 KiB	50.33 KiB	1.011× more
250	generated: UI labels	no	1.3360 ms	1.1707 ms	1.141× faster	91.52 KiB	92.79 KiB	1.014× more
250	generated: UI labels	yes	1.6381 ms	1.2588 ms	1.301× faster	99.68 KiB	99.91 KiB	1.002× more
250	generated: context labels	no	1.3728 ms	1.1463 ms	1.198× faster	79.27 KiB	79.81 KiB	1.007× more
250	generated: context labels	yes	1.4930 ms	1.4138 ms	1.056× faster	86.07 KiB	86.32 KiB	1.003× more
250	generated: help paragraphs	no	3.2016 ms	1.8306 ms	1.749× faster	551.65 KiB	568.42 KiB	1.030× more
250	generated: help paragraphs	yes	4.2914 ms	2.1651 ms	1.982× faster	610.86 KiB	632.32 KiB	1.035× more
250	generated: mixed catalog	no	2.9324 ms	2.0096 ms	1.459× faster	178.97 KiB	183.16 KiB	1.023× more
250	generated: mixed catalog	yes	3.3237 ms	2.3687 ms	1.403× faster	212.96 KiB	215.10 KiB	1.010× more
250	generated: plural status	no	2.5251 ms	2.0214 ms	1.249× faster	104.23 KiB	105.05 KiB	1.008× more
250	generated: plural status	yes	2.3970 ms	2.0610 ms	1.163× faster	113.79 KiB	114.88 KiB	1.010× more
348	Django German	no	4.7925 ms	4.3883 ms	1.092× faster	155.46 KiB	155.99 KiB	1.003× more
348	Django German	yes	4.4586 ms	3.9581 ms	1.126× faster	155.46 KiB	155.99 KiB	1.003× more
500	generated: UI labels	no	3.1018 ms	2.6990 ms	1.149× faster	174.82 KiB	175.48 KiB	1.004× more
500	generated: UI labels	yes	3.5530 ms	2.3261 ms	1.527× faster	193.21 KiB	194.26 KiB	1.005× more
500	generated: context labels	no	3.9200 ms	2.8876 ms	1.358× faster	150.04 KiB	150.26 KiB	1.001× more
500	generated: context labels	yes	3.3068 ms	2.4294 ms	1.361× faster	165.68 KiB	166.12 KiB	1.003× more
500	generated: help paragraphs	no	8.7462 ms	3.0473 ms	2.870× faster	1094.40 KiB	1095.82 KiB	1.001× more
500	generated: help paragraphs	yes	10.9009 ms	4.9355 ms	2.209× faster	1214.89 KiB	1221.64 KiB	1.006× more
500	generated: mixed catalog	no	6.5656 ms	4.3322 ms	1.516× faster	347.74 KiB	353.81 KiB	1.017× more
500	generated: mixed catalog	yes	7.9519 ms	4.8575 ms	1.637× faster	413.59 KiB	418.58 KiB	1.012× more
500	generated: plural status	no	4.4429 ms	3.5859 ms	1.239× faster	200.71 KiB	202.78 KiB	1.010× more
500	generated: plural status	yes	4.6811 ms	3.6184 ms	1.294× faster	221.96 KiB	222.51 KiB	1.002× more
902	Wagtail French	no	14.6092 ms	12.0440 ms	1.213× faster	459.64 KiB	462.06 KiB	1.005× more
902	Wagtail French	yes	15.4097 ms	10.8802 ms	1.416× faster	459.64 KiB	462.06 KiB	1.005× more
902	Wagtail German	no	13.9527 ms	10.9376 ms	1.276× faster	450.92 KiB	455.24 KiB	1.010× more
902	Wagtail German	yes	16.5029 ms	11.1853 ms	1.475× faster	450.92 KiB	455.24 KiB	1.010× more
1000	generated: UI labels	no	5.7973 ms	3.9999 ms	1.449× faster	343.63 KiB	345.79 KiB	1.006× more
1000	generated: UI labels	yes	6.1017 ms	3.9066 ms	1.562× faster	376.48 KiB	379.50 KiB	1.008× more
1000	generated: context labels	no	9.0295 ms	5.5931 ms	1.614× faster	293.81 KiB	294.86 KiB	1.004× more
