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Key Concepts

Duck Typing

Introduction

Duck typing is popular type system in Python. The language documentation defines duck typing as follows:

A programming style which does not look at an object's type to determine if it has the right interface; instead, the method or attribute is simply called or used (“If it looks like a duck and quacks like a duck, it must be a duck.”) By emphasizing interfaces rather than specific types, well-designed code improves its flexibility by allowing polymorphic substitution.

Duck-typing avoids tests using type() or isinstance(). (Note, however, that duck-typing can be complemented with abstract base classes.) Instead, it typically employs hasattr() tests or EAFP programming.


You can use duck-typing to add attributes to all non-primitive types in Python

Select Language: 
# Duck-typing enables you to add attributes to non-primitive types in Python.
class Duck:
    pass

duck = Duck()
duck.quacks = "Quack!"
print(duck.quacks)

Custom Security Properties

You can add attributes to the Security object. For example, you can add an exponential moving average.

Select Language: 
# Add an EMA indicator to the SPY Equity object.
equity = self.add_equity("SPY")
equity.ema = self.ema(equity.symbol, 10, Resolution.DAILY)

This feature is helpful because you can get the Security object from the securities object.

Select Language: 
# Get the SPY Equity object from the securities object to get the EMA indicator.
ema = self.securities["SPY"].ema.current.value

Other Types

You can add properties to all types. However, these properties live in the Python space, so you can't access them without a reference. The following example demonstrates that you can add an exponential moving average to the symbol attribute of a Security object. 

# Add an EMA to the SPY security.
equity = self.add_equity("SPY")
self._symbol = equity.symbol
self._symbol.ema = self.ema(self._symbol, 10)
ema = self._symbol.ema.current.value

ema = self.securities["SPY"].symbol.ema.current.value

Examples

The following examples demonstrate some common practices for duck typing.

Example 1: Store EMA Indicator

The following example implements a trend-following strategy using the EMA indicator for the top 20 liquid equities. It uses duck typing to store the EMA indicator in each Security object , so the indicator is discarded together with the Security when it is removed from the universe.

Select Language: 
class DuckTypingAlgorithm(QCAlgorithm):
    def initialize(self) -> None:
        self.set_start_date(2022, 1, 1)
        self.set_end_date(2022, 6, 1)
        # Weekly renewal on the universe to allow time to capitalize on the trend.
        self.universe_settings.schedule.on(self.date_rules.week_start())
        # Trade on the top 20 liquid equities since they have higher volume and hence price movements.
        self._universe = self.add_universe(self.universe.dollar_volume.top(20))

    def on_data(self, slice: Slice) -> None:
        for kvp in self._universe.members:
            symbol, security = kvp.key, kvp.value

            # Trade on updated price data.
            bar = slice.bars.get(symbol)
            if bar:
                ema = security.ema.current.value
                # Switch to long if the current price is above EMA, suggesting an uptrend.
                if bar.close > ema and not self.portfolio[symbol].is_long:
                    self.set_holdings(symbol, 0.025)
                elif bar.close < ema and not self.portfolio[symbol].is_short:
                    self.set_holdings(symbol, -0.025)
                
    def on_securities_changed(self, changes: SecurityChanges) -> None:
        for removed in changes.removed_securities:
            # Liquidate if it is not in the top 50 liquidity stocks anymore.
            self.liquidate(removed.symbol)
            # Release resources that update automatic indicators
            self.deregister_indicator(removed.ema)

        for added in changes.added_securities:
            # Use duck typing to create an EMA indicator for trend trading.
            # Discarded together when the security is removed from the universe, you won't need to handle it.
            added.ema = self.ema(added.symbol, 60, Resolution.DAILY)
            # Warm up the EMA indicator to ensure its readiness for immediate usage.
            self.warm_up_indicator(added.symbol, added.ema, Resolution.DAILY)

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