Source code for pyface.base_toolkit
""" Common toolkit loading utilities and classes
This module provides common code for ETS packages that need to do GUI toolkit
discovery and loading. The common patterns that ETS has settled on are that
where different GUI toolkits require alternative implementations of features
the toolkit is expected to provide a callable object which takes a relative
module path and an object name, separated by a colon and return the toolkit's
implementation of that object (usually this is a class, but it could be
anything). The assumption is that this is implemented by objects in
sub-modules of the toolkit, but plugin authors are free to use whatever methods
they like.
Which toolkit to use is specified via the :py:mod:`traits.etsconfig.etsconfig`
package, but if this is not explicitly set by an application at startup or via
environment variables, there needs to be a way of discovering and loading any
available working toolkit implementations. The default mechanism is via the
now-standard :py:mod:`pkg_resources` and :py:mod:`setuptools` "entry point"
system.
This module provides three things:
- a function :py:func:`import_toolkit` that attempts to find and load a toolkit
entry point for a specified toolkit name
- a function :py:func:`find_toolkit` that attempts to find a toolkit entry
point that works
- a class :py:class:`Toolkit` class that implements the standard logic for
finding toolkit objects.
These are done in a library-agnostic way so that the same tools can be used
not just for different pyface backends, but also for TraitsUI and ETS
libraries where we need to switch between different GUI toolkit
implementations.
Note that there is no requirement for new toolkit implementations to use this
:py:class:`Toolkit` implementation, but they should be compatible with it.
Default toolkit loading logic
-----------------------------
The :py_func:`find_toolkit` function uses the following logic when attempting
to load toolkits:
- if ETSConfig.toolkit is set, try to load a plugin with a matching name.
If it succeeds, we are good, and if it fails then we error out.
- after that, we try every 'pyface.toolkit' plugin we can find. If one
succeeds, we consider ourselves good, and set the ETSConfig.toolkit
appropriately. The order is configurable, and by default will try to load
the `qt4` toolkit first, `wx` next, then all others in arbitrary order,
and `null` last.
- finally, if all else fails, we try to load the null toolkit.
"""
import logging
import os
import pkg_resources
import sys
from traits.api import HasTraits, List, ReadOnly, Str, TraitError
from traits.etsconfig.api import ETSConfig
try:
provisional_toolkit = ETSConfig.provisional_toolkit
except AttributeError:
from contextlib import contextmanager
# for backward compatibility
@contextmanager
def provisional_toolkit(toolkit_name):
""" Perform an operation with toolkit provisionally set
This sets the toolkit attribute of the ETSConfig object set to the
provided value. If the operation fails with an exception, the toolkit
is reset to nothing.
"""
if ETSConfig.toolkit:
raise AttributeError("ETSConfig toolkit is already set")
ETSConfig.toolkit = toolkit_name
try:
yield
except:
# reset the toolkit state
ETSConfig._toolkit = ''
raise
logger = logging.getLogger(__name__)
TOOLKIT_PRIORITIES = {
'qt4': -2,
'wx': -1,
'null': float('inf')
}
default_priorities = lambda plugin: TOOLKIT_PRIORITIES.get(plugin.name, 0)
class Toolkit(HasTraits):
""" A basic toolkit implementation for use by specific toolkits.
This implementation uses pathname mangling to find modules and objects in
those modules. If an object can't be found, the toolkit will return a
class that raises NotImplementedError when it is instantiated.
"""
#: The name of the package (eg. pyface)
package = ReadOnly
#: The name of the toolkit
toolkit = ReadOnly
#: The packages to look in for implementations.
packages = List(Str)
def __init__(self, package, toolkit, *packages, **traits):
super(Toolkit, self).__init__(
package=package,
toolkit=toolkit,
packages=list(packages),
**traits
)
def __call__(self, name):
""" Return the toolkit specific object with the given name.
Parameters
----------
name : str
The name consists of the relative module path and the object name
separated by a colon.
