Files and errors
Introduced| English | Meaning |
|---|---|
| exception/ekˈsepʃn/ | An error signal raised while a program runs, which stops it unless the code handles it. |
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 withFileNotFoundError. - 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. Callline.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
A file holds three lines: Ana,72, Ben,55, Cai,90. The loop adds int(line.strip().split(",")[1]) for every line. What is the total?
Difficulty 1/5 · 100 coins + a small bonus on first completion today
$72 + 55 + 90 = 217$. strip() removes the "\n", split(",") separates the name from the mark, and [1] takes the mark.
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 writef.write(name + "\n")orf.write(f"{name},{mark}\n").
Match each mode to what it does.
Difficulty 2/5 · 140 coins + a small bonus on first completion today
"r" is the default. Choosing "w" for a log you meant to extend wipes it every run.
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.
Match each line to the exception it raises.
Difficulty 2/5 · 140 coins + a small bonus on first completion today
A ValueError is the right type with a bad value; a TypeError is the wrong type for the operation.
try and except
- Put the risky lines in a
try:block and the plan for one named error in anexcept ValueError:block. If that error happens, Python jumps straight to theexceptblock. - Lines in the
tryblock after the failing line are skipped. Code after the wholetry … exceptruns as normal.
try:
print("A")
n = int("seven") # ValueError here
print("B") # skipped
except ValueError:
print("C")
print("D") # prints A, C, D
What does the A/B/C/D program print? try: prints A, runs int("seven"), prints B; except ValueError: prints C; then D is printed after the block.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
A prints, then int("seven") raises. The rest of the try block, including B, is skipped; C runs; D runs because it is after the whole block.
Ask again until the input is good
- Put
try … exceptinside awhile True:loop. A good value reachesreturn, which leaves the loop and the function together. A bad value is caught, and the loop asks again. - The user who types
xand then4sees one message and the program carries on with4.
def read_count():
while True:
try:
return int(input("How many? "))
except ValueError:
print("Whole numbers only.")
A user types x, then 2.5, then 4 into read_count(). How many times is "Whole numbers only." printed?
Difficulty 2/5 · 140 coins + a small bonus on first completion today
int("x") and int("2.5") both raise ValueError, so the message appears twice. int("4") works and return ends the loop.
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.
- Read the text size from
settings.txt; use 12 if the file is missing or does not hold a whole number. try:open the file andsize = int(f.read().strip()).except FileNotFoundError: size = 12andexcept ValueError: size = 12.- A file holding
16gives 16; no file gives 12; a file holdingbiggives 12. New players no longer see a crash.
Put the lines of the settings reader in order.
Difficulty 3/5 · 200 coins + a small bonus on first completion today
The risky open-and-convert goes in try. One except can name two errors in a tuple when both get the same fallback.
Marks that slip away
- A bare
except:. It catches every error, including your own bugs, and hides them. Name the error you expect. - A huge
tryblock. Keep only the risky lines inside, so you know which line theexceptis answering. "w"when you meant"a"."w"wipes the old contents first.- Forgetting
strip()and"\n". Lines read in keep their line break; lines written out need one.
A bare except: is the safest choice, because it stops the program from ever crashing.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
It also swallows your own bugs, such as a misspelt variable name, so they fail silently. Name the error you expect and can answer.
You've got it
with open(...)closes the file for you."r"reads,"w"empties and writes,"a"appends; strip lines you read, add"\n"to lines you write.- An exception has a name.
try … except Name:catches that one error; the rest of thetryblock is skipped. - Catch only the errors you can answer, give each a fallback, and let unexpected errors crash so you can fix them.