kicad/Documentation/development/pcbnew-plugins.md

Python Plugin Development for Pcbnew

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Introduction

KiCad implements a Python plugin interface so that external Python plugins can be run from within Pcbnew. The interface is generated using the Simplified Wrapper and Interface Generator or SWIG. SWIG is instructed to translate specific C/C++ header files into other languages using interface files. These files ultimately decide what C/C++ functions, classes and other declarations are exported and can be found in pcbnew/swig/.

During build-time the SWIG interface files are used to generate the corresponding .py files. These files are installed into Python's system-wide dist-packages repository, thus they can be imported by any Python 2 interpreter installed on the system.

Existing Pcbnew Python API documentation

The Pcbnew Python API can be used stand-alone, i.e. no instance of Pcbnew is running and the board project to be manipulated is loaded and saved from and to file. This approach is shown with some examples in the user's documentation.

Another documentation source is the auto-generated Doxygen reference of the API. It can be found here.

Action Plugin Support

Besides the stand-alone usage of the generated Python plugin interface, additional support regarding online manipulation of board projects is available for Pcbnew. Plugins using this feature are called Action Plugins and they are accessible using a Pcbnew menu entry that can be found under Tools->External Plugins. KiCad plugins that follow the Action Plugin conventions can be made to show up as external plugins in that menu. The user can run the plugin resulting in calling a defined entry function in the Python plugin's code. This function can then be used to access and manipulate the currently loaded board from the Python script environment.

Typical Plugin Structure

The Action Plugin support is implemented in Pcbnew by discovering Python packages and Python script files in specific directories on startup. In order for the discovery process to work, the following requirements must be met.

• The plugin must be installed in the KiCad plugins search paths as documented in scripting/kicadplugins.i. (Currently on a Linux Mint Installation this is /usr/share/kicad/scripting/plugins/ and ~/.kicad_plugins/)
• Alternatively a symbolic link can be created in the KiCad plugin path link to the plugin file/folder in another location of the file system. This can be useful for development.
• The plugin must be written as a simple Python script (*.py) located in the plugin search path. Note that this method is preferred for small plugins consisting of a single .py file.
• Alternatively the plugin must be implemented as a Python package conforming to the Python package standard definitions (See 6.4. Packages). Note that this method is preferred for larger plugin projects consisting of multiple .py files and resource files such as dialogs or images.
• The Python plugin must contain a class derived from pcbnew.ActionPlugin and it's register() method must be called within the plugin.

The following examples demonstrate the plugin requirements.

Simple Plugin Example

The folder structure of the simple plugin is fairly straight forward. A single Python script file is placed into a directory that is present in the KiCad plugin path.

+ ~/.kicad_plugins/ # A folder in the KiCad plugin path
    - simple_plugin.py

The file simple_plugin.py contains the following.

import pcbnew

class SimplePlugin(pcbnew.ActionPlugin):
    def defaults(self):
        self.name = "Plugin Name as shown in Pcbnew: Tools->External Plugins"
        self.category = "A descriptive category name"
        self.description = "A description of the plugin and what it does"

    def Run(self):
        # The entry function of the plugin that is executed on user action
        print("Hello World")

SimplePlugin().register() # Instantiate and register to Pcbnew

Complex Plugin Example

The complex plugin example represents a single Python package that is imported on Pcbnew startup. When the Python package is imported, the __init__.py file is executed and is thus a perfect place to instantiate and register the plugin to Pcbnew. The big advantage here is, that you can modularize your plugin much better and include other files without cluttering the KiCad plugin directory. Additionally, the same plugin can be executed standalone using python -m e.g. to perform tests on the Python code. The following folder structure shows how complex plugins are implemented:

+ ~/.kicad_plugins/ # this directory has to be in the plugin path
    + complex_plugin/ # The plugin directory (A Python package)
        - __init__.py # This file is executed when the package is imported (on Pcbnew startup)
        - __main__.py # This file is optional. See below
        - complex_plugin_action.py # The ActionPlugin derived class lives here
        - complex_plugin_utils.py # Other Python parts of the plugin
        + otherstuff/
            - otherfile.png
            - misc.txt

It is recommended to name the file containing the ActionPlugin derived class as <package-name>_action.py. In this case the file is named complex_plugin_action.py with the following contents:

import pcbnew

class ComplexPluginAction(pcbnew.ActionPlugin)
    def defaults(self):
        self.name = "A complex action plugin"
        self.category = "A descriptive category name"
        self.description "A description of the plugin"

    def Run(self):
        # The entry function of the plugin that is executed on user action
        print("Hello World")

The __init__.py file is then used to instantiate and register the plugin to Pcbnew as follows.

from .complex_plugin_action import ComplexPluginAction # Note the relative import!
ComplexPluginAction().register() # Instantiate and register to Pcbnew

As described in PEP 338 Python can execute packages (or modules) as scripts. This can be useful to implement a command-line stand-alone version of your KiCad plugin with minimum effort. In order to implement this feature, a __main__.py file is created in the package directory. This file can be executed by running the following command.

python -m <package_name>

Make sure that your current directory is the parent directory of the package directory when running the command. In these examples, this would be ~/.kicad_plugins. When running the command the Python interpreter runs /complex_plugin/__init__.py followed by /complex_plugin/__main__.py.