Python Fundamentals

Python is a general-purpose programming language. You write source code as statements and expressions. A Python interpreter executes that code using objects, names, functions, modules, and exceptions.

Use one mental model:

source code → parser/compiler → bytecode or implementation form → interpreter
                                      ↓
                              objects + side effects

Python language behavior is defined by the language reference. CPython is the most common implementation, but implementation details are not universal language guarantees.

Names refer to objects

Assignment binds a name to an object.

items = [1, 2]
alias = items
alias.append(3)

Both names refer to the same list, so the mutation is visible through both names.

count = 2
count = count + 1

The second assignment binds count to a different integer object. It does not mutate the original integer.

This distinction explains many Python behaviors:

• parameters receive references to objects;
• mutable objects can change in place;
• multiple names can share one object;
• rebinding one name does not rebind another;
• identity and equality answer different questions.

Use == to compare values. Use is for object identity, most commonly with None.

Core scalar types

Python includes:

• None for absence of a value;
• bool for True and False;
• int for integers;
• float for binary floating-point numbers;
• complex for complex numbers;
• str for Unicode text;
• bytes for byte sequences.

Floating-point numbers cannot represent every decimal fraction exactly. Do not use binary floating point for exact decimal accounting without a suitable decimal representation and explicit rounding rules.

Text and bytes are different. Decode bytes into text with a known character encoding. Encode text into bytes when writing to a byte-oriented boundary.

Collections and mutability