Terry J. Reedy <[email protected]> added the comment:
3.2 doc entry:
random.triangular(low, high, mode)
Return a random floating point number N such that low <= N <= high and with the
specified mode between those bounds. The low and high bounds default to zero
and one. The mode argument defaults to the midpoint between the bounds, giving
a symmetric distribution.
3.2 behavior:
>>> from random import triangular
>>> triangular(1,1)
1.0
>>> triangular(1,1,1)
Traceback (most recent call last):
File "<pyshell#2>", line 1, in <module>
triangular(1,1,1)
File "C:\Programs\Python32\lib\random.py", line 346, in triangular
c = 0.5 if mode is None else (mode - low) / (high - low)
ZeroDivisionError: division by zero
I regard is as a bug that explicitly giving a 'default value' causes the
function to fail.
The last sentence of the doc is a lie: the actual default for mode is None:
>>> triangular(1,1,None)
1.0
and if it is None, it *not* calculated (low + .5(high-low)). The actual
internal third parameter is the fraction of the range (high-low) that is the up
slope versus the down slope of the distribution. The code calls that 'c', as
calculated by the line shown in the traceback. The fix is simple: add 'or
low==high' to the condition.
c = 0.5 if (mode is None or low==high) else (mode - low) / (high - low)
Contrary to the doc ('mode between those bounds'), the definition on Wikipedia
and code include the degenerate cases of mode == low or high. The code in
effect treats modes outside the range as being at an endpoint.
Suggested doc revision, with defaults given in the signature as normal:
random.triangular(low=0.0, high=1.0, mode=None)
Return a random floating point number N from a triangular distribution such
that low <= N <= high with the specified mode between or at those bounds. A
mode outside a bound is treated as being at the bound. The default mode
argument corresponds to the midpoint between the bounds, giving a symmetric
distribution.
----------
components: +Library (Lib) -Extension Modules
keywords: +patch
nosy: +terry.reedy
stage: -> needs patch
title: random.triangular error when low = mode -> random.triangular error when
low = high=mode
type: -> behavior
versions: +Python 2.7, Python 3.2, Python 3.3 -Python 2.6
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<http://bugs.python.org/issue13355>
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