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Run coverage and linting reports on diffs

Project description

Automatically find diff lines that need test coverage. Also finds diff lines that have violations (according to tools such as pycodestyle, pyflakes, flake8, or pylint). This is used as a code quality metric during code reviews.


Diff coverage is the percentage of new or modified lines that are covered by tests. This provides a clear and achievable standard for code review: If you touch a line of code, that line should be covered. Code coverage is every developer’s responsibility!

The diff-cover command line tool compares an XML coverage report with the output of git diff. It then reports coverage information for lines in the diff.

Currently, diff-cover requires that:

  • You are using git for version control.

  • Your test runner generates coverage reports in Cobertura, Clover or JaCoCo XML format.

Supported XML coverage reports can be generated with many coverage tools, including:

diff-cover is designed to be extended. If you are interested in adding support for other version control systems or coverage report formats, see below for information on how to contribute!


To install the latest release:

pip install diff_cover

To install the development version:

git clone
cd diff-cover
python install

Getting Started

  1. Set the current working directory to a git repository.

  2. Run your test suite under coverage and generate a [Cobertura, Clover or JaCoCo] XML report. For example, using pytest-cov:

pytest --cov --cov-report=xml

This will create a coverage.xml file in the current working directory.

NOTE: If you are using a different coverage generator, you will need to use different commands to generate the coverage XML report.

  1. Run diff-cover:

diff-cover coverage.xml

This will compare the current git branch to origin/main and print the diff coverage report to the console.

You can also generate an HTML, JSON or Markdown version of the report:

diff-cover coverage.xml --html-report report.html
diff-cover coverage.xml --json-report report.json
diff-cover coverage.xml --markdown-report

Multiple XML Coverage Reports

In the case that one has multiple xml reports form multiple test suites, you can get a combined coverage report (a line is counted as covered if it is covered in ANY of the xml reports) by running diff-cover with multiple coverage reports as arguments. You may specify any arbitrary number of coverage reports:

diff-cover coverage1.xml coverage2.xml

Quality Coverage

You can use diff-cover to see quality reports on the diff as well by running diff-quality.

diff-quality --violations=<tool>

Where tool is the quality checker to use. Currently pycodestyle, pyflakes, flake8, pylint, checkstyle, checkstylexml are supported, but more checkers can (and should!) be supported. See the section “Adding diff-quality` Support for a New Quality Checker”.

NOTE: There’s no way to run findbugs from diff-quality as it operating over the generated java bytecode and should be integrated into the build framework.

Like diff-cover, HTML, JSON or Markdown reports can be generated with

diff-quality --violations=<tool> --html-report report.html
diff-quality --violations=<tool> --json-report report.json
diff-quality --violations=<tool> --markdown-report

If you have already generated a report using pycodestyle, pyflakes, flake8, pylint, checkstyle, checkstylexml, or findbugs you can pass the report to diff-quality. This is more efficient than letting diff-quality re-run pycodestyle, pyflakes, flake8, pylint, checkstyle, or checkstylexml.

# For pylint < 1.0
pylint -f parseable > pylint_report.txt

# For pylint >= 1.0
pylint --msg-template="{path}:{line}: [{msg_id}({symbol}), {obj}] {msg}" > pylint_report.txt

# Use the generated pylint report when running diff-quality
diff-quality --violations=pylint pylint_report.txt

# Use a generated pycodestyle report when running diff-quality.
pycodestyle > pycodestyle_report.txt
diff-quality --violations=pycodestyle pycodestyle_report.txt

Note that you must use the -f parseable option to generate the pylint report for pylint versions less than 1.0 and the --msg-template option for versions >= 1.0.

diff-quality will also accept multiple pycodestyle, pyflakes, flake8, or pylint reports:

diff-quality --violations=pylint report_1.txt report_2.txt

If you need to pass in additional options you can with the options flag

diff-quality --violations=pycodestyle --options="--exclude='*/migrations*' --statistics" pycodestyle_report.txt

Compare Branch

By default, diff-cover compares the current branch to origin/main. To specify a different compare branch:

diff-cover coverage.xml --compare-branch=origin/release

Fail Under

To have diff-cover and diff-quality return a non zero status code if the report quality/coverage percentage is below a certain threshold specify the fail-under parameter

diff-cover coverage.xml --fail-under=80
diff-quality --violations=pycodestyle --fail-under=80

The above will return a non zero status if the coverage or quality score was below 80%.

Exclude/Include paths

Explicit exclusion of paths is possible for both diff-cover and diff-quality, while inclusion is only supported for diff-quality (since 5.1.0).

The exclude option works with fnmatch, include with glob. Both options can consume multiple values. Include options should be wrapped in double quotes to prevent shell globbing. Also they should be relative to the current git directory.

diff-cover coverage.xml --exclude
diff-quality --violations=pycodestyle --exclude

diff-quality --violations=pycodestyle --include project/foo/**

The following is executed for every changed file:

  1. check if any include pattern was specified

  2. if yes, check if the changed file is part of at least one include pattern

  3. check if the file is part of any exclude pattern

Ignore/Include based on file status in git

Both diff-cover and diff-quality allow users to ignore and include files based on the git status: staged, unstaged, untracked:

  • --ignore-staged: ignore all staged files (by default include them)

  • --ignore-unstaged: ignore all unstaged files (by default include them)

  • --include-untracked: include all untracked files (by default ignore them)

Quiet mode

Both diff-cover and diff-quality support a quiet mode which is disable by default. It can be enabled by using the -q/--quiet flag:

diff-cover coverage.xml -q
diff-quality --violations=pycodestyle -q

If enabled, the tool will only print errors and failures but no information or warning messages.

