Programming foundations: Python

Sefism.

Chapter 01

Language basics

The building blocks every program uses: values and types, decisions, loops, functions, strings and missing values. Get these automatic before anything else.

Variables and types

Also called data types, primitives

What
A variable is a name that refers to a value. The value has a type, such as integer, decimal number, boolean or string, which decides what you can do with it and how much memory it uses.
Why
Types stop you adding a name to a price by accident and tell the reader what a value means. Choosing the right one (a whole number for a count, a decimal type for money) avoids a whole class of bugs.
Python
name = "Ada"
age = 36
height = 1.70
is_admin = False
age += 1
print(f"{name} is {age}")

Watch out

Reusing one variable for different meanings in the same function, so a reader cannot tell what it holds at any line.

Static and dynamic typing

Also called type checking, type inference, dynamic typing, statically typed

What
Statically typed languages (Java, C++, C#) check types when the code is compiled. Dynamically typed languages (Python, JavaScript) check them while the program runs. Type hints and TypeScript add optional static checks to the dynamic ones.
Why
It tells you when a type mistake will surface: before the program runs, or only when that line executes. It also explains why editors can autocomplete so much more in typed code.

Watch out

Treating var or auto as dynamic typing: the type is still fixed at compile time, the compiler just works it out for you.

Conditionals

Also called if else, ternary, switch

What
Code that runs only when a condition is true, with else if branches checked in order and an optional else for everything left. The ternary form picks one of two values in a single expression.
Why
Every decision in a program, from validating input to choosing a price band, is a conditional. Clear ordering of branches is what keeps the logic readable.
Python
if score >= 90:
    grade = "A"
elif score >= 50:
    grade = "pass"
else:
    grade = "fail"

label = "adult" if age >= 18 else "minor"

Watch out

Ordering checks from loosest to strictest, so a later branch such as score >= 90 can never be reached.

Loops

Also called for loop, while loop, foreach, iteration

What
A loop repeats a block of code: a for-each loop visits every item in a collection, a counting for loop runs over a range of indexes, and a while loop repeats as long as a condition holds.
Why
Processing every order, retrying a request or reading a file line by line are all loops. Prefer for-each when you only need the items, because there is no index to get wrong.
Python
for name in names:
    print(name)

for i, name in enumerate(names):
    print(i, name)

while attempts < 3:
    attempts += 1

Watch out

Off-by-one errors: looping with <= over the length reads one element past the end.

Functions

Also called methods, parameters, return value

What
A named block of code that takes inputs (parameters), does one job and can return a result. In Java and C# functions live inside classes and are called methods.
Why
Functions let you name an idea, reuse it and test it on its own. A program built from small functions is far easier to read and change than one long script.
Python
def area(width: float, height: float = 1.0) -> float:
    return width * height

print(area(3, 4))
print(area(width=3))

Watch out

Writing a function that both calculates and prints: return the value and let the caller decide what to do with it.

Scope

Also called block scope, global variables, local variables

What
Scope is the part of the code where a name is visible. Variables declared inside a function or block exist only there; globals are visible everywhere in a file or program.
Why
Keeping variables in the smallest scope means fewer places can change them, so bugs are easier to find and names do not clash.
Example
A total declared inside a loop is reset on every pass and gone after the loop. Declare it before the loop if you need the sum afterwards.

Watch out

Relying on global mutable variables: any function can change them, so the order of calls silently changes results.

Strings and formatting

Also called string interpolation, f-strings, template literals

What
A string is a sequence of text characters. Languages give you methods to trim, split, join, search and slice them, plus interpolation to build text from values.
Why
Most input and output is text: file lines, form fields, JSON and log messages. Interpolation is clearer and safer than gluing pieces together with +.
Python
s = "  Hello, World  "
clean = s.strip().lower()
parts = clean.split(", ")
joined = "-".join(parts)
print(f"{joined} has {len(joined)} chars")
print(clean[:5])

Watch out

Building a big string with + inside a loop in Java, C# or Python copies it every time; use StringBuilder or a join.

Null and missing values

Also called None, null, undefined, nullptr, optional

What
A special value meaning "nothing here": None in Python, null and undefined in JavaScript, null in Java and C#. C++ uses std::optional for a value that may be absent.
Why
Lookups, searches and optional input often have no result. Handling that case on purpose is what prevents the most common runtime crash in most languages.
Python
user = users.get("ada")
if user is None:
    print("no such user")

name = user.name if user else "guest"

Watch out

Calling a method on a value that can be null without checking first, which throws a null reference error far from where the null came from.

Reading and validating input

Also called parsing, user input, stdin

What
Reading text typed by a user or passed to the program, then converting it to the type you need, such as an integer, and rejecting anything that does not fit.
Why
Input is where programs meet the messy world. Validating it at the edge means the rest of your code can trust its data.
Python
raw = input("Age: ")
try:
    age = int(raw)
except ValueError:
    print("Please type a whole number")

Watch out

Converting input without handling failure: users type spaces, letters and empty lines, and the program crashes.

Go deeper

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