"""
from importlib import import_module
mname, oname = name.split(':')
if not mname.startswith('.'):
mname = '.' + mname
for package in self.packages:
try:
module = import_module(mname, package)
except ImportError as exc:
# is the error while trying to import package mname or not?
if all(part not in exc.args[0] for part in mname.split('.')
if part):
# something else went wrong - let the exception be raised
raise
# Ignore *ANY* errors unless a debug ENV variable is set.
if 'ETS_DEBUG' in os.environ:
# Attempt to only skip errors in importing the backend modules.
# The idea here is that this only happens when the last entry in
# the traceback's stack frame mentions the toolkit in question.
import traceback
frames = traceback.extract_tb(sys.exc_traceback)
filename, lineno, function, text = frames[-1]
if not package in filename:
raise
else:
obj = getattr(module, oname, None)
if obj is not None:
return obj
toolkit = self.toolkit
class Unimplemented(object):
""" An unimplemented toolkit object
This is returned if an object isn't implemented by the selected
toolkit. It raises an exception if it is ever instantiated.
"""
def __init__(self, *args, **kwargs):
msg = "the %s %s backend doesn't implement %s"
raise NotImplementedError(msg % (toolkit, package, name))
return Unimplemented
def import_toolkit(toolkit_name, entry_point='pyface.toolkits'):
""" Attempt to import an toolkit specified by an entry point.
Parameters
----------
toolkit_name : str
The name of the toolkit we would like to load.
entry_point : str
The name of the entry point that holds our toolkits.
Returns
-------
toolkit_object : callable
A callable object that implements the Toolkit interface.
Raises
------
RuntimeError
If no toolkit is found, or if the toolkit cannot be loaded for some
reason.
"""
plugins = list(pkg_resources.iter_entry_points(entry_point, toolkit_name))
if len(plugins) == 0:
msg = 'No {} plugin found for toolkit {}'
msg = msg.format(entry_point, toolkit_name)
logger.debug(msg)
raise RuntimeError(msg)
elif len(plugins) > 1:
msg = ("multiple %r plugins found for toolkit %r: %s")
modules = ', '.join(plugin.module_name for plugin in plugins)
logger.warning(msg, entry_point, toolkit_name, modules)
for plugin in plugins:
try:
toolkit_object = plugin.load()
return toolkit_object
except (ImportError, AttributeError) as exc:
msg = "Could not load plugin %r from %r"
logger.info(msg, plugin.name, plugin.module_name)
logger.debug(exc, exc_info=True)
msg = 'No {} plugin could be loaded for {}'
msg = msg.format(entry_point, toolkit_name)
logger.info(msg)
raise RuntimeError(msg)
def find_toolkit(entry_point, toolkits=None, priorities=default_priorities):
""" Find a toolkit that works.
If ETSConfig is set, then attempt to find a matching toolkit. Otherwise
try every plugin for the entry_point until one works. The ordering of the
plugins is supplied via the priorities function which should be suitable
for use as a sorting key function. If all else fails, explicitly try to
load the "null" toolkit backend. If that fails, give up.
Parameters
----------
entry_point : str
The name of the entry point that holds our toolkits.
toolkits : collection of strings
Only consider toolkits which match the given strings, ignore other
ones.
priorities : callable
A callable function that returns an priority for each plugin.
Returns
-------
toolkit : Toolkit instance
A callable object that implements the Toolkit interface.
Raises
------
TraitError
If no working toolkit is found.
RuntimeError
If no ETSConfig.toolkit is set but the toolkit cannot be loaded for
some reason.
"""
if ETSConfig.toolkit:
return import_toolkit(ETSConfig.toolkit, entry_point)
entry_points = [
plugin for plugin in pkg_resources.iter_entry_points(entry_point)
if toolkits is None or plugin.name in toolkits
]
for plugin in sorted(entry_points, key=priorities):
try:
with ETSConfig.provisional_toolkit(plugin.name):
toolkit = plugin.load()
return toolkit
except (ImportError, AttributeError, RuntimeError) as exc:
msg = "Could not load %s plugin %r from %r"
logger.info(msg, entry_point, plugin.name, plugin.module_name)
logger.debug(exc, exc_info=True)
# if all else fails, try to import the null toolkit.
with ETSConfig.provisional_toolkit('null'):
return import_toolkit('null', entry_point)
raise TraitError("Could not import any {} toolkit.".format(entry_point))
