Dict when defining not returning multi value key error

[Steven D'Aprano]

On Thu, 31 Jul 2014 20:12:12 -0700, Dan Stromberg wrote:

> I removed some quotes, and noticed that 1 and 1.0 hash the same. That's
> a bit unexpected, but I suppose it's not completely unreasonable.


You should expect that two equal objects should hash to the same value in 
different versions of Python, or even from one run of Python to the next. 
But within a single session, then far from being "not completely 
unreasonable", having equal items hash to the same value is the only 
reasonable thing to do. It is part of the dict API that if x == y, then 
dict[x] == dict[y], and that cannot happen unless x and y hash to the 
same thing.

In general, it's not entirely possible to enforce that, especially given 
how flexible custom __eq__ methods can be, so Python doesn't even try. 
But with *numbers* (or at least those which are part of the numeric 
tower), Python does insist that each distinct value should hash to the 
same result no matter what type that value happens to be:


py> n = 23
py> hash(n) == hash(long(n)) == hash(float(n)) == hash(complex(n))
True
py> from fractions import Fraction
py> from decimal import Decimal
py> hash(n) == hash(Fraction(n)) == hash(Decimal(n))
True


With the possible exception of Decimal, which is not fully integrated 
with the numeric tower, all numbers in the standard library will obey the 
condition that if x == y, hash(x) == hash(y), regardless of type.


-- 
Steven