Writing sustainable Python scripts

Vincent Bernat

Python is a great language to write a standalone script. Getting to the result can be a matter of a dozen to a few hundred lines of code and, moments later, you can forget about it and focus on your next task.

Six months later, a co-worker asks you why the script fails and you don’t have a clue: no documentation, hard-coded parameters, nothing logged during the execution and no sensible tests to figure out what may go wrong.

Turning a “quick-and-dirty” Python script into a sustainable version, which will be easy to use, understand and support by your co-workers and your future self, only takes some moderate effort. As an illustration, let’s start from the following script solving the infamous Fizz-Buzz test:

import sys
for n in range(int(sys.argv[1]), int(sys.argv[2])):
    if n % 3 == 0 and n % 5 == 0:
        print("fizzbuzz")
    elif n % 3 == 0:
        print("fizz")
    elif n % 5 == 0:
        print("buzz")
    else:
        print(n)

Documentation⚓︎

I find useful to write documentation before coding: it makes the design easier and it ensures I will not postpone this task indefinitely. The documentation can be embedded at the top of the script:

#!/usr/bin/env python3

"""Simple fizzbuzz generator.

This script prints out a sequence of numbers from a provided range
with the following restrictions:

 - if the number is divisble by 3, then print out "fizz",
 - if the number is divisible by 5, then print out "buzz",
 - if the number is divisible by 3 and 5, then print out "fizzbuzz".
"""

The first line is a short summary of the script purpose. The remaining paragraphs contain additional details on its action.

Command-line arguments⚓︎

The second task is to turn hard-coded parameters into documented and configurable values through command-line arguments, using the argparse module. In our example, we ask the user to specify a range and allow them to modify the modulo values for “fizz” and “buzz”.

import argparse
import sys


class CustomFormatter(argparse.RawDescriptionHelpFormatter,
                      argparse.ArgumentDefaultsHelpFormatter):
    pass


def parse_args(args=sys.argv[1:]):
    """Parse arguments."""
    parser = argparse.ArgumentParser(
        description=sys.modules[__name__].__doc__,
        formatter_class=CustomFormatter)

    g = parser.add_argument_group("fizzbuzz settings")
    g.add_argument("--fizz", metavar="N",
                   default=3,
                   type=int,
                   help="Modulo value for fizz")
    g.add_argument("--buzz", metavar="N",
                   default=5,
                   type=int,
                   help="Modulo value for buzz")

    parser.add_argument("start", type=int, help="Start value")
    parser.add_argument("end", type=int, help="End value")

    return parser.parse_args(args)


options = parse_args()
for n in range(options.start, options.end + 1):
    # ...

The added value of this modification is tremendous: parameters are now properly documented and are discoverable through the --help flag. Moreover, the documentation we wrote in the previous section is also displayed:

$ ./fizzbuzz.py --help
usage: fizzbuzz.py [-h] [--fizz N] [--buzz N] start end

Simple fizzbuzz generator.

This script prints out a sequence of numbers from a provided range
with the following restrictions:

 - if the number is divisble by 3, then print out "fizz",
 - if the number is divisible by 5, then print out "buzz",
 - if the number is divisible by 3 and 5, then print out "fizzbuzz".

positional arguments:
  start         Start value
  end           End value

optional arguments:
  -h, --help    show this help message and exit

fizzbuzz settings:
  --fizz N      Modulo value for fizz (default: 3)
  --buzz N      Modulo value for buzz (default: 5)

The argparse module is quite powerful. If you are not familiar with it, skimming through the documentation is helpful. I like to use the ability to define sub-commands and argument groups.

Logging⚓︎

A nice addition to a script is to display information during its execution. The logging module is a good fit for this purpose. First, we define the logger:

import logging
import logging.handlers
import os
import sys

logger = logging.getLogger(os.path.splitext(os.path.basename(sys.argv[0]))[0])

Then, we make its verbosity configurable: logger.debug() should output something only when a user runs our script with --debug and --silent should mute the logs unless an exceptional condition occurs. For this purpose, we add the following code in parse_args():

# In parse_args()
g = parser.add_mutually_exclusive_group()
g.add_argument("--debug", "-d", action="store_true",
               default=False,
               help="enable debugging")
g.add_argument("--silent", "-s", action="store_true",
               default=False,
               help="don't log to console")

We add this function to configure logging:

def setup_logging(options):
    """Configure logging."""
    root = logging.getLogger("")
    root.setLevel(logging.WARNING)
    logger.setLevel(options.debug and logging.DEBUG or logging.INFO)
    if not options.silent:
        ch = logging.StreamHandler()
        ch.setFormatter(logging.Formatter(
            "%(levelname)s[%(name)s] %(message)s"))
        root.addHandler(ch)

