isympy -I: A saner interactive environment  As promised, here is
another post describing a new feature in the upcoming SymPy
0.7.2.  Automatic Symbol Definition  While not as ground
breaking as the feature I described in my last post, this
feature is still quite useful. As you may know, SymPy is
inherently a Python library, meaning that it lives by the rules
of Python. If you want to use any name, whether it be a Symbol
or a function (like cos), you need to define it (in the case of
Symbols), or import it (in the case of functions that come with
SymPy). We provide the script isympy with SymPy to assist with
this. This script automatically runs IPython (if it’s
installed), imports all names from sympy (from sympy import *),
and defines common symbol names (like x, y, and z).  But if you
want to use a Symbol that is not one of the ones predefined by
isympy, you will get something like  In [1]: r*x NameError
Traceback (most recent call last)  in () ----> 1 r*x  NameError:
name 'r'