A better way to deal with filesystem-required cli

[HansBug]

When we need to use click to build CLI applications, it is often inevitable to read and write external files. In fact, there is special support for such a common situation in click, which is called isolated file system.

This is a demo of cli implement (provided by click offical documentation).

import click

@click.command()
@click.argument('f', type=click.File())
def cat(f):
   click.echo(f.read())

This is the unittest code for the cli code above.

from click.testing import CliRunner
from cat import cat

def test_cat():
   runner = CliRunner()
   with runner.isolated_filesystem():
      with open('hello.txt', 'w') as f:
          f.write('Hello World!')

      result = runner.invoke(cat, ['hello.txt'])
      assert result.exit_code == 0
      assert result.output == 'Hello World!\n'

Its principle is to create a temporary path in the context of the with block and temporarily change the current working directory to the temporary diretory. Therefore, the running test involving file IO can be realized, and the running files will not appear under the current code path to ensure the cleanness of the repository.

Similarly, when we need to test the other functions which involve file IO as well, we can manually build a tempfile-based test block like the code below.

import tempfile


def iofunc(ifile, ofile):  # just a copy
    with open(ifile, 'rb') as ifd:
        with open(ofile, 'wb') as ofd:
            ofd.write(ifd.read())


def test_iofunc():
    with tempfile.NamedTemporaryFile() as itf:
        with tempfile.NamedTemporaryFile() as otf:
            with open(itf.name, 'wb') as ifd:
                ifd.write(b'1234567')

            iofunc(itf.name, otf.name)

            with open(otf.name, 'rb') as ofd:
                _all_bytes = ofd.read()
                assert b'1234567' in _all_bytes

References:

• File System Isolation
• tempfile — Generate temporary files and directories