PCC·10 · Chapter record

Files and Exceptions

Chapter 10 — saving data and handling things going wrong

Done

Five sessions on files and errors: reading a file into memory, writing and appending to one, handling exceptions with try/except/else, failing silently on purpose, and saving data with JSON. Since this page runs entirely in your browser, each demo uses a small simulated file rather than your real filesystem — the Python code shown is exactly what you'd write for a real file. Each session explains the concept, lists the key terms, then gives you exercises — try them yourself before revealing the code.

10.1

Reading from a File

Concept

with open(filename) as file_object: opens a file — the with block closes it automatically when done, even if something goes wrong inside.
.read() pulls the entire file into memory as one string; loop directly over the file object (or use .readlines()) to work through it one line at a time.
Every line from a text file carries an invisible trailing newline — chain .rstrip() to strip it back off when printing.

Key Terms

open() / with: opens a file and closes it automatically at the end of the block
.read(): reads the whole file into one string
.readlines(): reads the file into a list, one entry per line

Exercise

Activity 1

Edit the simulated file below to a short 3-line text of your choice, then compare .read() against the line-by-line loop.

Activity 2

Notice the extra whitespace without rstrip() — explain in one sentence why it shows up.

10.2

Writing to a File

Concept

open(filename, 'w') opens (or creates) a file in write mode — but any existing content is erased first.
.write() doesn't add newlines on its own — include \n yourself if you want each call to land on its own line.
open(filename, 'a') opens in append mode instead — new writes are added to the end, and nothing existing is erased.

Key Terms

'w' mode: write mode — creates the file if needed, erases existing content first
'a' mode: append mode — adds new content to the end without erasing

Exercise

Activity 3

Write a line with mode 'w', then write a different line with mode 'w' again — confirm the first one disappeared.

Activity 4

Now use mode 'a' twice in a row and confirm both new lines get added without erasing what came before.

10.3

try / except / else

Concept

Wrap risky code in try: — Python jumps to a matching except: block instead of crashing with a traceback.
else: holds code that should run only if the try block succeeded — it keeps the "what if it works" logic separate from the "what if it fails" logic.
FileNotFoundError is raised when open() can't find the file you asked for — exactly the kind of thing try/except is built for.

Key Terms

try / except / else: attempt code, handle its failure, and run success-only code separately
FileNotFoundError: raised when open() can't find the requested file

Exercise

Activity 5

Divide two numbers below. Try dividing by zero, and try typing text instead of a number — read the different message for each.

Activity 6

Try opening a file that doesn't exist in the widget below, then one that does — compare the two outcomes.

10.4

Failing Silently

Concept

pass in an except block tells Python to do nothing at all — the program moves on as if the error never happened.
Useful when a missing file isn't worth bothering the user about — like skipping a book that isn't there while still reporting word counts for the ones that are.
It's a judgment call: report errors that matter to your user, and fail silently on the ones that don't.

Key Terms

pass: a statement that does nothing — a deliberate no-op
failing silently: catching an error and continuing without telling the user

Exercise

Activity 7

Add a filename that doesn't exist in the simulated library below to the list, and confirm it gets skipped with no error message at all.

10.5

Storing Data with JSON

Concept

json.dump(data, f) writes a Python list or dictionary into a file as JSON text — simple structures translate over almost exactly as written.
json.load(f) reads that JSON text back into a real Python list or dictionary — a clean way to hand data between two runs of a program, or between two different programs entirely.
A common pattern: try to load remembered data, and if the file doesn't exist yet, fall back to asking for it fresh.

Key Terms

json.dump(): writes Python data into a file as JSON text
json.load(): reads JSON text from a file back into Python data

Exercise

Final challenge

Save a username with json.dump(), then load it back to confirm it survived. Then forget it and try loading again — what would a real program do at that point?