Configuration files

Both tools allow users to specify the options in a configuration file with –config-file/-c:

diff-cover coverage.xml --config-file myconfig.toml
diff-quality --violations=pycodestyle --config-file myconfig.toml

Currently, only TOML files are supported. Please note, that only non-mandatory options are supported. If an option is specified in the configuration file and over the command line, the value of the command line is used.

TOML configuration

The parser will only react to configuration files ending with .toml. To use it, install diff-cover with the extra requirement toml.

The option names are the same as on the command line, but all dashes should be underscores. If an option can be specified multiple times, the configuration value should be specified as a list.

compare_branch = <s>"origin/feature"</s>
quiet = true

compare_branch = <s>"origin/feature"</s>
ignore_staged = true


Issue: diff-cover always reports: “No lines with coverage information in this diff.”

Solution: diff-cover matches source files in the coverage XML report with source files in the git diff. For this reason, it’s important that the relative paths to the files match. If you are using to generate the coverage XML report, then make sure you run diff-cover from the same working directory.

Issue: GitDiffTool._execute() raises the error:

fatal: ambiguous argument 'origin/main...HEAD': unknown revision or path not in the working tree.

This is known to occur when running diff-cover in Travis CI

Solution: Fetch the remote main branch before running diff-cover:

git fetch origin master:refs/remotes/origin/main

Issue: diff-quality reports “diff_cover.violations_reporter.QualityReporterError: No config file found, using default configuration”

Solution: Your project needs a pylintrc file. Provide this file (it can be empty) and diff-quality should run without issue.

Issue: diff-quality reports “Quality tool not installed”

Solution: diff-quality assumes you have the tool you wish to run against your diff installed. If you do not have it then install it with your favorite package manager.

Issue: diff-quality reports no quality issues

Solution: You might use a pattern like diff-quality --violations foo *.py. The last argument is not used to specify the files but for the quality tool report. Remove it to resolve the issue


The code in this repository is licensed under the Apache 2.0 license. Please see LICENSE.txt for details.

How to Contribute

Contributions are very welcome. The easiest way is to fork this repo, and then make a pull request from your fork.

NOTE: diff-quality supports a plugin model, so new tools can be integrated without requiring changes to this repo. See the section “Adding diff-quality` Support for a New Quality Checker”.

Setting Up For Development

This project is managed with poetry this can be installed with pip poetry manages a python virtual environment and organizes dependencies. It also packages this project.

pip install poetry
poetry install

I would also suggest running this command after. This will make it so git blame ignores the commit that formatted the entire codebase.

git config blame.ignoreRevsFile .git-blame-ignore-revs

Adding diff-quality` Support for a New Quality Checker

Adding support for a new quality checker is simple. diff-quality supports plugins using the popular Python pluggy package.

If the quality checker is already implemented as a Python package, great! If not, create a Python package to host the plugin implementation.

In the Python package’s file, define an entry point for the plugin, e.g.

        'diff_cover': [
            'sqlfluff = sqlfluff.diff_quality_plugin'


  • The dictionary key for the entry point must be named diff_cover

  • The value must be in the format TOOL_NAME = YOUR_PACKAGE.PLUGIN_MODULE

When your package is installed, diff-quality uses this information to look up the tool package and module based on the tool name provided to the --violations option of the diff-quality command, e.g.:

$ diff-quality --violations sqlfluff

The plugin implementation will look something like the example below. This is a simplified example based on a working plugin implementation.

from diff_cover.hook import hookimpl as diff_cover_hookimpl
from diff_cover.violationsreporters.base import BaseViolationReporter, Violation

class SQLFluffViolationReporter(BaseViolationReporter):
    supported_extensions = ['sql']

    def __init__(self):
        super(SQLFluffViolationReporter, self).__init__('sqlfluff')

    def violations(self, src_path):
        return [
            Violation(violation.line_number, violation.description)
            for violation in get_linter().get_violations(src_path)

    def measured_lines(self, src_path):
        return None

    def installed():
        return True

def diff_cover_report_quality():
    return SQLFluffViolationReporter()

Important notes:

  • diff-quality is looking for a plugin function:

    • Located in your package’s module that was listed in the entry point.

    • Marked with the @diff_cover_hookimpl decorator

    • Named diff_cover_report_quality. (This distinguishes it from any other plugin types diff_cover may support.)

  • The function should return an object with the following properties and methods:

    • supported_extensions property with a list of supported file extensions

    • violations() function that returns a list of Violation objects for the specified src_path. For more details on this function and other possible reporting-related methods, see the BaseViolationReporter class here.

Special Thanks

Shout out to the original author of diff-cover Will Daly and the original author of diff-quality Sarina Canelake.

Originally created with the support of edX.

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