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bpo-36027: Extend three-argument pow to negative second argument #13266

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Merged
mdickinson merged 11 commits into from
Jun 2, 2019
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bpo-36027: Extend three-argument pow to negative second argument #13266

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5b2f2bc
Extend three-argument pow to allow the second argument to be negative.
mdickinson May 12, 2019
0a45fb8
Remove excessive whitespace.
mdickinson May 12, 2019
5f372ad
Tidy up imports.
mdickinson May 12, 2019
11dec1e
Remove a redundant test.
mdickinson May 12, 2019
12b51aa
Merge remote-tracking branch 'origin/master' into invmod
mdickinson Jun 1, 2019
fca728e
Clarify that the negative third argument only applies for int type op…
mdickinson Jun 1, 2019
0d70b1b
Minor test updates: restore the test for 'pow(-1, -2, 3)' in test_bui…
mdickinson Jun 1, 2019
4ea0bf0
Cleaner wording in what's new entry.
mdickinson Jun 1, 2019
be89d23
Fix typo in what's new entry
mdickinson Jun 1, 2019
4aaec00
DOC: add example of computing an inverse; use inv_x instead of ix.
mdickinson Jun 2, 2019
345cfcc
Merge remote-tracking branch 'origin/master' into invmod
mdickinson Jun 2, 2019
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Clarify that the negative third argument only applies for int type op… 
…erands.
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@mdickinson
mdickinson committed Jun 1, 2019
commit fca728e26b9cf72c29ea694b588517546ad051a1
13 changes: 7 additions & 6 deletions Doc/library/functions.rst
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Expand Up @@ -1261,14 +1261,15 @@ are always available. They are listed here in alphabetical order.
converted to float and a float result is delivered. For example, ``10**2``
returns ``100``, but ``10**-2`` returns ``0.01``.

If *z* is present, all three of *x*, *y* and *z* must be of integer type
and *z* must be nonzero. If *z* is present and *y* is negative, *x* must
be relatively prime to *z*. In that case, ``pow(ix, -y, z)`` is returned,
where *ix* is an inverse to *x* modulo *z*.
For :class:`int` operands *x* and *y*, if *z* is present, *z* must also be
of integer type and *z* must be nonzero. If *z* is present and *y* is
negative, *x* must be relatively prime to *z*. In that case, ``pow(ix, -y,

@rhettinger rhettinger Jun 1, 2019

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Consider changing ix to inv_x for readability.

z)`` is returned, where *ix* is an inverse to *x* modulo *z*.

@rhettinger rhettinger Jun 1, 2019

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Perhaps this needs an example for clarity:

# Multiplicative inverse of 38 in mod 97
>>> pow(38, -1, 97)
23
>>> 23 * 38 % 97 == 1
True


.. versionchanged:: 3.8
The three-argument form now allows the second argument to be negative,
permitting computation of modular inverses.
For :class:`int` operands, the three-argument form of ``pow`` now allows
the second argument to be negative, permitting computation of modular
inverses.


.. function:: print(*objects, sep=' ', end='\\n', file=sys.stdout, flush=False)
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