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pycobertura logo

A code coverage diff tool for Cobertura reports.

Travis PyPI


pycobertura is a generic Cobertura report parser. It was also designed to help prevent code coverage from decreasing with the pycobertura diff command: any line changed should be tested and uncovered changes should be clearly visible without letting legacy uncovered code get in the way so developers can focus solely on their changes.


  • show coverage summary of a cobertura file
  • output in plain text or HTML
  • compare two cobertura files and show changes in coverage
  • colorized diff output
  • diff exit status of non-zero if coverage worsened or if any changes were left uncovered
  • fail based on uncovered lines rather than on decrease of coverage rate (see why)

NOTE: The API is unstable any may be subject to changes until it reaches 1.0.


$ pip install pycobertura

CLI usage

pycobertura provides a command line interface to report on coverage files.

Help commands

Different help screens are available depending on what you need help about.

$ pycobertura --help
$ pycobertura show --help
$ pycobertura diff --help

Command show

The show command displays the report summary of a coverage file.

$ pycobertura show coverage.xml
Filename                     Stmts    Miss  Cover    Missing
-------------------------  -------  ------  -------  ---------
pycobertura/          1       0  100.00%
pycobertura/              18       0  100.00%
pycobertura/        93       0  100.00%
pycobertura/       129       0  100.00%
pycobertura/            12       0  100.00%
TOTAL                          253       0  100.00%

The following is a screenshot of the HTML version of another coverage file which also include the source code with highlighted source code to indicate whether lines were covered (green) or not (red).

pycobertura show --format html --output coverage.html coverage.xml

Command diff

You can also use the diff command to show the difference between two coverage files. To properly compute the Missing column, it is necessary to provide the source code that was used to generate each of the passed Cobertura reports (see why).

$ pycobertura diff coverage.old.xml --source1 old_source/ --source2 new_source/
Filename          Stmts    Miss    Cover     Missing
----------------  -------  ------  --------  ---------
dummy/    -        -2      +50.00%   -2, -5
dummy/   +2       -       +100.00%
TOTAL             +2       -2      +50.00%

The column Missing will show line numbers prefixed with either a plus sign + or a minus sign -. When prefixed with a plus sign, the line was introduced as uncovered and is shown in red, when prefixed as a minus sign, the line is no longer uncovered and is rendered in green.

This screenshot shows how the HTML output only applies coverage highlighting to the parts of the code where the coverage has changed (from covered to uncovered, or vice versa).

pycobertura diff --format html --output coverage.html ./master/coverage.xml ./myfeature/coverage.xml

diff exit codes

Upon exit, the diff command may return various exit codes:

  • 0: all is good
  • 1: some exception occurred (likely due to inappropriate usage or a bug in pycobertura)
  • 2: coverage worsened (implies 3)
  • 3: not all changes are covered

Library usage

Using it as a library in your Python application is easy:

from pycobertura import Cobertura
cobertura = Cobertura('coverage.xml')

cobertura.version == '4.0.2'
cobertura.line_rate() == 1.0  # 100%
cobertura.files() == [
cobertura.line_rate('pycobertura/') == 1.0

from pycobertura import TextReporter
tr = TextReporter(cobertura)
tr.generate() == """\
Filename                     Stmts    Miss  Cover    Missing
-------------------------  -------  ------  -------  ---------
pycobertura/          1       0  100.00%
pycobertura/              18       0  100.00%
pycobertura/        93       0  100.00%
pycobertura/       129       0  100.00%
pycobertura/            12       0  100.00%
TOTAL                          253       0  100.00%"""

from pycobertura import TextReporterDelta

coverage1 = Cobertura('coverage1.xml')
coverage2 = Cobertura('coverage2.xml')
delta = TextReporterDelta(coverage1, coverage2)
delta.generate() == """\
Filename          Stmts    Miss    Cover     Missing
----------------  -------  ------  --------  ---------
dummy/    -        -2      +50.00%   -2, -5
dummy/   +2       -       +100.00%
TOTAL             +2       -2      +50.00%"""

How to contribute?

Found a bug/typo? Got a patch? Have an idea? Please use Github issues or fork pycobertura and submit a pull request (PR). All contributions are welcome!

If you submit a PR:

  • ensure the description of your PR illustrates your changes clearly by showing what the problem was and how you fixed it (before/after)
  • make sure your changes are covered with one or more tests
  • add a descriptive note in the CHANGES file under the Unreleased section
  • update the README accordingly if your changes outdate the documentation
  • make sure all tests are passing using tox
pip install tox


How does pycobertura work?

Pycobertura takes two different Cobertura reports and compares them line by line. If the coverage status of a line changed from covered to uncovered or vice versa, then pycobertura will report it. Sometimes you have no code changes at all, the only changes were to add more tests and pycobertura will show you the progress.

Pycobertura was initially designed as a general purpose Cobertura parser and can generate a summary table for a single Cobertura file (the show command).

I only have one Cobertura report, can I just see my uncovered changes?

Yes. All you have to do is pass your same coverage report twice and provide the path to the two different code bases:

pycobertura diff coverage.xml coverage.xml --source1 master/ --source2 myfeature/

But keep in mind that this will not show you if your changes have introduced a drop in coverage elsewhere in the code base.

Why doesn't pycobertura use git to diff the source given revision SHAs rather than passing paths to the source code?

