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Failures that matter

Real files fail in the middle. A half-written file is worse than no file, so the plan has to say what a failure leaves behind.

What you will see

About 2 min of reading

Quick check

Before this new lesson, one small question about the lesson you just finished. Recall it, do not look it up.

Try It

Plan a safe write.

Rows kept.

rows = ["a", "b", "c"] half = rows[:1] print(len(half))

Before you run it, say out loud what it will print. Then open the answer.

What it prints

Rename runs.

import os os.replace("a", "a") print("moved")

Before you run it, say out loud what it will print. Then open the answer.

What it prints

moved

Bytes written.

out = "data" print(len(out))

Before you run it, say out loud what it will print. Then open the answer.

What it prints

Backup grew.

rows = ["a"] backup = list(rows) backup.append("b") print(len(backup))

Before you run it, say out loud what it will print. Then open the answer.

What it prints

Put it in order

Before you write anything, put these lines back in the right order. Click a line to move it up one step. This support step is easier than writing code from scratch, and it is here to make the last example stick.

Data Flow

A partial file is worse than none. Write to a temp name, then rename, so the switch is one atomic step.

1 build the new text

↓

2 write a temp file

↓

3 rename into place

↓

4 report a failure

Line by line

Read the first example one line at a time. A line that prints shows what appears on screen; the rest set things up quietly.

rows = ["a", "b", "c"] sets nothing on screen

half = rows[:1] sets nothing on screen

print(len(half)) 1

Why this works

Atomic means one switch

A rename is the point where old becomes new. Everything before it can fail with no visible change.

Two states are valid

While a write runs, both the old file and the new one are readable. Readers must accept either, not assume one.

Examples

Half a file

rows = ["a", "b", "c"]
half = rows[:1]
print(len(half))

Writing row by row can stop in the middle. The reader then sees a truncated file.

Write to a temp name

import os
os.replace("a", "a")
print("moved")

Write a temp file, then rename it into place. A rename is the switch point.

Flush is not enough

out = "data"
print(len(out))

A flush pushes bytes out. The file is only complete once the name is swapped in.

Read the same rules

import os
print(os.path.exists("a"))

A reader must handle the old file and the new one. Both are valid states at once.

Keep the old one

rows = ["a"]
backup = list(rows)
backup.append("b")
print(len(backup))

Write the new version before removing the old. A crash then leaves a working file.

Say what failed

import sys
print("out.txt: cannot write", file=sys.stderr)

Name the path and the reason. A silent partial file is a wrong answer.

Words this lesson uses

Each one is defined here on this site. Open the entry for the full definition and the other words it points at.

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