On Sep 10, 2010, at 11:19 PM, Bill Hart wrote:
> What isn't clear is how expressions like
>
> 2*x + 3*y + 4*z
>
> get handled without creating intermediate classes. Do the Add and
> Mult classes combine subexpressions together within the __new__
> method?
I think it is impossible to create that all in one fell-swoop in Python just by
typing 2*x + 3*y + 4*z because everything is handled by Python's operator
evaluation precedence. So in this case, Python will evaluate 2*x, 3*x, 2*x +
3*x, 4*z, (2*x + 3*x) + 4*z, in that order, and there is really no way to make
it do otherwise.
Now, I suppose we could attempt to make sympify() smarter when passed a string
form of an expression (sympify('2*x + 3*y + 4*z')), but right now, it isn't
(more or less, it just calls ast parse, which does the same thing as regular
Python except it converts x, y, and z into Symbols first) . I don't know how
worth it it would be to make it smarter, since the string version is rarely
used compared to the standard version.
Aaron Meurer
>
> On Sep 10, 5:10 pm, Ondrej Certik <[email protected]> wrote:
>> On Fri, Sep 10, 2010 at 3:40 PM, Bill Hart <[email protected]> wrote:
>>> I recently viewed Ondrej's 2007 presentation on SymPy on the web:
>>> http://www.google.com/url?sa=t&source=web&cd=6&ved=0CCoQFjAF&url=http....
>>> ]
>>
>>> I'm curious about the details of the expression management in sympy,
>>> which are discussed on slide 14. Ondrej notes that the sympy strategy
>>> avoids the creation of intermediate classes, which I think is a great
>>> idea. However, I don't quite see how this is done when I look at the
>>> code. Can you provide more details about how this is achieved?
>>
>> We use the __new__ method of Python classes, which is responsible for
>> classes creation, and so if you do:
>>
>> a-a
>>
>> Add.__new__() get's called and inside that class it gets decided to
>> create a class Integer(0) instead (without creating the intermediate
>> Add class), in this case, "0" is also cached, but that's a detail.
>>
>>
>>
>>> I'm interested because I've run into performance bottlenecks in my
>>> Coopr software (specifically in coopr.pyomo), where I'm building
>>> algebraic expressions and I intermediate class construction is a major
>>> bottleneck!
>>
>> Ondrej
>
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