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PEP 786: Precision and Modulo-Precision Flag format specifiers for integer fields #4416

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PEP 786: Precision and Modulo-Precision Flag format specifiers for integer fields #4416

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9c0ef26
First main commit to PEP 791
jb2170 Apr 11, 2025
4e0e75e
791 -> 786
AA-Turner May 6, 2025
bd628a0
Add Alyssa as the sponsor of PEP 786
jb2170 May 8, 2025
ac5024d
Update .github/CODEOWNERS adding @ncoghlan for PEP 786
jb2170 May 8, 2025
3ca6120
Address AA-Turner's notes
jb2170 May 8, 2025
bd29ea4
Remove TODO bullet point for `z` and `#` commutativity
jb2170 May 8, 2025
d7d9cf0
Apply the more trivial suggestions from code review
jb2170 May 16, 2025
d1e47e4
Hyphenate 8-bit etc when appropriate
jb2170 May 16, 2025
be6f1dd
Correct 'defacto' -> 'de facto'
jb2170 May 16, 2025
b5c3b24
Reword the verdict of '!' syntax to not sound like there already is c…
jb2170 May 16, 2025
17a5532
Remove TODO bullet point for authorship
jb2170 May 16, 2025
a06e982
Reword 45 and -19 being indistinguishable modulo 64, '101101'
jb2170 May 16, 2025
2086547
Remove Sergey from Thanks as per his request
jb2170 May 22, 2025
d477f4c
Restructure Abstract and remove mentions to divmod
jb2170 May 22, 2025
e49ebdf
Fix quoting style; when quoting a whole sentence one should keep the …
jb2170 May 22, 2025
9cd0077
Remove TODO bullet point about RFC 2119
jb2170 May 22, 2025
565466d
Format to ~80 chars
jb2170 May 22, 2025
045df95
Restructure section about two cases of truncation
jb2170 May 22, 2025
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Ready for review
jb2170 May 22, 2025
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Hyphenate 8-bit etc when appropriate
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jb2170 committed May 16, 2025
commit d1e47e47f4bdfaa4a007c913a6bc9b887e107d8b
18 changes: 9 additions & 9 deletions peps/pep-0786.rst
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Expand Up @@ -90,12 +90,12 @@ Observe that one can always extend a signed number's binary representation by ex

.. code-block:: text

45 (8 bit) 00101101
45 (9 bit) 000101101
-19 (8 bit) 11101101
-19 (9 bit) 111101101
45 (8-bit) 00101101
45 (9-bit) 000101101
-19 (8-bit) 11101101
-19 (9-bit) 111101101

For non-negative numbers this is obvious. For negative numbers this is because the erstwhile leading column of an ``n`` bit representation goes from having a value of ``-2 ** (n-1)``, to ``+2 ** (n-1)``, with a new ``n+1``\ th column of value ``-2 ** n`` prefixed on, the overall sum unaffected.
For non-negative numbers this is obvious. For negative numbers this is because the erstwhile leading column of an ``n``\ -bit representation goes from having a value of ``-2 ** (n-1)``, to ``+2 ** (n-1)``, with a new ``n+1``\ th column of value ``-2 ** n`` prefixed on, the overall sum unaffected.

This is what C's ``printf`` does, working with powers of two as the numbers of digits:
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@skirpichev skirpichev Jun 1, 2025

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You might note, that "b" format type is available since the C23. (Works for me with -std=c2x one recent gcc.)


Expand All @@ -113,10 +113,10 @@ Conversely it should be clear that one can losslessly truncate a signed number's

.. code-block:: text

45 (8 bit) 00101101
45 (7 bit) 0101101
-19 (8 bit) 11101101
-19 (7 bit) 1101101
45 (8-bit) 00101101
45 (7-bit) 0101101
-19 (8-bit) 11101101
-19 (7-bit) 1101101

If one were to truncate another digit off of these examples, then both would end up as ``101101``, 45 indistinguishable from -19 when using only 6 binary digits because they are both the same modulo ``2 ** 6 = 64``. Therefore to losslessly and unambiguously represent a signed integer ``x`` as a binary string which is rendered to the end user, we have a defacto 'minimal width' representation convention, using ``n`` digits, where ``n`` is the smallest integer such that ``x`` is in ``range(-2 ** (n-1), 2 ** (n-1))``.

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