Circuit.free_symbols() should not return a set

[xenoicwyce]

The method pytket.Circuit.free_symbols() should either return a list or a tuple (ordered iterables) instead of a set. When iterating a set, the order is different for every new instance of Python, causing inconsistent assignment of parameter values to the symbols. When assigning parameters using Circuit.symbol_substitution(), the users usually zip the free_symbols() and an array of parameter values to be assigned. But since the order of the symbols are not fixed, it will assign to different parameters given the same array.

[cqc-alec]

When assigning parameters using Circuit.symbol_substitution(), the users usually zip the free_symbols() and an array of parameter values to be assigned.

This isn't a good idea, for the reason you say. The user should be explicit about what value to assign to what symbol.

Since there is no canonical order for the symbols, a set seems like the appropriate container. (Of course you can always sort it if you want a predictable order.)

[xenoicwyce]

But wouldn't it be more convenient if I can know the order when I iterate through the symbols? For example if I zip(circ.free_symbols(), param_values) to create a dictionary for symbol substitution. Is there particular reason for the necessity of symbols to be a set?

[cqc-alec]

One reason is that the underlying symengine function returns a set. Yes, we could convert it to a sorted list before returning it to the user, but the ordering has no intrinsic meaning (and we'd lose the benefit of efficient membership testing etc).