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Basic

Reading line by line

Text files are lines. Reading them one at a time keeps memory flat and lets you stop early, which a whole-file read cannot do.

What you will see

2

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

Walk lines and count them.

1

Split three rows.

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

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

What it prints
3
2

Lengths with and without the ending.

line = "hello\n" print(len(line), len(line.strip()))

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

What it prints
6 5
3

Rows with a trailing ending.

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

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

What it prints
3
4

Blank lines filtered.

lines = ["a", "", "b"] kept = [x for x in lines if x] print(len(kept))

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

What it prints
2

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 file iterated gives one line at a time. Each line still carries its ending, so strip before comparing.
1 open the file
↓
2 iterate lines
↓
3 strip each one
↓
4 stop when done
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.

text = "a\nb\nc" sets nothing on screen
print(len(text.splitlines())) 3
Why this works
Streaming keeps memory flat

Iterating a file holds one line at a time. Reading it all holds the whole file, which is the difference at a million lines.

The ending is part of the line

A line includes its newline until you strip it. Comparing without stripping fails on every row.

Examples

Loop over lines

with open("notes.txt") as f:
    for line in f:
        print(line.strip())

Iterating a file yields one line at a time. Memory stays small whatever the size.

Lines into a list

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

splitlines cuts on any line ending. The count is the number of rows.

Strip the ending

line = "hello\n"
print(len(line), len(line.strip()))

Every line carries its ending. Strip removes it before you compare.

Stop at the first hit

for line in ["x", "find", "y"]:
    if line == "find":
        print(line)
        break

A loop can stop early. A whole-file read already paid for every byte.

Count the rows

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

A trailing ending does not add an empty row. The count stays three.

Skip blank lines

lines = ["a", "", "b"]
kept = [x for x in lines if x]
print(len(kept))

A blank line is a real line with nothing in it. Filter it when you want rows only.

One pass, two counts

lines = ["ok", "bad", "ok"]
bad = sum(1 for x in lines if x == "bad")
print(len(lines), bad)

Count while you read. Storing the lines to count later costs memory.

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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