When asking for a symbol .free_symbols I get something in curly braces (which is a set).
If I use this set as list of arguments for lambdify of sympy it seems it is converted into a list. This is hinted in the doc but I suggest a warning to be given here when this conversion is made. A good reason for this is that the ordering of the symbols may be altered in this conversion.
In my case
_chiSquare.free_symbols gives {c_95_0, c_95_1}
but
list({'c_95_0', 'c_95_1'}) gives ['c_95_1', 'c_95_0']
I like to automate the making of numerical functions using .free_symbols but this is hard to work with if order of variable is changed without notice.
My question is how one is supposed to deal with free_symbols and lambdify in a way that arguments order is kept fixed.
I think it is better to keep track of the symbols explicitly rather than using free_symbols.
If you must work from free_symbols then you can sort them to get a consistent ordering:
In [3]: sorted((x*y).free_symbols, key=lambda s: s.name)
Out[3]: [x, y]