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RC2_IND_MIXED_RETURN

compute() mixes single-value returns with dict returns. Use one or the other.

What it means

The shape of what compute() returns is what decides the shape of the indicator. Return a number and it is single-value: strategies read it as .value and the chart draws one line. Return a dict and it is multi-output: strategies read .upper, .mid, .lower, and the chart draws one series per key.

That decision is made once, at compile time, from all the return statements in the method — because it becomes the C# type the value record is emitted as. A method that returns a number down one branch and a dict down another has no single such type, so it is refused.

Why it happens

Almost always a guard clause that was written before the indicator grew its extra outputs:

class MyBands(Indicator):
    position = ON_CHART
    period = Param(20, min=2, max=200)
    buffer = Series(period)


    def compute(self, close):
        self.buffer.add(close)
        mid = sum(self.buffer) / len(self.buffer)
        if not self.is_ready:
            return close                  # ❌ a bare number on this path
        return {"upper": mid + 1, "mid": mid, "lower": mid - 1}

The early return close was correct when the indicator returned a single average. Once the main path started returning three bands, it stopped being a warmup shortcut and became a second, incompatible shape.

How to fix it

Return the same dict on every path — fill the keys with whatever the early branch would have meant.

class MyBands(Indicator):
    position = ON_CHART
    period = Param(20, min=2, max=200)
    buffer = Series(period)


    def compute(self, close):
        self.buffer.add(close)
        mid = sum(self.buffer) / len(self.buffer)
        if not self.is_ready:
            return {"upper": close, "mid": close, "lower": close}
        return {"upper": mid + 1, "mid": mid, "lower": mid - 1}

If the indicator was only ever meant to produce one number, go the other way and delete the dict — the branch that returns three values is then the one that does not belong.

The one exception that looks like a mix

return {"value": v, "color": c} is not a multi-output return — those two keys are reserved for the coloured single-value shape used by histogram bars. Mixing that dict with a bare number raises this same diagnostic, and the fix is the same: pick one shape.