Expand SWEEP_LIKE to work for {'t': [0, 1, 3]}

[Strilanc]

Currently if you make a call like sampler.sample(circuit, params={'t': range(5)}) it doesn't work, because it expects e.g. a list of dictionaries instead of a dictionary containing lists. For convenience we should just support both cases. Check if the input is a dictionary with list values (and possibly singletons too), and if so take the cartesian product of the various entries and turn it into a sweep.

Test cases:

assert cirq.to_sweeps({'t': [0, 2, 3]}) == (
    cirq.to_sweeps([{'t': 0}, {'t': 2}, {'t': 3}]}))

assert cirq.to_sweeps({'t': [0, 1], 's': [2, 3], 'r': 4}) == (
    cirq.to_sweeps([
        {'t': 0, 's': 2, 'r': 4},
        {'t': 0, 's': 3, 'r': 4},
        {'t': 1, 's': 2, 'r': 4},
        {'t': 1, 's': 3, 'r': 4},
    ]))

[crystalzhaizhai]

Could you please assign it to me?

[kevinsung]

The following code still doesn't work:

import cirq
import sympy

a = cirq.LineQubit(0)
circuit = cirq.Circuit(cirq.X(a)**sympy.Symbol('t'),
                        cirq.measure(a, key='m'))
params = {'t': [0.3, 0.5]}
sim = cirq.Simulator()
result = sim.run_sweep(circuit, params=params)

[maffoo]

I think we decided not to do this so that you can resolve list-valued parameters (we have use cases for this internally). Also, there's an ambiguity if you have multiple parameters; given {'a': [1, 2], 'b': [3, 4]} should that do a cartesian product sweep over a and b or a zip sweep? Instead you need to make the sweep explicit at the top level, either as [{'t': 0.3}, {'t': 0.5}] or using actual sweep objects: cirq.Points('t', [0.3, 0.5]).

[kevinsung]

If that's the case, then we need to roll back #2786. That PR already partially implemented this feature, using the Cartesian product definition.

[dstrain115]

Should we put this on the list of issues to discuss at the next cirq cync, @balopat ?