The main body of our script becomes this:

if __name__ == "__main__":
    options = parse_args()
    setup_logging(options)

    try:
        logger.debug("compute fizzbuzz from {} to {}".format(options.start,
                                                             options.end))
        for n in range(options.start, options.end + 1):
            # ...
    except Exception as e:
        logger.exception("%s", e)
        sys.exit(1)
    sys.exit(0)

If the script may run unattended—e.g. from a crontab, we can make it log to syslog:

def setup_logging(options):
    """Configure logging."""
    root = logging.getLogger("")
    root.setLevel(logging.WARNING)
    logger.setLevel(options.debug and logging.DEBUG or logging.INFO)
    if not options.silent:
        if not sys.stderr.isatty():
            facility = logging.handlers.SysLogHandler.LOG_DAEMON
            sh = logging.handlers.SysLogHandler(address='/dev/log',
                                                facility=facility)
            sh.setFormatter(logging.Formatter(
                "{0}[{1}]: %(message)s".format(
                    logger.name,
                    os.getpid())))
            root.addHandler(sh)
        else:
            ch = logging.StreamHandler()
            ch.setFormatter(logging.Formatter(
                "%(levelname)s[%(name)s] %(message)s"))
            root.addHandler(ch)

For this example, this is a lot of code just to use logger.debug() once, but in a real script, this will come handy to help users understand how the task is completed.

$ ./fizzbuzz.py --debug 1 3
DEBUG[fizzbuzz] compute fizzbuzz from 1 to 3
1
2
fizz

Tests⚓︎

Unit tests are very useful to ensure an application behaves as intended. It is not common to use them in scripts, but writing a few of them greatly improves their reliability. Let’s turn the code in the inner “for” loop into a function with some interactive examples of use to its documentation:

def fizzbuzz(n, fizz, buzz):
    """Compute fizzbuzz nth item given modulo values for fizz and buzz.

    >>> fizzbuzz(5, fizz=3, buzz=5)
    'buzz'
    >>> fizzbuzz(3, fizz=3, buzz=5)
    'fizz'
    >>> fizzbuzz(15, fizz=3, buzz=5)
    'fizzbuzz'
    >>> fizzbuzz(4, fizz=3, buzz=5)
    4
    >>> fizzbuzz(4, fizz=4, buzz=6)
    'fizz'

    """
    if n % fizz == 0 and n % buzz == 0:
        return "fizzbuzz"
    if n % fizz == 0:
        return "fizz"
    if n % buzz == 0:
        return "buzz"
    return n

pytest can ensure the results are correct:1

$ python3 -m pytest -v --doctest-modules ./fizzbuzz.py
============================ test session starts =============================
platform linux -- Python 3.7.4, pytest-3.10.1, py-1.8.0, pluggy-0.8.0 -- /usr/bin/python3
cachedir: .pytest_cache
rootdir: /home/bernat/code/perso/python-script, inifile:
plugins: xdist-1.26.1, timeout-1.3.3, forked-1.0.2, cov-2.6.0
collected 1 item

fizzbuzz.py::fizzbuzz.fizzbuzz PASSED                                  [100%]

========================== 1 passed in 0.05 seconds ==========================

In case of an error, pytest displays a message describing the location and the nature of the failure:

$ python3 -m pytest -v --doctest-modules ./fizzbuzz.py -k fizzbuzz.fizzbuzz
============================ test session starts =============================
platform linux -- Python 3.7.4, pytest-3.10.1, py-1.8.0, pluggy-0.8.0 -- /usr/bin/python3
cachedir: .pytest_cache
rootdir: /home/bernat/code/perso/python-script, inifile:
plugins: xdist-1.26.1, timeout-1.3.3, forked-1.0.2, cov-2.6.0
collected 1 item

fizzbuzz.py::fizzbuzz.fizzbuzz FAILED                                  [100%]

================================== FAILURES ==================================
________________________ [doctest] fizzbuzz.fizzbuzz _________________________
100
101     >>> fizzbuzz(5, fizz=3, buzz=5)
102     'buzz'
103     >>> fizzbuzz(3, fizz=3, buzz=5)
104     'fizz'
105     >>> fizzbuzz(15, fizz=3, buzz=5)
106     'fizzbuzz'
107     >>> fizzbuzz(4, fizz=3, buzz=5)
108     4
109     >>> fizzbuzz(4, fizz=4, buzz=6)
Expected:
    fizz
Got:
    4

/home/bernat/code/perso/python-script/fizzbuzz.py:109: DocTestFailure
========================== 1 failed in 0.02 seconds ==========================

We can also write unit tests as code. Let’s suppose we want to test the following function:

def main(options):
    """Compute a fizzbuzz set of strings and return them as an array."""
    logger.debug("compute fizzbuzz from {} to {}".format(options.start,
                                                         options.end))
    return [str(fizzbuzz(i, options.fizz, options.buzz))
            for i in range(options.start, options.end+1)]

