Yes, this is absolutely intended. Things like
if x < y:
...
no longer work, unless x - y is explicitly known to be negative or
nonnegative. This fixes a lot of subtle bugs that pop up, because
people will write things like the above expecting x and y to be
numbers, but when symbols are passed in, it would arbitrarily be True
or False.
This also fits with the Python 3 idiom that you can't order things
unless they have a well-defined order to them.
If you want a canonical (but arbitrary) way to order SymPy
expressions, use default_sort_key, like
sorted(syms, key=default_sort_key)
It was designed for this exact purpose.
Aaron Meurer
On Sat, Aug 31, 2013 at 1:35 AM, Matthew Brett <[email protected]> wrote:
> Hi,
>
> I just noticed this in current master (not in 0.7.3):
>
> In [4]: from sympy import symbols
>
> In [5]: syms = symbols('c, b, a')
>
> In [6]: sorted(syms)
> ---------------------------------------------------------------------------
> TypeError Traceback (most recent call last)
> <ipython-input-6-dcb7d6a54d2e> in <module>()
> ----> 1 sorted(syms)
>
> /Users/mb312/usr/local/lib/python2.7/site-packages/sympy/core/relational.py
> in __nonzero__(self)
> 224
> 225 def __nonzero__(self):
> --> 226 raise TypeError("symbolic boolean expression has no
> truth value.")
> 227
> 228 __bool__ = __nonzero__
>
> TypeError: symbolic boolean expression has no truth value.
>
> Is this intended? It's fairly easy to work around but it broke some
> of our code and I suppose it might be surprising.
>
> Cheers,
>
> Matthew
>
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