all Python 3Files & I/Ofunction

readline()

reading

Read a single line including its trailing '\n'; at end of file it returns an empty string, which is how EOF is detected.

Syntax

f.readline(size=-1)

A blank line in the middle of a file comes back as '\n', which is truthy; only real end of file yields '', so a while line := f.readline() loop will not stop early on an empty line. If the file does not end with a newline, the last line arrives without '\n', which is why rstrip('\n') is safer than slicing off [:-1]. To walk the whole file, for line in f does the same job lazily and without a manual exit condition.

Examples

with open('demo.txt', 'w') as f:
    f.write('line1\nline2\n')
with open('demo.txt') as f:
    first = f.readline()
    second = f.readline()
print(repr(first))   # → 'line1\n'
print(repr(second))  # → 'line2\n'

See also

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