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Coding guide

Pull request checklist

This checklist should be consulted when creating pull requests to make sure they are complete before merging. These are not intended to be rigidly followed—it’s just an attempt to list in one place all of the items that are necessary for a good pull request. Of course, some items will not always apply.

Branch selection

  • In general, simple bugfixes that are unlikely to introduce new bugs of their own should be merged onto the maintenance branch. New features, or anything that changes the API, should be made against master. The rules are fuzzy here – when in doubt, try to get some consensus.
    • Once changes are merged into the maintenance branch, they should be merged into master.


  • Formatting should follow PEP8. Exceptions to these rules are acceptable if it makes the code objectively more readable.

    • You should consider installing/enabling automatic PEP8 checking in your editor. Part of the test suite is checking PEP8 compliance, things go smoother if the code is mostly PEP8 compliant to begin with.
  • No tabs (only spaces). No trailing whitespace.

    • Configuring your editor to remove these things upon saving will save a lot of trouble.
  • Import the following modules using the standard scipy conventions:

    import numpy as np
    import as ma
    import matplotlib as mpl
    from matplotlib import pyplot as plt
    import matplotlib.cbook as cbook
    import matplotlib.collections as mcol
    import matplotlib.patches as mpatches
  • See below for additional points about Keyword argument processing, if code in your pull request does that.

  • Adding a new pyplot function involves generating code. See Writing a new pyplot function for more information.


  • Every new feature should be documented. If it’s a new module, don’t forget to add a new rst file to the API docs.

  • Docstrings should be in numpydoc format. Don’t be thrown off by the fact that many of the existing docstrings are not in that format; we are working to standardize on numpydoc.

    Docstrings should look like (at a minimum):

    def foo(bar, baz=None):
        This is a prose description of foo and all the great
        things it does.
        bar : (type of bar)
            A description of bar
        baz : (type of baz), optional
            A description of baz
        foobar : (type of foobar)
            A description of foobar
        foobaz : (type of foobaz)
            A description of foobaz
        # some very clever code
        return foobar, foobaz
  • Each high-level plotting function should have a simple example in the Example section of the docstring. This should be as simple as possible to demonstrate the method. More complex examples should go in the examples tree.

  • Build the docs and make sure all formatting warnings are addressed.

  • See Documenting matplotlib for our documentation style guide.

  • If your changes are non-trivial, please make an entry in the CHANGELOG.

  • If your change is a major new feature, add an entry to doc/users/whats_new.rst.

  • If you change the API in a backward-incompatible way, please document it in doc/api/api_changes.rst.


Using the test framework is discussed in detail in the section Testing.

  • If the PR is a bugfix, add a test that fails prior to the change and passes with the change. Include any relevant issue numbers in the docstring of the test.
  • If this is a new feature, add a test that exercises as much of the new feature as possible. (The --with-coverage option may be useful here).
  • Make sure the Travis tests are passing before merging.
    • The Travis tests automatically test on all of the Python versions matplotlib supports whenever a pull request is created or updated. The tox support in matplotlib may be useful for testing locally.


  • If you have added new files or directories, or reorganized existing ones, make sure the new files included in the match patterns in, and/or in package_data in

C/C++ extensions

  • Extensions may be written in C or C++.
  • Code style should conform to PEP7 (understanding that PEP7 doesn’t address C++, but most of its admonitions still apply).
  • Interfacing with Python may be done either with the raw Python/C API or Cython.
  • Python/C interface code should be kept separate from the core C/C++ code. The interface code should be named FOO_wrap.cpp or FOO_wrapper.cpp.
  • Header file documentation (aka docstrings) should be in Numpydoc format. We don’t plan on using automated tools for these docstrings, and the Numpydoc format is well understood in the scientific Python community.

Style guide

Keyword argument processing

Matplotlib makes extensive use of **kwargs for pass-through customizations from one function to another. A typical example is in matplotlib.pylab.text(). The definition of the pylab text function is a simple pass-through to matplotlib.axes.Axes.text():

# in
def text(*args, **kwargs):
    ret =  gca().text(*args, **kwargs)
    return ret

text() in simplified form looks like this, i.e., it just passes all args and kwargs on to matplotlib.text.Text.__init__():

# in
def text(self, x, y, s, fontdict=None, withdash=False, **kwargs):
    t = Text(x=x, y=y, text=s, **kwargs)

and __init__() (again with liberties for illustration) just passes them on to the matplotlib.artist.Artist.update() method:

# in
def __init__(self, x=0, y=0, text='', **kwargs):

update does the work looking for methods named like set_property if property is a keyword argument. i.e., no one looks at the keywords, they just get passed through the API to the artist constructor which looks for suitably named methods and calls them with the value.

As a general rule, the use of **kwargs should be reserved for pass-through keyword arguments, as in the example above. If all the keyword args are to be used in the function, and not passed on, use the key/value keyword args in the function definition rather than the **kwargs idiom.

In some cases, you may want to consume some keys in the local function, and let others pass through. You can pop the ones to be used locally and pass on the rest. For example, in plot(), scalex and scaley are local arguments and the rest are passed on as Line2D() keyword arguments:

# in
def plot(self, *args, **kwargs):
    scalex = kwargs.pop('scalex', True)
    scaley = kwargs.pop('scaley', True)
    if not self._hold: self.cla()
    lines = []
    for line in self._get_lines(*args, **kwargs):

Note: there is a use case when kwargs are meant to be used locally in the function (not passed on), but you still need the **kwargs idiom. That is when you want to use *args to allow variable numbers of non-keyword args. In this case, python will not allow you to use named keyword args after the *args usage, so you will be forced to use **kwargs. An example is matplotlib.contour.ContourLabeler.clabel():

# in
def clabel(self, *args, **kwargs):
    fontsize = kwargs.get('fontsize', None)
    inline = kwargs.get('inline', 1)
    self.fmt = kwargs.get('fmt', '%1.3f')
    colors = kwargs.get('colors', None)
    if len(args) == 0:
        levels = self.levels
        indices = range(len(self.levels))
    elif len(args) == 1:


This section describes how to add certain kinds of new features to matplotlib.

Developing a new backend

If you are working on a custom backend, the backend setting in matplotlibrc (Customizing matplotlib) supports an external backend via the module directive. if is a matplotlib backend in your PYTHONPATH, you can set use it on one of several ways

  • in matplotlibrc:

    backend : module://my_backend
  • with the MPLBACKEND environment variable:

    > export MPLBACKEND="module://my_backend"
    > python
  • from the command shell with the -d flag:

    > python -dmodule://my_backend
  • with the use directive in your script:

    import matplotlib

Writing examples

We have hundreds of examples in subdirectories of matplotlib/examples, and these are automatically generated when the website is built to show up both in the examples and gallery sections of the website.

Any sample data that the example uses should be kept small and distributed with matplotlib in the lib/matplotlib/mpl-data/sample_data/ directory. Then in your example code you can load it into a file handle with:

import matplotlib.cbook as cbook
fh = cbook.get_sample_data('mydata.dat')

Writing a new pyplot function

A large portion of the pyplot interface is automatically generated by the script (in the root of the source tree). To add or remove a plotting method from pyplot, edit the appropriate list in and then run the script which will update the content in lib/matplotlib/ Both the changes in and lib/matplotlib/ should be checked into the repository.

Note: looks for changes in the installed version of matplotlib and not the source tree. If you expect the file to show your new changes, but they are missing, this might be the cause.

Install your new files by running python build and python install followed by python The new file should now have the latest changes.