Developing Morepath


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Install Morepath for development

Clone Morepath from github:

$ git clone

If this doesn’t work and you get an error ‘Permission denied (publickey)’, you need to upload your ssh public key to github.

Then go to the morepath directory:

$ cd morepath

Make sure you have virtualenv installed.

Create a new virtualenv for Python 3 inside the morepath directory:

$ virtualenv -p python3 env/py3

Activate the virtualenv:

$ source env/py3/bin/activate

Make sure you have recent setuptools and pip installed:

$ pip install -U setuptools pip

Install the various dependencies and development tools from requirements/develop.txt:

$ pip install -Ur requirements/develop.txt --src src

This needs your ssh key installed in github to work.

The --src src option makes sure that the dependent reg, dectate and importscan projects are checked out in the src directory. You can make changes to them during development too.

For upgrading the sources and requirements just run the command again.

If you want to test Morepath with Python 2.7 as well you can create a second virtualenv for it:

$ virtualenv -p python2.7 env/py27

You can then activate it:

$ source env/py27/bin/activate

Then uprade setuptools and pip and install the develop requirements as described above.


The following commands work only if you have the virtualenv activated.

Running the tests

You can run the tests using py.test:

$ py.test

To generate test coverage information as HTML do:

$ py.test --cov --cov-report html

You can then point your web browser to the htmlcov/index.html file in the project directory and click on modules to see detailed coverage information.


flake8 is a tool that can do various checks for common Python mistakes using pyflakes and checks for PEP8 style compliance. We want a codebase where there are no flake8 messages.

To do pyflakes and pep8 checking do:

$ flake8 morepath


radon is a tool that can check various measures of code complexity.

To check for cyclomatic complexity (excluding the tests):

$ radon cc morepath -e "morepath/tests*"

To filter for anything not ranked A:

$ radon cc morepath --min B -e "morepath/tests*"

And to see the maintainability index:

$ radon mi morepath -e "morepath/tests*"

Running the documentation tests

The documentation contains code. To check these code snippets, you can run this code using this command:

(py3) $ sphinx-build -b doctest doc doc/build/doctest

Or alternatively if you have Make installed:

(py3) $ cd doc
(py3) $ make doctest

Or from the Morepath project directory:

(py3) $ make -C doc doctest

Since the sample code in the documentation is maintained in Python 3 syntax, we do not support running the doctests with Python 2.7.

Building the HTML documentation

To build the HTML documentation (output in doc/build/html), run:

$ sphinx-build doc doc/build/html

Or alternatively if you have Make installed:

$ cd doc
$ make html

Or from the Morepath project directory:

$ make -C doc html

Developing Reg, Dectate or Importscan

If you need to adjust the sources of Reg, Dectate or Importscan and test them together with Morepath, they’re available in the src directory. You can edit them and test changes in the Morepath project directly.

If you want to run the tests for one of them, let’s say Reg, do:

$ cd src/reg
$ py.test


With tox you can test Morepath under different Python environments.

We have Travis continuous integration installed on Morepath’s github repository and it runs the same tox tests after each checkin.

First you should install all Python versions which you want to test. The versions which are not installed will be skipped. You should at least install Python 3.5 which is required by flake8, coverage and doctests and Python 2.7 for testing Morepath with Python 2.

One tool you can use to install multiple versions of Python is pyenv.

To find out which test environments are defined for Morepath in tox.ini run:

$ tox -l

You can run all tox tests with:

$ tox

You can also specify a test environment to run e.g.:

$ tox -e py35
$ tox -e pep8
$ tox -e docs

To find out which dependencies and which versions tox installs in the testenv, you can use:

$ tox -e freeze


In some cases we have to make changes that break compatibility and break user code. We mark these in CHANGES.txt (CHANGES) using breaking change, deprecated or removed.

These entries should explain the change, and also tell the user what to do to upgrade their code. Do include an before/after code example as that makes it much easier, even if it’s a simple import change.

We like to keep things moving and reserve the right to introduce breaking changes. When we do make a breaking change it should be marked clearly in CHANGES.txt (CHANGES) with a Breaking change marker.

If it is not a great burden we use deprecations. Morepath in this case retains the old APIs but issues a deprecation warning. See Upgrading to a new Morepath version for the notes for end-users concerning this. Here is the deprecation procedure for developers:

  • Add a Deprecated entry in CHANGES.txt that describes what to do, as in a breaking change.

  • Issue a deprecation warning in the code that is deprecated.

  • Put a **Deprecated** entry in the docstring of whatever got deprecated with a brief comment on what to do.

  • Put an issue labeled remove deprecation in the tracker for one release milestone after the upcoming release that states we should remove the deprecation. Create the milestone if needed.

    This way we don’t maintain deprecated code and their warnings indefinitely – one release later we remove the backwards compatibility code and deprecation warnings.

  • Once we go and remove code, we repeat the information on what to do in a new Removed* entry in CHANGES.txt; treat it just like Breaking change and recycle the text written for the previous Deprecated entry for the stuff we’re now removing.