Files and errors

Why does the game crash for new players?

• A game reads the player's text size from settings.txt. It works on the developer's computer. On a new player's first run there is no file yet, and the game stops with FileNotFoundError.
• Programs meet a messy world: missing files, typing mistakes, bad data. This lesson shows how to read and write files, and how to plan for the errors you can expect.

Reading a file

• with open("scores.txt") as f: opens the file and closes it for you when the block ends, even if an error happens inside.
• Looping for line in f: gives one line at a time, with its "\n" on the end. Call line.strip() before you split or convert it.

total = 0
with open("scores.txt") as f:          # Ana,72  Ben,55  Cai,90
    for line in f:
        name, mark = line.strip().split(",")
        total += int(mark)
print(total)   # 217

Writing a file

• open(path, "w") empties the file first, then writes. open(path, "a") appends to the end and keeps what was there.
• f.write() takes a string and adds no line break, so write f.write(name + "\n") or f.write(f"{name},{mark}\n").

Errors have names

• When something goes wrong, Python raises an exception 异常, and each kind has a name: FileNotFoundError (no such file), ValueError (int("x")), KeyError (missing dictionary key), TypeError ("a" + 1), ZeroDivisionError (1 / 0).
• The name is the most useful line of an error message. Read it first, then the line number above it.

try and except

• Put the risky lines in a try: block and the plan for one named error in an except ValueError: block. If that error happens, Python jumps straight to the except block.
• Lines in the try block after the failing line are skipped. Code after the whole try … except runs as normal.

try:
    print("A")
    n = int("seven")     # ValueError here
    print("B")           # skipped
except ValueError:
    print("C")
print("D")               # prints A, C, D

Ask again until the input is good

• Put try … except inside a while True: loop. A good value reaches return, which leaves the loop and the function together. A bad value is caught, and the loop asks again.
• The user who types x and then 4 sees one message and the program carries on with 4.

def read_count():
    while True:
        try:
            return int(input("How many? "))
        except ValueError:
            print("Whole numbers only.")

One outcome, several roads

• Robust code tries the normal path, names each failure it expects and gives each one a sensible fallback. The caller always gets a usable result.
• Catch only what you have a plan for. A typing mistake in your own code should still crash loudly so you can find it.

Marks that slip away

You've got it