If statements and comparisons: let the program decide

A comparison is a question with a yes or no answer: is this price above that level? Python answers True or False, and an if statement runs a block of code only when the answer is True.

With if, elif and else the tests are tried from the top and only the first one that is true runs. The order you write them in is part of the logic, so read the chain in the script as a list of questions asked one after another.

The price and the levels in the script are invented, and the tests are there to show the syntax. They are not rules for trading. The last two lines show a trap worth meeting early: two decimal numbers that should be equal often are not, quite.

The code

# If statements and comparisons: let the program choose what to do.
# No broker, no account, no internet. The price and the levels are invented, and the tests
# below are examples of syntax, not rules to trade by.

price = 104.3
stop_level = 101.0
limit_level = 108.0
direction = "BUY"
market_open = True

# A comparison is a question whose answer is True or False.
print("price > stop_level:", price > stop_level)
print("price == limit_level:", price == limit_level)
print("direction != 'SELL':", direction != "SELL")

# if, elif and else: the first branch whose test is True runs, and the rest are skipped.
if price <= stop_level:
    state = "at or below the stop"
elif price >= limit_level:
    state = "at or above the limit"
else:
    state = "between the stop and the limit"
print("The price is", state)

# and, or and not join tests together.
if market_open and direction == "BUY":
    print("The market is open and the direction is BUY")
if price < stop_level or price > limit_level:
    print("Outside the range")
else:
    print("Inside the range")
if not market_open:
    print("The market is closed")

# A chained comparison reads like the maths.
print("stop_level < price < limit_level:", stop_level < price < limit_level)

# Do not test floats with ==. Arithmetic on floats is very slightly inexact.
total = 0.1 + 0.2
print("0.1 + 0.2 == 0.3:", total == 0.3)
print("Close enough to 0.3:", abs(total - 0.3) < 0.000001)

What you should see

price > stop_level: True
price == limit_level: False
direction != 'SELL': True
The price is between the stop and the limit
The market is open and the direction is BUY
Inside the range
stop_level < price < limit_level: True
0.1 + 0.2 == 0.3: False
Close enough to 0.3: True

Run with Python 3.12 on Windows. No network and no account.

Check yourself

Answer each question from the code above before you open it. The answers were written from the script on this page and what it printed.

Exercise

Change price to 108.0, the same as the limit level. Predict which lines of output change, then run it.

price > stop_level: True
price == limit_level: True
direction != 'SELL': True
The price is at or above the limit
The market is open and the direction is BUY
Inside the range
stop_level < price < limit_level: False
0.1 + 0.2 == 0.3: False
Close enough to 0.3: True

# Exercise solution: the lesson script with the one change the exercise asks for.
# If statements and comparisons: let the program choose what to do.
# No broker, no account, no internet. The price and the levels are invented, and the tests
# below are examples of syntax, not rules to trade by.

price = 108.0
stop_level = 101.0
limit_level = 108.0
direction = "BUY"
market_open = True

# A comparison is a question whose answer is True or False.
print("price > stop_level:", price > stop_level)
print("price == limit_level:", price == limit_level)
print("direction != 'SELL':", direction != "SELL")

# if, elif and else: the first branch whose test is True runs, and the rest are skipped.
if price <= stop_level:
    state = "at or below the stop"
elif price >= limit_level:
    state = "at or above the limit"
else:
    state = "between the stop and the limit"
print("The price is", state)

# and, or and not join tests together.
if market_open and direction == "BUY":
    print("The market is open and the direction is BUY")
if price < stop_level or price > limit_level:
    print("Outside the range")
else:
    print("Inside the range")
if not market_open:
    print("The market is closed")

# A chained comparison reads like the maths.
print("stop_level < price < limit_level:", stop_level < price < limit_level)

# Do not test floats with ==. Arithmetic on floats is very slightly inexact.
total = 0.1 + 0.2
print("0.1 + 0.2 == 0.3:", total == 0.3)
print("Close enough to 0.3:", abs(total - 0.3) < 0.000001)

Your progress

Next: Lists and loops

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Where this sits in the path

Stage 1 of 4: First steps. Values, decisions, loops and functions, and the habit of testing what you wrote. No files and no account.

1. 1First Python script
2. 2Variables and types
3. 3If and comparisonsYou are here
4. 4Lists and loops
5. 5Functions and sizing
6. 6Test your functions

The whole path, every stage