== does exact structural equality checking. a == b is basically the
same as
a.func == b.func and a.args == b.args.
The issue here is that Add sorts its arguments (so that x + y == y + x
gives True), but when evaluate=False is used, the sorting is bypassed:
In [2]: sympify('2*x-4', evaluate=False).args
Out[2]: (2⋅x, -4)
In [3]: (2*x - 4).args
Out[3]: (-4, 2⋅x)
The same issue would occur with Mul.
Aaron Meurer
On Mon, Oct 10, 2016 at 1:58 PM, Robert Lee <[email protected]> wrote:
> Thanks Aaron. The evaluate context is incredibly useful...
>
> Another interesting I've noted is that equality checking of expressions
> (using == operator, not equals) seem to fail if one subexpression has
> evaluate set to False whereas an identical expression has the same
> subexpression set to True. Is this by design -- for more complicated core
> types -- or a bug?
>
> Here's some sample code to reproduce the issue:
>
>>>> expr_eval_false = sympify('2*x-4', evaluate=False)
>
>>>> expr_eval_false
>
> 2*x - 4
>
>>>> expr_eval_true = sympify('2*x-4', evaluate=True)
>
>>>> expr_eval_false == expr_eval_true
>
> False
>
>>>> expr_eval_false.equals(expr_eval_true)
>
> True
>
>
> I'm interested in comparing sympy expression trees so look forward to some
> independent work in that area.
>
>
> On Friday, October 7, 2016 at 4:17:14 PM UTC-4, Aaron Meurer wrote:
>>
>> You can use with evaluate(False):
>>
>> In [11]: with evaluate(False):
>> ....: print(x - sin(x - x))
>> ....:
>> x - sin(-x + x)
>>
>> Aaron Meurer
>>
>> On Fri, Oct 7, 2016 at 3:29 PM, Robert Lee <[email protected]> wrote:
>> > I want the ability to be able to recursively pass flags like
>> > evaluate=False
>> > through all the sub-expressions. Is this currently possible? The best
>> > hack I
>> > have is actually going through and reforming each sub-expression
>> > recursively
>> > and setting the appropriate flags.
>> >
>> >
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