I used to use a lot of SymPy, but finally switched to Sage. This is Python as well, but has some syntax extensions and sensible defaults that reduce many quirks of Python. For example, you can write 1/2 instead of of Rational(1,2) and a^b instead of ab.

Moreover, Sage integrates lots of other libraries seamlessly. I remember doing lots of polynomial calculations and groebner basis stuff, and generating Singular script from Python seemed to be very elegant in the beginning. However, this quickly showed severe drawbacks. With Sage this all became easy and seamless again.