Adding a New Linter to the Tree¶
A linter is a yaml file with a .yml
extension. Depending on how the type of linter, there may
be python code alongside the definition, pointed to by the ‘payload’ attribute.
Here’s a trivial example:
no-eval.yml
Now no-eval.yml
gets passed into LintRoller.read()
.
Linter Types¶
There are four types of linters, though more may be added in the future.
- string - fails if substring is found
- regex - fails if regex matches
- external - fails if a python function returns a non-empty result list
- structured_log - fails if a mozlog logger emits any lint_error or lint_warning log messages
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. The signature for this python
function is lint(files, config, **kwargs)
, where files
is a list of
files to lint and config
is the linter definition defined in the .yml
file.
Structured log linters are much like external linters, but suitable
for cases where the linter code is using mozlog and emits
lint_error
or lint_warning
logging messages when the lint
fails. This is recommended for writing novel gecko-specific lints. In
this case the signature for lint functions is lint(files, config, logger,
**kwargs)
.
Linter Definition¶
Each .yml
file must have at least one linter defined in it. Here are the supported keys:
- 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)
- extensions - A list of file extensions to be considered (optional)
- setup - A function that sets up external dependencies (optional)
In addition to the above, some .yml
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)
For structured_log lints the following additional keys apply:
- logger - A StructuredLog object to use for logging. If not supplied one will be created (optional)
Example¶
Here is an example of an external linter that shells out to the python flake8 linter,
let’s call the file flake8_lint.py
:
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, config, **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_config(config, **res))
return results
Now here is the linter definition that would call it:
Notice the payload has two parts, delimited by ‘:’. The first is the module path, which
mozlint
will attempt to import (e.g, the name of a function to call). The second is
the object path within that module. It is up to consumers of mozlint
to ensure the
module is in sys.path
. Structured log linters use the same import mechanism.