Python: your first language

The first programming language you learn, from the very first line: running a program, decisions and loops, functions, lists and dictionaries, errors, then files and JSON. Eight lessons, each with code you run yourself in the page.

OverviewEight lessons from your first line of Python: decisions and loops, functions, lists and dictionaries, errors, files and JSON, and how to read an error message. Each lesson opens with an animation and code you run yourself.
Environment & First ProgramsHow the REPL differs from a script file, and why every project gets its own virtual environment instead of polluting the system Python.
Control FlowHow a program makes decisions with conditions and repeats work with loops, when while beats for , and the off-by-one traps.
Functions & ModularityWrapping repeated logic in a function that returns a value instead of printing it, splitting code across importable files, and what the __main__ guard is for.
Core Data StructuresList, tuple, dict and set: what separates them, and when the wrong choice is the reason your program is slow.
Exception HandlingCatching only the error you expect instead of swallowing everything, how except , else and finally differ, and why silent failure is worse than a crash.
File HandlingHow to open a file, read it three different ways, write to it, and guarantee it always closes, even if an error happens halfway through.
SerializationHow to turn a Python object in memory into a format you can save to a file or send over a network, and reconstruct it exactly as it was.
Debugging StrategiesFrom confidently reading a traceback, to smart print debugging, to interactive pdb, to professional logging, the tools ordered from simplest to deepest.
Wrap-up and resourcesThe best outside resources for these lessons, classified by kind and level. All are open to everyone and need no account on any platform. Use the filters to narrow the list.