Booleans and comparisons

True and False

A boolean (bool) has exactly two values: True and False, written with a capital letter. Every trading rule ends in one: "is the market crossed?", "can we afford this order?". Comparing two values produces a boolean, so ask > bid is not a decision yet, it is a value you can store, print and combine.

bid, ask = 184.25, 184.5
print(ask > bid)        # True
print(type(ask > bid))  # <class 'bool'>

Comparison operators

Python compares with == (equal), != (not equal), <, <=, > and >=. A single = is assignment, not comparison, so price = 5 stores a value while price == 5 asks a question. Comparisons can be chained: low <= price <= high reads like the maths and checks both sides at once.

# Comparison operators give True or False
price = 184.4
print(price == 184.4)         # True
print(price != 184.25)        # True
print(184.25 <= price < 185)  # True (chained)

Combining conditions: and, or, not

a and b is True only when both are. a or b is True when at least one is. not a flips the value. Together they express real rules, such as "the market is open, the stock is not halted, and we have the cash".

# Combine conditions with and, or, not
market_open, halted, cash = True, False, 500.0
can_buy = market_open and not halted and cash >= 184.5
print(can_buy)               # True
print(halted or cash < 100)  # False

and and or short-circuit: Python stops as soon as the answer is known, and does not evaluate the rest. That is useful as a guard, putting the safe check first so the risky one only runs when it is allowed to.

# Short-circuit: the division never runs when quantity is 0
quantity = 0
print(quantity > 0 and 100 / quantity > 1)  # False

Truthiness

Any value can be used as a condition. 0, 0.0, the empty text "" and None count as false; almost everything else counts as true. bool(value) shows how Python reads a value. Be careful: quantity and price > 0 may give back 0 instead of False, so write the comparison out when a real bool is needed.

# Zero, empty text and None count as False
print(bool(0), bool(0.0), bool(""), bool(None))  # False False False False
print(bool(200), bool("AAPL"))                    # True True

Comparing floats, and None

== between floats is risky because of rounding error (see integers and floats): use math.isclose for "equal enough". == compares values; the word is asks whether two names point at the very same object, and you only need it for one thing: x is None, to test for "no value".

# Floats need a tolerance; None is tested with "is"
import math

print(0.1 + 0.2 == 0.3)              # False
print(math.isclose(0.1 + 0.2, 0.3))  # True
fill_price = None
print(fill_price is None)            # True

Next up: conditionals, where a boolean decides which lines of code run: if, elif and else.

Lab · what's graded

• Detect a crossed market with is_crossed25 pts
• Check a price is inside a band with within_band25 pts
• Combine the trading conditions with can_trade25 pts
• Validate an order with is_valid_order25 pts