At the end of the script,2 we add the following unit tests, leveraging pytest’s parametrized test functions:

# Unit tests
import pytest                   # noqa: E402
import shlex                    # noqa: E402


@pytest.mark.parametrize("args, expected", [
    ("0 0", ["fizzbuzz"]),
    ("3 5", ["fizz", "4", "buzz"]),
    ("9 12", ["fizz", "buzz", "11", "fizz"]),
    ("14 17", ["14", "fizzbuzz", "16", "17"]),
    ("14 17 --fizz=2", ["fizz", "buzz", "fizz", "17"]),
    ("17 20 --buzz=10", ["17", "fizz", "19", "buzz"]),
])
def test_main(args, expected):
    options = parse_args(shlex.split(args))
    options.debug = True
    options.silent = True
    setup_logging(options)
    assert main(options) == expected

The test function runs once for each of the provided parameters. The args part is used as input for the parse_args() function to get the appropriate options we need to pass to the main() function. The expected part is compared to the result of the main() function. When everything works as expected, pytest says:

python3 -m pytest -v --doctest-modules ./fizzbuzz.py
============================ test session starts =============================
platform linux -- Python 3.7.4, pytest-3.10.1, py-1.8.0, pluggy-0.8.0 -- /usr/bin/python3
cachedir: .pytest_cache
rootdir: /home/bernat/code/perso/python-script, inifile:
plugins: xdist-1.26.1, timeout-1.3.3, forked-1.0.2, cov-2.6.0
collected 7 items

fizzbuzz.py::fizzbuzz.fizzbuzz PASSED                                  [ 14%]
fizzbuzz.py::test_main[0 0-expected0] PASSED                           [ 28%]
fizzbuzz.py::test_main[3 5-expected1] PASSED                           [ 42%]
fizzbuzz.py::test_main[9 12-expected2] PASSED                          [ 57%]
fizzbuzz.py::test_main[14 17-expected3] PASSED                         [ 71%]
fizzbuzz.py::test_main[14 17 --fizz=2-expected4] PASSED                [ 85%]
fizzbuzz.py::test_main[17 20 --buzz=10-expected5] PASSED               [100%]

========================== 7 passed in 0.03 seconds ==========================

When an error occurs, pytest provides a useful assessment of the situation:

$ python3 -m pytest -v --doctest-modules ./fizzbuzz.py
[...]
================================== FAILURES ==================================
__________________________ test_main[0 0-expected0] __________________________

args = '0 0', expected = ['0']

    @pytest.mark.parametrize("args, expected", [
        ("0 0", ["0"]),
        ("3 5", ["fizz", "4", "buzz"]),
        ("9 12", ["fizz", "buzz", "11", "fizz"]),
        ("14 17", ["14", "fizzbuzz", "16", "17"]),
        ("14 17 --fizz=2", ["fizz", "buzz", "fizz", "17"]),
        ("17 20 --buzz=10", ["17", "fizz", "19", "buzz"]),
    ])
    def test_main(args, expected):
        options = parse_args(shlex.split(args))
        options.debug = True
        options.silent = True
        setup_logging(options)
>       assert main(options) == expected
E       AssertionError: assert ['fizzbuzz'] == ['0']
E         At index 0 diff: 'fizzbuzz' != '0'
E         Full diff:
E         - ['fizzbuzz']
E         + ['0']

fizzbuzz.py:160: AssertionError
----------------------------- Captured log call ------------------------------
fizzbuzz.py                125 DEBUG    compute fizzbuzz from 0 to 0
===================== 1 failed, 6 passed in 0.05 seconds =====================

The call to logger.debug() is included in the output. This is another good reason to use the logging feature! If you want to know more about the wonderful features of pytest, have a look at “Testing network software with pytest and Linux namespaces.”


To sum up, enhancing a Python script to make it more sustainable can be done in four steps:

  1. add documentation at the top,
  2. use the argparse module to document the different parameters,
  3. use the logging module to log details about progress, and
  4. add some unit tests.

You can find the complete example on GitHub and use it as a template!

Update (2019.06)

There are some interesting threads about this article on Lobsters and Reddit. While the addition of documentation and command-line arguments seems to be well-received, logs and tests are sometimes reported as too verbose. Dan Connolly write “Practical production python scripts” as an answer to this post.


  1. This requires the script name to end with .py. I dislike appending an extension to a script name: the language is a technical detail that shouldn’t be exposed to the user. However, it seems to be the easiest way to let test runners, like pytest, discover the enclosed tests. ↩︎

  2. Because the script ends with a call to sys.exit(), when invoked normally, the additional code for tests will not be executed. This ensures pytest is not needed to run the script. ↩︎

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