This extension registers an IHttpListener configured to execute a custom script editable via the Script tab added to Burp. The script is executed in the context with the following global and local variables (extender, callbacks, helpers, toolFlag, messageIsRequest, messageInfo).

burpscript.py

from java.awt import Font
from javax.swing import JScrollPane, JTextPane
from javax.swing.text import SimpleAttributeSet

from burp import IBurpExtender, IExtensionStateListener, IHttpListener, ITab

import base64
import traceback


class BurpExtender(IBurpExtender, IExtensionStateListener, IHttpListener, ITab):
    def registerExtenderCallbacks(self, callbacks):
        self.callbacks = callbacks
        self.helpers = callbacks.helpers

        self.scriptpane = JTextPane()
        self.scriptpane.setFont(Font('Monospaced', Font.PLAIN, 11))

        self.scrollpane = JScrollPane()
        self.scrollpane.setViewportView(self.scriptpane)

        self._code = compile('', '<string>', 'exec')
        self._script = ''

        script = callbacks.loadExtensionSetting('script')

        if script:
            script = base64.b64decode(script)

            self.scriptpane.document.insertString(
                    self.scriptpane.document.length,
                    script,
                    SimpleAttributeSet())

            self._script = script
            self._code = compile(script, '<string>', 'exec')

        callbacks.registerExtensionStateListener(self)
        callbacks.registerHttpListener(self)
        callbacks.customizeUiComponent(self.getUiComponent())
        callbacks.addSuiteTab(self)

        self.scriptpane.requestFocus()

    def extensionUnloaded(self):
        try:
            self.callbacks.saveExtensionSetting(
                    'script', base64.b64encode(self._script))
        except Exception:
            traceback.print_exc(file=self.callbacks.getStderr())
        return

    def processHttpMessage(self, toolFlag, messageIsRequest, messageInfo):
        try:
            globals_ = {}
            locals_  = {'extender': self,
                        'callbacks': self.callbacks,
                        'helpers': self.helpers,
                        'toolFlag': toolFlag,
                        'messageIsRequest': messageIsRequest,
                        'messageInfo': messageInfo
                        }
            exec(self.script, globals_, locals_)
        except Exception:
            traceback.print_exc(file=self.callbacks.getStderr())
        return

    def getTabCaption(self):
        return 'Script'

    def getUiComponent(self):
        return self.scrollpane

    @property
    def script(self):
        end = self.scriptpane.document.length
        _script = self.scriptpane.document.getText(0, end)

        if _script == self._script:
            return self._code

        self._script = _script
        self._code = compile(_script, '<string>', 'exec')
        return self._code