Files, exceptions and context managers

Reading and writing files safely, and handling errors without hiding bugs.

12 min read · Lesson 6 of 6 · Pro

Reading a file

Python 3

with open("marks.txt", "r") as f:
    for line in f:
        print(line.rstrip())

Three things are happening. open returns a file object. for line in f reads it one line at a time, so a 2 GB file uses almost no memory. line.rstrip() removes the trailing newline that readline leaves on.

The with is the important part. When the block ends - normally, or because an exception was raised - the file is closed automatically. Compare:

Python 3

f = open("marks.txt", "r")
data = f.read()
f.close()          # never runs if f.read() raises

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