Please note the deliberate mistake of "y = Symbol('x')", which you may have
missed when interpreting my question. You have corrected this deliberate
mistake in your answer. My code was valid (it ran and gave the results I
quoted) and highlighted what was, to me, a surprising behaviour.
As to this being the same as "a = 1; b = 1; a == b; a is b" that is true
for immutables, but not so for e.g. "a = []; b = []" for which vanilla
Python gives "a == b" => True and "a is b" => False.
>From this can I infer that the SymPy Symbol class is immutable? That might
clear it up in my mind.
On Wednesday, June 3, 2020 at 9:09:19 PM UTC+1, Jason Moore wrote:
>
> Actually, I don't think your code is valid. You create two distinctly
> different symbols:
>
> In [1]: import sympy as sm
>
>
> In [2]: x = sm.Symbol('x')
>
>
> In [3]: y = sm.Symbol('y')
>
>
> In [4]: x == y
>
> Out[4]: False
>
> In [5]: x is y
>
> Out[5]: False
>
> In [6]: x + y
>
> Out[6]: x + y
>
> Jason
> moorepants.info
> +01 530-601-9791
>
>
> On Wed, Jun 3, 2020 at 1:06 PM Jason Moore <[email protected]
> <javascript:>> wrote:
>
>> Yes, this is intentional. It is really no different than this:
>>
>> In [1]: a = 1
>>
>>
>> In [2]: b = 1
>>
>>
>> In [3]: type(a)
>>
>> Out[3]: int
>>
>> In [4]: type(b)
>>
>> Out[4]: int
>>
>> In [5]: a == b
>>
>> Out[5]: True
>>
>> In [6]: a is b
>>
>> Out[6]: True
>>
>> Jason
>> moorepants.info
>> +01 530-601-9791
>>
>>
>> On Wed, Jun 3, 2020 at 12:53 PM James Bateman <[email protected]
>> <javascript:>> wrote:
>>
>>> I've just discovered a bug in my code which boiled down to the
>>> following, where a symbol "y" was given the same SymPy name as an existing
>>> symbol.
>>>
>>> import sympy as sp
>>> x = sp.Symbol('x')
>>> y = sp.Symbol('y')
>>>
>>> x == y # True
>>> x is y # True; expected False
>>> x + y # 2*x; expected x + x (which would have made the bug in my code
>>> more apparent)
>>>
>>> The behaviour here is very surprising to me. I would have expected x
>>> and y to be different Python objects with __repr__ methods which just so
>>> happen to return the same string. Instead, x and y are apparently
>>> different Python names for the same object (x is y).
>>>
>>> Is this intentional? I think I must misunderstand some deep design
>>> choice in SymPy, and I can't express my confusion well enough to Google
>>> it. Please help!
>>>
>>>
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>>> .
>>>
>>
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