Adding a New Linter to the Tree

A linter is a python file with a .lint extension and a global dict called LINTER. Depending on how complex it is, there may or may not be any actual python code alongside the LINTER definition.

Here’s a trivial example:

no-eval.lint

LINTER = {
    'name': 'EvalLinter',
    'description': "Ensures the string 'eval' doesn't show up."
    'include': "**/*.js",
    'type': 'string',
    'payload': 'eval',
}

Now no-eval.lint gets passed into LintRoller.read().

Linter Types

There are three types of linters, though more may be added in the future.

  1. string - fails if substring is found
  2. regex - fails if regex matches
  3. external - fails if a python function returns a non-empty result list

As seen from the example above, string and regex linters are very easy to create, but they should be avoided if possible. It is much better to use a context aware linter for the language you are trying to lint. For example, use eslint to lint JavaScript files, use flake8 to lint python files, etc.

Which brings us to the third and most interesting type of linter, external. External linters call an arbitrary python function which is responsible for not only running the linter, but ensuring the results are structured properly. For example, an external type could shell out to a 3rd party linter, collect the output and format it into a list of ResultContainer objects.

LINTER Definition

Each .lint file must have a variable called LINTER which is a dict containing metadata about the linter. Here are the supported keys:

  • name - The name of the linter (required)
  • description - A brief description of the linter’s purpose (required)
  • type - One of ‘string’, ‘regex’ or ‘external’ (required)
  • payload - The actual linting logic, depends on the type (required)
  • include - A list of glob patterns that must be matched (optional)
  • exclude - A list of glob patterns that must not be matched (optional)
  • setup - A function that sets up external dependencies (optional)

In addition to the above, some .lint files correspond to a single lint rule. For these, the following additional keys may be specified:

  • message - A string to print on infraction (optional)
  • hint - A string with a clue on how to fix the infraction (optional)
  • rule - An id string for the lint rule (optional)
  • level - The severity of the infraction, either ‘error’ or ‘warning’ (optional)

Example

Here is an example of an external linter that shells out to the python flake8 linter:

import json
import os
import subprocess
from collections import defaultdict

from mozlint import result


FLAKE8_NOT_FOUND = """
Could not find flake8! Install flake8 and try again.
""".strip()


def lint(files, **lintargs):
    import which

    binary = os.environ.get('FLAKE8')
    if not binary:
        try:
            binary = which.which('flake8')
        except which.WhichError:
            print(FLAKE8_NOT_FOUND)
            return 1

    # Flake8 allows passing in a custom format string. We use
    # this to help mold the default flake8 format into what
    # mozlint's ResultContainer object expects.
    cmdargs = [
        binary,
        '--format',
        '{"path":"%(path)s","lineno":%(row)s,"column":%(col)s,"rule":"%(code)s","message":"%(text)s"}',
    ] + files

    proc = subprocess.Popen(cmdargs, stdout=subprocess.PIPE, env=os.environ)
    output = proc.communicate()[0]

    # all passed
    if not output:
        return []

    results = []
    for line in output.splitlines():
        # res is a dict of the form specified by --format above
        res = json.loads(line)

        # parse level out of the id string
        if 'code' in res and res['code'].startswith('W'):
            res['level'] = 'warning'

        # result.from_linter is a convenience method that
        # creates a ResultContainer using a LINTER definition
        # to populate some defaults.
        results.append(result.from_linter(LINTER, **res))

    return results


LINTER = {
    'name': "flake8",
    'description': "Python linter",
    'include': ['**/*.py'],
    'type': 'external',
    'payload': lint,
}