Loop bounds must be constant: Vyper compiler static analysis

Answer

The most gas‑efficient way to iterate over a dynamic list while satisfying Vyper’s static‑analysis rule is to fix a compile‑time upper bound (e.g., MAX_ITEMS) and break out of the loop as soon as the actual length is reached.

Pattern

MAX_ITEMS: constant(uint256) = 100  # adjust to your realistic maximum

items: public(DynArray[uint256, MAX_ITEMS])

@external
@view
def process_items() -> uint256:
    total: uint256 = 0
    for i in range(MAX_ITEMS):
        if i >= len(self.items):
            break
        total += self.items[i]
    return total

Handling a much smaller actual length

• The loop still executes up to MAX_ITEMS iterations, but the break stops further work as soon as i >= len(items).
• Because the loop body is minimal (just a bounds check and an addition), the extra iterations add only a few gas units each.
• If the expected maximum is large and the typical size is tiny, consider lowering MAX_ITEMS to a tighter bound that still covers the worst‑case you anticipate, or redesign the logic to batch process off‑chain.

Verification steps

1. Copy the snippet above into a file process.vy.
2. Compile with Vyper ≥0.2.0: vyper process.vy.
3. Confirm the compiler succeeds; any attempt to replace MAX_ITEMS with a state variable or function argument will produce a static‑analysis error about non‑constant loop bounds.