1000	generated: context labels	yes	7.1513 ms	5.0712 ms	1.410× faster	321.12 KiB	322.66 KiB	1.005× more
1000	generated: help paragraphs	no	26.4305 ms	7.2473 ms	3.647× faster	2182.11 KiB	2196.45 KiB	1.007× more
1000	generated: help paragraphs	yes	44.0453 ms	8.6429 ms	5.096× faster	2419.15 KiB	2444.96 KiB	1.011× more
1000	generated: mixed catalog	no	19.5093 ms	9.5673 ms	2.039× faster	689.44 KiB	698.28 KiB	1.013× more
1000	generated: mixed catalog	yes	24.3490 ms	10.2310 ms	2.380× faster	821.26 KiB	828.86 KiB	1.009× more
1000	generated: plural status	no	12.3560 ms	8.4057 ms	1.470× faster	395.89 KiB	397.25 KiB	1.003× more
1000	generated: plural status	yes	12.0427 ms	7.8518 ms	1.534× faster	434.54 KiB	436.84 KiB	1.005× more
2500	generated: UI labels	no	21.1395 ms	10.9906 ms	1.923× faster	830.44 KiB	835.67 KiB	1.006× more
2500	generated: UI labels	yes	26.5648 ms	11.5095 ms	2.308× faster	903.12 KiB	911.85 KiB	1.010× more
2500	generated: context labels	no	18.6855 ms	11.4761 ms	1.628× faster	701.51 KiB	704.84 KiB	1.005× more
2500	generated: context labels	yes	24.1964 ms	11.4877 ms	2.106× faster	768.65 KiB	773.18 KiB	1.006× more
2500	generated: help paragraphs	no	328.6664 ms	20.8280 ms	15.780× faster	5425.60 KiB	5513.45 KiB	1.016× more
2500	generated: help paragraphs	yes	563.0075 ms	24.2740 ms	23.194× faster	6008.79 KiB	6130.16 KiB	1.020× more
2500	generated: mixed catalog	no	80.3307 ms	25.4874 ms	3.152× faster	1714.36 KiB	1730.85 KiB	1.010× more
2500	generated: mixed catalog	yes	103.8611 ms	28.2105 ms	3.682× faster	2024.54 KiB	2061.41 KiB	1.018× more
2500	generated: plural status	no	37.7302 ms	19.2432 ms	1.961× faster	969.71 KiB	980.55 KiB	1.011× more
2500	generated: plural status	yes	42.8890 ms	20.4280 ms	2.100× faster	1057.86 KiB	1071.85 KiB	1.013× more
5000	generated: UI labels	no	66.9249 ms	22.2688 ms	3.005× faster	1505.45 KiB	1517.30 KiB	1.008× more
5000	generated: UI labels	yes	67.0938 ms	24.8941 ms	2.695× faster	1651.78 KiB	1670.93 KiB	1.012× more
5000	generated: context labels	no	47.4505 ms	22.4813 ms	2.111× faster	1390.62 KiB	1398.35 KiB	1.006× more
5000	generated: context labels	yes	58.6467 ms	22.8965 ms	2.561× faster	1543.03 KiB	1553.26 KiB	1.007× more
5000	generated: help paragraphs	no	2799.6683 ms	42.0053 ms	66.650× faster	10695.09 KiB	10918.81 KiB	1.021× more
5000	generated: help paragraphs	yes	3994.5715 ms	44.5307 ms	89.704× faster	11862.42 KiB	12160.32 KiB	1.025× more
5000	generated: mixed catalog	no	321.7073 ms	48.1212 ms	6.685× faster	3259.16 KiB	3303.50 KiB	1.014× more
5000	generated: mixed catalog	yes	429.5797 ms	57.2891 ms	7.498× faster	3878.69 KiB	3967.72 KiB	1.023× more
5000	generated: plural status	no	97.1637 ms	35.9960 ms	2.699× faster	1789.74 KiB	1812.31 KiB	1.013× more
5000	generated: plural status	yes	121.2796 ms	36.1377 ms	3.356× faster	1967.64 KiB	1996.67 KiB	1.015× more

