4. Control flow: loops and list comprehensions

Level 2: Python from Scratch for Finance

Any trading strategy is, at heart, a chain of decisions of the form 'if this happens, do that'. In Python those decisions are written with if, elif (short for 'else if') and else.

current_price = 183.20
moving_average = 180.00

if current_price > moving_average:
    print("Buy signal: price is above the moving average")
elif current_price == moving_average:
    print("No signal: price is exactly at the average")
else:
    print("Sell signal: price is below the moving average")

Here we meet one of Python's most distinctive traits, and one that surprises anyone coming from another language: blocks of code are not delimited with curly braces {}, they are delimited by indentation. Everything indented by the same number of spaces belongs to the same block. If you get the indentation wrong, Python raises a syntax error outright rather than producing an unexpected result — one of its most useful protections, however awkward it feels at first.

To repeat an action over each element of a sequence you use the for loop, the tool you will use most for iterating over lists of prices, tickers or dates.

week_prices = [182.52, 184.10, 179.88, 181.45, 183.20]

for price in week_prices:
    if price > 183:
        print(f"High price detected: {price}")

When you need both the value and its position in the list, enumerate gives you both at once, instead of having to keep a manual counter:

for day, price in enumerate(week_prices):
    print(f"Day {day}: {price}")

The while loop, by contrast, repeats as long as a condition holds, no matter how many times — useful, for example, to simulate a position until the stop-loss triggers:

capital = 10000
day = 0

while capital > 5000 and day < 30:
    capital = capital * 1.01  # simulated growth of 1% per day
    day += 1

print(f"Simulation finished on day {day}, final capital: {capital:.2f}")

A construct very characteristic of Python, and one you will see constantly in real financial code, is the list comprehension: a compact way of creating a new list by applying an operation to each element of another, in a single line instead of a full for loop.

daily_returns = [(week_prices[i] - week_prices[i-1]) / week_prices[i-1]
                  for i in range(1, len(week_prices))]

That single line computes the percentage return of each day relative to the previous one, running through the whole list. It is exactly the same result as writing a for loop with an empty list you keep appending to, but shorter and, in most cases, faster to run.