Python from zero

What a program is

A program is a text file of instructions that a machine performs exactly and in order. The machine here is the interpreter, a program named python3 that reads your file top to bottom and does what each line says: compute a value, print text, wait, or, from the First light page on, pull a GPIO pin high.

Coming from ladder logic, one difference deserves naming: a PLC scan loops over all rungs continuously by itself. A Python script runs once, top to bottom, then exits, unless you write the loop yourself. The "scan cycle" of your IoT programs will be a while loop you can see and control, which is why loops get their own section below.

Two ways to run Python

$ python3
Python 3.11.2 (main, ...) on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 3.3 - 2.0
1.2999999999999998
>>> (3.3 - 2.0) / 330        # the LED current calculation from the GPIO page, in amps
0.00393939393939394
>>> exit()

Variables and types

A variable is a name attached to a value. Naming values is how programs stay readable and changeable: alter delay once and every use follows.

>>> delay = 0.5              # a float (decimal number)
>>> blinks = 10              # an int (whole number)
>>> name = "lastname-pi"     # a str (text, in quotes)
>>> running = True           # a bool: True or False, capitalized
>>> blinks * delay
5.0
>>> print(f"{name} will blink {blinks} times")
lastname-pi will blink 10 times

• = means assign, "attach this name to this value", not mathematical equality. (Equality testing is ==, next section.)
• The main types this course uses: int, float, str, bool. type(delay) tells you what you have.
• print() writes to the terminal; an f"..." string with {name} inside splices values into text. You will format every sensor reading this way.
• Names: lowercase with underscores (press_count), chosen to make the code read like the Week 5 pseudocode it came from.

Decisions: if and else

temperature = 31.5

if temperature > 30:
    print("Too hot, fan on")
elif temperature < 18:
    print("Too cold, heater on")
else:
    print("In range")

Three things to absorb, because every control program is made of them:

• The condition (temperature > 30) is an expression that evaluates to True or False. Comparisons: > < >= <= == (equals) != (not equals); combine with and, or, not.
• The colon and the indent define the block: everything indented under the if belongs to it. This is the thermostat logic of Assignment 2 in miniature.
• elif chains further tests; else catches everything remaining. At most one branch runs.

Repetition: while and for

from time import sleep

count = 0
while count < 5:
    print(f"blink {count}")
    sleep(0.5)
    count = count + 1

for count in range(5):
    print(f"blink {count}")

for colour in ["red", "green", "blue"]:
    print(colour)

The sleep() call comes from the time module via an import: from time import sleep at the top of the file makes the function available. Imports are how all borrowed code arrives, including gpiozero two pages from now; the Environments page explains where imported code actually lives.

Functions: naming a recipe

from time import sleep

def blink_message(times, delay):
    """Print a blink message 'times' times, 'delay' seconds apart."""
    for n in range(times):
        print(f"blink {n}")
        sleep(delay)

blink_message(3, 0.5)     # fast
blink_message(2, 1.0)     # slow

A function wraps a block under a name, with parameters for the parts that vary. Defining it does nothing; calling it runs it. Functions are how programs stop being one long scroll: the same recipe, reused with different values, testable on its own. A function can also return a value to its caller (def celsius(raw): return raw * 0.1), which is how sensor-reading code will hand you numbers. Week 6 builds classes out of exactly this idea.

Tracebacks: errors that explain themselves

You will cause errors today, on purpose and otherwise. Python reports them with a traceback, and like Git's messages in Week 1, it is written to be read:

$ python3 blink.py
Traceback (most recent call last):
  File "/home/username/iot/week4/blink.py", line 4, in <module>
    print(f"blink {countt}")
                   ^^^^^^
NameError: name 'countt' is not defined. Did you mean: 'count'?

• Read from the bottom. The last line is the diagnosis: the error type (NameError) and the explanation, here with the fix suggested.
• The lines above give the address: file and line number. Open the file at that line; the answer is almost always there or one line earlier.
• The frequent ones this week: SyntaxError (typo in the grammar, often a missing colon or quote), IndentationError (see the stop box above), NameError (misspelled name, or used before assignment), ModuleNotFoundError (import failed, an environment question, not a code one).

Checklist for this stage

• I opened the REPL, used it as a calculator, and quit it
• I assigned variables of all four types and printed them with an f-string
• I wrote an if/elif/else that reacts to a temperature value I change
• I wrote a while loop and a for loop that count with sleep(), and stopped a running one with Ctrl+C
• I defined a function with parameters and called it twice with different values
• I caused a NameError on purpose and fixed it by reading the traceback bottom-up

Check yourself