Public fixture sources:

• Django German (unchanged contents; source SHA256 in JSON).
• Wagtail German (unchanged contents; source SHA256 in JSON).
• Wagtail French (unchanged contents; source SHA256 in JSON).

[akx]

Please remove the new tests, I don't think they're necessary?

[harshitgupta31415]

@akx updated as per your request.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.51%. Comparing base (9a3ef47) to head (acb11a9).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1376   +/-   ##
=======================================
  Coverage   93.51%   93.51%           
=======================================
  Files          27       27           
  Lines        4932     4933    +1     
=======================================
+ Hits         4612     4613    +1     
  Misses        320      320           

Flag	Coverage Δ
macos-14-3.10	92.60% <100.00%> (+<0.01%)	⬆️
macos-14-3.11	92.54% <100.00%> (+<0.01%)	⬆️
macos-14-3.12	92.74% <100.00%> (+<0.01%)	⬆️
macos-14-3.13	92.74% <100.00%> (+<0.01%)	⬆️
macos-14-3.14	92.72% <100.00%> (+<0.01%)	⬆️
macos-14-3.15	92.72% <100.00%> (+<0.01%)	⬆️
macos-14-3.8	92.47% <100.00%> (+<0.01%)	⬆️
macos-14-3.9	92.53% <100.00%> (+<0.01%)	⬆️
macos-14-pypy3.10	92.60% <100.00%> (+<0.01%)	⬆️
ubuntu-24.04-3.10	92.62% <100.00%> (+<0.01%)	⬆️
ubuntu-24.04-3.11	92.56% <100.00%> (+<0.01%)	⬆️
ubuntu-24.04-3.12	92.76% <100.00%> (+<0.01%)	⬆️
ubuntu-24.04-3.13	92.76% <100.00%> (+<0.01%)	⬆️
ubuntu-24.04-3.14	92.74% <100.00%> (+<0.01%)	⬆️
ubuntu-24.04-3.15	92.74% <100.00%> (+<0.01%)	⬆️
ubuntu-24.04-3.8	92.49% <100.00%> (+<0.01%)	⬆️
ubuntu-24.04-3.9	92.55% <100.00%> (+<0.01%)	⬆️
ubuntu-24.04-pypy3.10	92.62% <100.00%> (+<0.01%)	⬆️
windows-2022-3.10	92.61% <100.00%> (+<0.01%)	⬆️
windows-2022-3.11	92.55% <100.00%> (+<0.01%)	⬆️
windows-2022-3.12	92.75% <100.00%> (+<0.01%)	⬆️
windows-2022-3.13	92.75% <100.00%> (+<0.01%)	⬆️
windows-2022-3.14	92.73% <100.00%> (+<0.01%)	⬆️
windows-2022-3.15	92.73% <100.00%> (+<0.01%)	⬆️
windows-2022-3.8	92.58% <100.00%> (+<0.01%)	⬆️
windows-2022-3.9	92.54% <100.00%> (+<0.01%)	⬆️
windows-2022-pypy3.10	92.61% <100.00%> (+<0.01%)	⬆️

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[codspeed]

Merging this PR will not alter performance

✅ 88 untouched benchmarks

---

_{Comparing harshitgupta31415:harshitgupta31415/speed-up-mo-file-writing (acb11a9) with master (9a3ef47)}