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Building Command Line Tools with Python


Python Makes CLIs Easier!


11 Jul 2017


WRITING PROGRAMMING PYTHON TOOLS

Completed project: GitHub

Unix has a command touch which is commonly used to create files. Let’s create a Python program with similar functionality that can be called as if it were a built-in Unix command.


pytouch

Here begins the story of our CLI tool pytouch. For simplicity, let’s let pytouch only take one argument filename.

When we enter pytouch sample.txt, we should expect to create a new file called sample.txt. If such a file already exists, we’ll truncate it.

Installation method

We’ll Python’s in-built setuptools to allow our tool to be installed with pip, the standard package management tool for Python.

Python version compatibility

When we distribute a command line tool like this, we only know that our user has installed Python; we don’t know what version of Python is installed or what packages are available to us. We’ll use tox to ensure that our tool can run on several different machines.

Directory structure

Let’s create a directory called pytouch-tool which will house all of our code. Create files until your folder structure looks like:

pytouch-tool
+-- pytouch
|   +-- __init__.py
|   +-- pytouch.py
+-- setup.py

Inside pytouch/pytouch.py

This file will contain the main method of our tool. It’ll handle parsing the one filename argument and creating that file in the current working directory. Add this to pytouch/pytouch.py:

import sys

def main():
    # sys.argv[0] is the path to the python interpreter
    # sys.argv[1] should contain the name of the file we want to create
    if len(sys.argv) != 3:
        raise ValueError("Please provide the name of the file to create.")

    filename = sys.argv[1]

    # 'w+' erases the file if it exists and creates it if the file doesn't exist
    with open(filename, 'w+'):
        pass

I hope this code is pretty self explanatory.

Inside pytouch/__init__.py

We’ll keep this file blank. This is just Python’s way of knowing that it should treat the containing pytouch folder as a Python package. It’s important to treat the pytouch directory as a package as we’ll see next.

Inside setup.py

We’re going to fill out this file so that we can use pip to simply install this package. Add the following to setup.py:

from setuptools import setup

setup(
    # The name of our pip package
    name='pytouch',
    # The Python packages in this project
    packages=[
        # This is the `pytouch` folder that contains __init__.py and pytouch.py
        'pytouch',
    ],
    version="0.0.1",
    entry_points={
        'console_scripts': [
            # We use this line to map our `main()` method in pytouch.py
            # to a shell command `pytouch`
            'pytouch = pytouch.pytouch:main',
        ],
    },
)

Learn more about the setuptools package here.


Installation

Go to the root of our project, the pytouch-tool directory. pip documentation tells us that we can run pip install . to install our package. You should see output like

Processing /path/to/pytouch-tools
Installing collected packages: pytouch
  Running setup.py install for pytouch ... done
Successfully installed pytouch-0.0.1

Now, if you run pip freeze | grep pytouch, you should see our tool!

$ pip freeze | grep pytouch
...
pytouch==0.0.1
...

Running pytouch

Take ‘er for a spin!

$ pytouch sample.txt
$ ls
...
sample.txt
...

Un-installation

Run pip uninstall pytouch and our tool is removed.


Further Steps

In a soon to come future post, we’ll dive into testing tools that will ensure our project can work with any version of Python. 2017/07/16 Edit: See Testing Python Packages.


© Pramod Kotipalli 2017

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