Basic

Writing a file

Writing is the same door in reverse. The mode you open with decides whether you replace the file or add to it, and that choice is easy to get wrong.

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

Write and check the bytes.

Length of joined rows.

rows = ["a", "b", "c"] out = "\n".join(rows) + "\n" print(len(out))

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

What it prints

Rows in that text.

text = "a\nb\n" print(len(text.splitlines()))

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

What it prints

Characters written.

out = "hello" print(len(out))

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

What it prints

Two writes appended.

out = "a" + "b" print(out)

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

Opening with w replaces the file and a adds to it. The endings you write are the endings you choose.

1 pick a mode

↓

2 build the text

↓

3 write it once

↓

4 read it back

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

out = "\n".join(rows) + "\n" sets nothing on screen

print(len(out)) 7

Why this works

Mode is a choice, not a default

w truncates and a appends. Opening with the wrong one silently destroys or grows a file, so state the mode every time.

Write once, not per line

Building the whole text and writing it in one call is simpler and lets one failure leave a whole file missing rather than half a file.

Examples

Write replaces

with open("out.txt", "w") as f:
    f.write("first")
print("done")

Mode w truncates the file first. Anything that was there is gone.

Append adds

with open("out.txt", "a") as f:
    f.write("more")
print("done")

Mode a keeps what exists and adds after it. Use it for logs.

Newlines are yours

with open("out.txt", "w") as f:
    f.write("a\nb\n")
print("done")

write does not add an ending. Put it in the string or lines run together.

Write many lines

rows = ["a", "b", "c"]
out = "\n".join(rows) + "\n"
print(len(out))

Join then write once. A loop of writes does the same with more calls.

A file that is new

with open("new.txt", "w") as f:
    f.write("hi")
print("made")

Mode w creates the file when it does not exist. It also empties it when it does.

Read back what you wrote

text = "a\nb\n"
print(len(text), len(text.splitlines()))

The bytes you wrote are the bytes you read. Counting them checks the write.

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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