René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Improve `printf`-style formatting docs for non-built-in types","articleBody":"# Add a description about how Decimal behaves differently with __format__ and % formatting in https://docs.python.org/3/library/string.html#format-specification-mini-language\n\nFor \\_\\_format\\_\\_, Decimal is formatted ~~with the highest possible precision~~ as it is.\n\nFor % formatting, Decimal is converted to float before formatting.\n\nThese behaviors have not changed since they were added.\n- The Decimal class was added in Python 2.4 (https://docs.python.org/2.4/whatsnew/whatsnew24.html).\n- The \\_\\_format\\_\\_ method was added in Python 2.6 (https://docs.python.org/2.6/whatsnew/2.6.html).\n\nCompare:\n| `sys.version` | `'%f' % 11111111111111111111.0` | `'%f' % Decimal('11111111111111111111.0')` | `Decimal('11111111111111111111.0').__format__('f')` | `'%e' % 0.0` | `'%e' % Decimal('0.0')` | `Decimal('0.0').__format__('e')` | `'%.20e' % 11111111111111111111.0` | `'%.20e' % Decimal('11111111111111111111.0')` | `Decimal('11111111111111111111.0').__format__('.20e')` | `'%.20g' % 11111111111111111111.0` | `'%.20g' % Decimal('11111111111111111111.0')` | `Decimal('11111111111111111111.0').__format__('.20g')` |\n| ---: | ---: | ---: | ---: | ---: | ---: | ---: | ---: | ---: | ---: | ---: | ---: | ---: |\n| '2.4.4 (#71, Oct 18 2006, 08:34:43) [MSC v.1310 32 bit (Intel)]' | '11111111111111111000.000000' | '11111111111111111000.000000' | Exception | '0.000000e+000' | '0.000000e+000' | Exception | '1.11111111111111110000e+019' | '1.11111111111111110000e+019' | Exception | '11111111111111111000' | '11111111111111111000' | Exception |\n| '2.6.6 (r266:84297, Aug 24 2010, 18:13:38) [MSC v.1500 64 bit (AMD64)]' | '11111111111111111000.000000' | '11111111111111111000.000000' | '11111111111111111111.0' | '0.000000e+00' | '0.000000e+00' | '0e-1' | '1.11111111111111110000e+19' | '1.11111111111111110000e+19' | '1.11111111111111111110e+19' | '11111111111111111000' | '11111111111111111000' | '11111111111111111111' |\n| '2.7.18 (v2.7.18:8d21aa21f2, Apr 20 2020, 13:25:05) [MSC v.1500 64 bit (AMD64)]' | '11111111111111110656.000000' | '11111111111111110656.000000' | '11111111111111111111.0' | '0.000000e+00' | '0.000000e+00' | '0e-1' | '1.11111111111111106560e+19' | '1.11111111111111106560e+19' | '1.11111111111111111110e+19' | '11111111111111110656' | '11111111111111110656' | '11111111111111111111' |\n| '3.14.0 (tags/v3.14.0:ebf955d, Oct  7 2025, 10:06:33) [MSC v.1944 32 bit (Intel)]' | '11111111111111110656.000000' | '11111111111111110656.000000' | '11111111111111111111.0' | '0.000000e+00' | '0.000000e+00' | '0e-1' | '1.11111111111111106560e+19' | '1.11111111111111106560e+19' | '1.11111111111111111110e+19' | '11111111111111110656' | '11111111111111110656' | '11111111111111111111' |\n| '3.14.0 (tags/v3.14.0:ebf955d, Oct  7 2025, 10:15:03) [MSC v.1944 64 bit (AMD64)]' | '11111111111111110656.000000' | '11111111111111110656.000000' | '11111111111111111111.0' | '0.000000e+00' | '0.000000e+00' | '0e-1' | '1.11111111111111106560e+19' | '1.11111111111111106560e+19' | '1.11111111111111111110e+19' | '11111111111111110656' | '11111111111111110656' | '11111111111111111111' |\n| '3.15.0a2 (tags/v3.15.0a2:a625628, Nov 18 2025, 18:12:01) [MSC v.1944 64 bit (AMD64)]' | '11111111111111110656.000000' | '11111111111111110656.000000' | '11111111111111111111.0' | '0.000000e+00' | '0.000000e+00' | '0e-1' | '1.11111111111111106560e+19' | '1.11111111111111106560e+19' | '1.11111111111111111110e+19' | '11111111111111110656' | '11111111111111110656' | '11111111111111111111' |\n\n```python\n# ./test.py\nimport sys\nfrom decimal import Decimal\n\nfor expr in (\n    \"sys.version\",\n\n    \"'%f' % 12345123451234512345.0\",\n    \"'%f' % Decimal('12345123451234512345.0')\",\n    \"Decimal('12345123451234512345.0').__format__('f')\",\n\n    \"'%e' % 0.0\",\n    \"'%e' % Decimal('0.0')\",\n    \"Decimal('0.0').__format__('e')\",\n\n    \"'%.20e' % 12345123451234512345.0\",\n    \"'%.20e' % Decimal('12345123451234512345.0')\",\n    \"Decimal('12345123451234512345.0').__format__('.20e')\",\n\n    \"'%.20g' % 12345123451234512345.0\",\n    \"'%.20g' % Decimal('12345123451234512345.0')\",\n    \"Decimal('12345123451234512345.0').__format__('.20g')\",\n):\n    print(expr)\n    try:\n        print(repr(eval(expr)))\n    except Exception:\n        print('Exception')\n    print('')\n```\n\n```python\n# ./test_runner.py\nfrom pathlib import Path\nfrom subprocess import run, PIPE\n\nPYTHON_DIR_ROOT = r\"C:\\python_dir_root\"\n\ndef print_row(row, file):\n    for v in row: print(f\"| {v} \", end='', file=file)\n    print(\"|\", file=file)\n\ndef print_md_table(headers, rows, file):\n    print_row(headers, file=file)\n    print_row(['---:'] * len(headers), file=file)\n    for row in rows: print_row(row, file=file)\n    print(file=file)\n\nrows = []\nfor dir in Path(PYTHON_DIR_ROOT).iterdir():\n    out = run([str(dir/\"python.exe\"), \"./test.py\"], stdout=PIPE).stdout\n    out = out.decode('utf-8').splitlines()\n    headers = [f\"`{h}`\" for h in out[::3]]\n    rows.append(out[1::3])\n\nwith open(\"./test_result.md\", 'w', encoding='utf-8') as fp:\n    print_md_table(headers, rows, file=fp)\n```\n\n\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-142117\n* gh-142126\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/1nftf","@type":"Person","name":"1nftf"},"datePublished":"2025-11-30T08:54:58.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":18},"url":"https://github.com/142108/cpython/issues/142108"}