[ python-Bugs-1025872 ] X to the power of 0 may give wrong answer

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Bugs item #1025872, was opened at 2004-09-10 09:53
Message generated for change (Comment added) made by tim_one
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Category: Python Interpreter Core
>Group: Not a Bug
>Status: Closed
>Resolution: Invalid
Priority: 5
Submitted By: Nick Coghlan (ncoghlan)
Assigned to: Nobody/Anonymous (nobody)
Summary: X to the power of 0 may give wrong answer

Initial Comment:
According to Python 2.3 and 2.4, -1 ** 0 = -1 ! It
appears to do this for any negative value.

The answer to x ** 0 should be 1, regardless of the
sign of x.

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>Comment By: Tim Peters (tim_one)
Date: 2004-09-10 10:07

Message:
Logged In: YES 
user_id=31435

Precedence.  From the language ref:  "The power operator 
binds more tightly than unary operators on its left; it binds 
less tightly than unary operators on its right.".

Python doesn't have negative numeric literals.  -1 is the 
unary minus operator applied to the positive literal 1.  So

-1**0

parses as

-(1**0)

and -1 is the correct result.  Add parens if you want a 
different evaluation order:

>>> (-1)**0
1
>>>

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Comment By: Nick Coghlan (ncoghlan)
Date: 2004-09-10 10:00

Message:
Logged In: YES 
user_id=1038590

The plot thickens. . . if I use the operator module, it works:

Python 2.3.4 (#1, Jun 15 2004, 21:38:43)
[GCC 3.4.0 20040613 (Red Hat Linux 3.4.0-5)] on linux2
Type "help", "copyright", "credits" or "license" for more
information.
>>> -4.3 ** 0
-1.0
>>> from operator import __pow__
>>> __pow__(-1, 0)
1
>>> -1 ** 0
-1
>>>


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