fix(backtest): guard zero cost_ratio in _get_buy_amount_by_cash_limit

`Exchange._get_buy_amount_by_cash_limit` computes the cash level at which the
proportional fee overtakes `min_cost`:

    critical_price = self.min_cost / cost_ratio + self.min_cost

`cost_ratio` is `open_cost + impact_cost`, and `impact_cost` defaults to 0.0, so
any backtest configured with `open_cost=0` raises ZeroDivisionError as soon as a
buy order exceeds available cash — the branch that calls this helper. Setting
`min_cost=0` as well does not help, since 0.0 / 0.0 raises too.

A zero-cost run is a routine baseline for isolating how much of a strategy's
result is being consumed by frictions, and it currently crashes.

When there is no proportional fee the service fee is always `min_cost`, so no
critical price exists and the min_cost branch is the correct one. Returning it
directly keeps every non-zero `cost_ratio` path byte-identical.

Adds unit tests over the default fee schedule, the above/below critical-price
branches, zero cost_ratio with and without min_cost, and cash below min_cost.

Author: mateosandoval10

qlib/backtest/exchange.py

@@ -846,6 +846,11 @@ def _get_buy_amount_by_cash_limit(self, trade_price: float, cash: float, cost_ra
         """
         max_trade_amount = 0.0
         if cash >= self.min_cost:
+            if cost_ratio <= 0:
+                # Without a proportional fee the service fee is always `min_cost`,
+                # so there is no critical price to compare against.
+                max_trade_amount = (cash - self.min_cost) / trade_price
+                return max_trade_amount
             # critical_price means the stock transaction price when the service fee is equal to min_cost.
             critical_price = self.min_cost / cost_ratio + self.min_cost
             if cash >= critical_price:

tests/backtest/test_exchange_cost.py

@@ -0,0 +1,60 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+
+import unittest
+
+from qlib.backtest.exchange import Exchange
+
+
+def make_exchange(min_cost: float) -> Exchange:
+    """Build an Exchange for testing `_get_buy_amount_by_cash_limit` only.
+
+    The method is pure arithmetic over `min_cost`, so the instance is created
+    without `__init__` to keep the test free of the data layer.
+    """
+    exchange = object.__new__(Exchange)
+    exchange.min_cost = min_cost
+    return exchange
+
+
+class TestBuyAmountByCashLimit(unittest.TestCase):
+    """`_get_buy_amount_by_cash_limit` must handle a zero proportional fee."""
+
+    TRADE_PRICE = 10.0
+    CASH = 1000.0
+
+    def test_default_cost_ratio(self):
+        """With the default fees, the min_cost branch applies at this cash level."""
+        exchange = make_exchange(min_cost=5.0)
+        # critical_price = 5 / 0.0015 + 5 = 3338.3 > cash, so the fee is min_cost.
+        amount = exchange._get_buy_amount_by_cash_limit(self.TRADE_PRICE, self.CASH, cost_ratio=0.0015)
+        self.assertAlmostEqual(amount, (self.CASH - 5.0) / self.TRADE_PRICE)
+
+    def test_cost_ratio_above_critical_price(self):
+        """Above the critical price the proportional fee applies."""
+        exchange = make_exchange(min_cost=5.0)
+        cash = 10_000.0
+        amount = exchange._get_buy_amount_by_cash_limit(self.TRADE_PRICE, cash, cost_ratio=0.0015)
+        self.assertAlmostEqual(amount, cash / 1.0015 / self.TRADE_PRICE)
+
+    def test_zero_cost_ratio_with_min_cost(self):
+        """A zero proportional fee must not divide by zero; min_cost still applies."""
+        exchange = make_exchange(min_cost=5.0)
+        amount = exchange._get_buy_amount_by_cash_limit(self.TRADE_PRICE, self.CASH, cost_ratio=0.0)
+        self.assertAlmostEqual(amount, (self.CASH - 5.0) / self.TRADE_PRICE)
+
+    def test_frictionless(self):
+        """With no fee at all the whole cash balance is investable."""
+        exchange = make_exchange(min_cost=0.0)
+        amount = exchange._get_buy_amount_by_cash_limit(self.TRADE_PRICE, self.CASH, cost_ratio=0.0)
+        self.assertAlmostEqual(amount, self.CASH / self.TRADE_PRICE)
+
+    def test_cash_below_min_cost(self):
+        """Cash that cannot even cover the minimum fee buys nothing."""
+        exchange = make_exchange(min_cost=5.0)
+        amount = exchange._get_buy_amount_by_cash_limit(self.TRADE_PRICE, 1.0, cost_ratio=0.0)
+        self.assertEqual(amount, 0.0)
+
+
+if __name__ == "__main__":
+    unittest.main()