Context Managers

Thomas J. Kennedy

Contents:

1 Motivation

Python provides the with statement (construct). This allows the setup and teardown involved in using resources (e.g., files, sockets, and database connections) to be handled elsewhere.

This has two main benefits:

There is less boilerplate code.
It is impossible to forget to close/deallocate a resource.

The with construct can be used with many types of resources (e.g., files and network sockets). For this lecture we will focus purely on file IO.

2 Writing to a File

Suppose that we want to output the numbers one (1) through ninety-nine (99) to a file… along with whether the number is even or odd.

To generate such a file, one might write:

Python File IO - Basic

text_file = open("even_or_odd_file.txt", "w")

for number in range(1, 100):
    even_or_odd = "even" if number % 2 == 0 else "odd"
    text_file.write(f"{number} - {even_or_odd}\n")

Did you notice the missing fclose(text_file)?

Python File IO - Add close

text_file = open("even_or_odd_file.txt", "w")

for number in range(1, 100):
    even_or_odd = "even" if number % 2 == 0 else "odd"
    text_file.write(f"{number} - {even_or_odd}\n")

close(text_file)

One small with allows the file close operation can be handled automatically.

Python File IO - Using with

with open("even_or_odd_file.txt", "w") as text_file:
    for number in range(1, 100):
        even_or_odd = "even" if number % 2 == 0 else "odd"
        text_file.write(f"{number} - {even_or_odd}\n")

Note the indentation. The file is closed as soon as the indented block under with ends.

3 Writing to a Gzipped Text File

This also works for other types of files–including compressed files.

Python File IO - Using with and gzip

import gzip

with gzip.open("even_or_odd_file.txt.gz", "wt") as text_file:
    for number in range(1, 100):
        text_file.write(f"{number} - {even_or_odd}\n")

4 Reading from a File

As one might expect… the with context manager can be used when reading data from a file.

import sys
import pprint as pp


def main():
    input_filename = sys.argv[1]

    with open(input_filename, "r") as input_file:
        for line in input_file:
            line = line.strip()

            # Skip blank lines
            if not line:
                continue

            tokens = line.split()

            pp.pprint(tokens, indent=2)


if __name__ == "__main__":
    main()

This code snippet is one that I found myself writing quite a bit for debugging purposes. The code will:

Open a file

Read each line

Skip blank lines

Split a line into tokens (essentially words)

Use the Python Pretty Printer to output a readable list of tokens for each line