On Thu, Jun 26, 2008 at 7:13 PM, Kirill Smelkov
<[EMAIL PROTECTED]> wrote:
>
> On Fri, Jun 20, 2008 at 05:30:29AM -0700, Pearu Peterson wrote:
>>
>>
>>
>> On Jun 20, 11:42 am, "Ondrej Certik" <[EMAIL PROTECTED]> wrote:
>> > >> Also, let's get rid of set_repr_level()? This is quite an important
>> > >> change, so I'd like to hear opinions of others as well.
>> >
>> > > We already had this discussion in the past:
>> >
>> > >http://code.google.com/p/sympy/issues/detail?id=697
>> >
>> > > And I think repr(x) should be "Symbol('x')", str(x) should be "x" and
>> > > there should be no Basic.set_repr_level.
>> >
>> > Agree. So you mean this, right Kirill?
>> >
>> > >>> repr(x**2/2)
>> >
>> > Mul(Half(1, 2), Pow(Symbol('x'), Integer(2)))
>
> Yes, exactly.
>
>> There is a problem with this representation - it will
>> be implementation dependent (say, one will change
>> the Half to something else like Rational).
>
> I don't think there is a problem that repr is implementation dependent --
> in fact repr is usually used for debugging and it is more convenient
> that it'll tell us what exactly some symbolic tree is.
>
> So, if repr says there is a Half - I think it is good that we could
> distinguish it from some Rational.
>
>> Another solution to the repr probelm is to return
>>
>> sympify('x**2/2')
>>
>> that will satisfy the basic assumption behind repr:
>>
>> eval(repr(obj)) == obj
>>
>> (assuming that the parser works correctly) and
>> is well readable.
>
> I think we could have
>
> eval(repr(obj)) == obj
>
> with the above approach too.
>
> At present
>
> eval('Half')
>
> produces NameError, but I think this should be fixed.
This is fixed trivially just by importing Half:
In [1]: sympify(Half)
---------------------------------------------------------------------------
NameError Traceback (most recent call last)
/home/ondra/sympy/<ipython console> in <module>()
NameError: name 'Half' is not defined
In [2]: from sympy.core.numbers import Half
In [3]: sympify(Half)
Out[3]: <class 'sympy.core.numbers.Half'>
>
>
> ----
>
> I'm for:
>
> >>> repr(x**2/2)
>
> Mul(Half(1, 2), Pow(Symbol('x'), Integer(2)))
The Pearu's proposition has the advantage that it's shorther. However
I think the repr should be in a way that you can copy & paste for
example into our test suite. Neither sympify() nor Mul(Half(1, 2),
Pow(Symbol('x'), Integer(2))) could (I mean of course you can do that,
but it looks ugly). Another option is this:
>>> repr(x**2/2)
Symbol("x")**2/2
>>> repr(x**2/2 /x**2)
sympify(1)/2
>>> repr(x/x)
sympify(1)
>>> repr(sin(x).series(x))
Symbol("x") - Symbol("x")**3/6 + Symbol("x")**5/120 + O(Symbol("x")**6)
And actually, how about using "x" instead of "Symbol("x")" for all
symbols that are loaded by "from sympy.abc import *" ? The repr would
return a very natural expressions in 95% of cases and it could always
(100%) be evaluated by:
from sympy.abc import *
from sympy import *
eval( repr(something) )
Would that be pythonic? If not, we may for example import "x" (maybe
also y and z) by "from sympy import *", so that x, y, z would be just
that and all other symbols by Symbol("a").
Ondrej
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