Because we would have to support N version control systems (VCS). It is easy enough to generate a directory that contains the source code at a given commit or branch name that it's not a top priority for pycobertura to be VCS-aware:

git archive --prefix=source1/ ${BRANCH_OR_COMMIT1} | tar -xf -
git archive --prefix=source2/ ${BRANCH_OR_COMMIT2} | tar -xf -
pycobertura diff --source1 source1/ --source2 source2/ coverage1.xml coverage2.xml -o output.html
rm -rf source1/ source2/

Mercurial has hg archive and Subversion has svn export. These are simple pre-steps to running pycobertura diff.

Also, the code repository may not always be available at the time pycobertura is run. Typically, in Continuous Delivery pipelines, only artifacts are available.

Why do I need to provide the path to the source code directory?

With the command pycobertura show, you don't need to provide the source code directory, unless you want the HTML output which will conveniently render the highlighted source code for you.

But with pycobertura diff, if you care about which lines are covered or uncovered (and not just a global count), then you will need to provide the source for each of the reports.

To better understand why, let's assume we have 2 Cobertura reports with the following info:

Report A:

line 1, hit
line 2, miss
line 3, hit

and Report B:

line 1, hit
line 2, miss
line 3, hit
line 4, miss
line 5, hit

How can you tell which lines need to be highlighted? Naively, you'd assume that lines 4-5 were added and these should be the highlighted lines, the ones part of your coverage diff. Well, that doesn't quite work.

The code for Report A is:

if foo is True:  # line 1
    total += 1   # line 2
return total     # line 3

The code for Report B is:

if foo is False:   # line 1  # new line
    total -= 1     # line 2  # new line
elif foo is True:  # line 3  # modified line
    total += 1     # line 4, unchanged
return total       # line 5, unchanged

The code change are lines 1-3 and these are the ones you want to highlight. Lines 4-5 don't need to be highlighted (unless coverage status changed in-between).

So, to accurately highlight the lines that have changed, the coverage reports alone are not sufficient and this is why you need to provide the path to the source that was used to generate each of the Cobertura reports and diff them to see which lines actually changed to report accurate coverage.

When should I use pycobertura?

pycobertura was built as a tool to educate developers about the testing culture in such way that any code change should have one or more tests along with it.

You can use pycobertura in your Continuous Integration (CI) or Continuous Delivery (CD) pipeline which would fail a build if the code changes worsened the coverage. For example, when a pull request is submitted, the new code should have equal or better coverage than the branch it's going to be merged into. Or if code navigates through a release pipeline and the new code has worse coverage than what's already in Production, then the release is aborted.

When a build is triggered by your CI/CD pipeline, each testing stage would typically store an artifact of the source code and another one of the Cobertura report. An extra stage in the pipeline could ensure that the coverage did not worsen. This can be done by retrieving the artifacts of the current build as well as the "target" artifacts (code and Cobertura report of Production or target branch of a pull request). Then pycobertura diff will take care of failing the build if the coverage worsened (return a non-zero exit code) and then the pycobertura report can be published as an artifact to make it available to developers to look at.

The step could look like this:

# Download artifacts of current build
curl -o coverage.${BUILD_ID}.xml https://ciserver/artifacts/${BUILD_ID}/coverage.xml
curl -o source.${BUILD_ID}.zip https://ciserver/artifacts/${BUILD_ID}/

# Download artifacts of already-in-Prod build
curl -o coverage.${PROD_BUILD}.xml https://ciserver/artifacts/${PROD_BUILD}/coverage.xml
curl -o source.${PROD_BUILD}.zip https://ciserver/artifacts/${PROD_BUILD}/

unzip source.${BUILD_ID}.zip -d source.${BUILD_ID}
unzip source.${PROD_BUILD}.zip -d source.${PROD_BUILD}

# Compare
pycobertura diff --format html \
                 --output pycobertura-diff.${BUILD_ID}.html \
                 --source1 source.${PROD_BUILD} \
                 --source2 source.${BUILD_ID} \
                 coverage.${PROD_BUILD}.xml \

# Upload the pycobertura report artifact
curl -F filedata=@pycobertura-diff.${BUILD_ID}.html http://ciserver/artifacts/${BUILD_ID}/

Why is the number of uncovered lines used as the metric to check if code coverage worsened rather than the line rate?

The line rate (percentage of covered lines) can legitimately go down for a number of reasons. To illustrate, suppose we have this code coverage report for version A of our code:

line 1: hit
line 2: hit
line 3: miss
line 4: hit
line 5: hit

Here, the line rate is 80% and uncovered lines is 1 (miss). Later in version B of our code, we legitimately delete a covered line and the following coverage report is generated:

line 1: hit
### line deleted ###
line 2: miss
line 3: hit
line 4: hit

The line rate decreased from 80% to 75% but uncovered lines is still at 1. In this case, failing the build based on line rate is inappropriate, thus making the line rate the wrong metric to look at when validating coverage.

The basic idea is that a code base may have technical debt of N uncovered lines and you want to prevent N from ever going up.

pycobertura sounds cool, but how to I generate a Cobertura file?

Depending on your programing language, you need to find a tool that measures code coverage and generates a Cobertura report which is an XML representation of your code coverage results.

In Python, is a great tool for measuring code coverage and plugins such as pytest-cov for pytest and nosexcover for nose are available to generate Cobertura reports while running tests.

Istanbul can generate Cobertura reports for Javascript and also has plugins for CoffeeScript if that is your cup of tea.

Cobertura is a very common file format available in many testing tools for pretty much all programing languages. pycobertura is language agnostic and should work with reports generated by tools in any language. But it was mostly developed and tested against reports generated with the pytest-cov plugin in Python. If you see issues, please create a ticket.

Logo credit

The pycobertura logo, Aysha, was graciously donated by Open Logos. Check them out!


A code coverage diff tool for Cobertura reports




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