# Initial implementation of the query system

Category: General, sympy, Tecnología

I sent some patches to sympy-patches with an initial implementation
of the query system. You can check it out by pulling from my branch:
`git pull http://fseoane.net/git/sympy.git master` into your sympy
repo. Some examples of what you can do (sample isympy session):
`In [1]: query(x, positive=True)` Returns None, as we do not know
whether x is positive or not.
`` In [2]: query(abs(x), positive=True) Out[2]: True`` because abs() is
always positive. Because exp() is always positive, the following should
also be True: `` In [3]: query(exp(x), positive=True)`` but why then
does it return None ?. Well, it simply is not True that exp() is always
positive, it is always positive for real values, but SymPy does not
assume that x is real, so you would have to specify that. This is now
done with the keyword assumptions:
`` In [5]: query(exp(x), positive=True, assumptions=Assume(x, real=True)) Out[5]: True``
As you can see, now assumptions are independent objects and are not tied
to symbols any more. For more examples, see the file
sympy/query/tests/test_query.py still in the TODO list: - support for
global assumptions - solve more complex implications, like
`query(x, positive=True, assumptions=Assume(x, even=True))` where it
should build the chain of implications even => integer => rational =>
real. This chain of implications currently stops at rational for
efficiency reasons, because the number of facts grows in each step which
makes the number of possible paths grow exponentially.