Metadata-Version: 2.1
Name: lazy-import
Version: 0.2.2
Summary: A module for lazy loading of Python modules
Home-page: https://github.com/mnmelo/lazy_import
Author: Manuel Nuno Melo
Author-email: manuel.nuno.melo@gmail.com
License: GPL
Description: lazy_import
        ===========
        
        |Build Status|
        
        ``lazy_import`` provides a set of functions that load modules, and related
        attributes, in a lazy fashion. This allows deferring of ``ImportErrors`` to
        actual module use-time. Likewise, actual module initialization only takes place
        at use-time. This is useful when using optional dependencies with heavy loading
        times and/or footprints, since that cost is only paid if the module is actually
        used.
        
        For minimal impact to other code running in the same session ``lazy_import``
        functionality is implemented without the use of import hooks.
        
        ``lazy_import`` is compatible with Python ≥ 2.7 or ≥ 3.4.
        
        Examples: lazy module loading
        -----------------------------
        
        .. code:: python
        
            import lazy_import
            np = lazy_import.lazy_module("numpy")
            # np is now available in the namespace and is listed in sys.modules under
            #  the 'numpy' key:
            import sys
            sys.modules['numpy']
            # The module is present as "Lazily-loaded module numpy"
        
            # Subsequent imports of the same module return the lazy version present
            #  in sys.modules
            import numpy # At this point numpy and np point to the same lazy module.
            # This is true for any import of 'numpy', even if from other modules!
        
            # Accessing attributes causes the full loading of the module ...
            np.pi
            # ... and the module is changed in place. np and numpy are now 
            #  "<module 'numpy' from '/usr/local/lib/python/site-packages/numpy/__init__.py'>"
        
            # Lazy-importing a module that's already fully loaded returns the full
            #  module instead (even if it was loaded elsewhere in the current session)
            #  because there's no point in being lazy in this case:
            os = lazy_import.lazy_module("os")
            # "<module 'os' from '/usr/lib/python/os.py'>"
        
        In the above code it can be seen that issuing
        ``lazy_import.lazy_module("numpy")`` registers the lazy module in the
        session-wide ``sys.modules`` registry. This means that *any* subsequent import
        of ``numpy`` in the same session, while the module is still not fully loaded,
        will get served a lazy version of the ``numpy`` module. This will happen also
        outside the code that calls ``lazy_module``:
        
        .. code:: python
           
            import lazy_import
            np = lazy_import.lazy_module("numpy")
            import module_that_uses_numpy # This module will get a lazy module upon
                                          # 'import numpy'
        
        Normally this is ok because the lazy module will behave pretty much as the real
        thing once fully-loaded. Still, it might be a good practice to document that
        you're lazily importing modules so-and-so, so that users are warned.
        
        Further uses are to delay ``ImportErrors``:
        
        .. code:: python
        
            import lazy_import
            # The following succeeds even when asking for a module that's not available
            missing = lazy_import.lazy_module("missing_module")
        
            missing.some_attr # This causes the full loading of the module, which now fails.
            "ImportError: __main__ attempted to use a functionality that requires module
             missing_module, but it couldn't be loaded. Please install missing_module and retry."
        
        
        Submodules work too:
        
        .. code:: python
        
            import lazy_import
            mod = lazy_import.lazy_module("some.sub.module")
            # mod now points to the some.sub.module lazy module
            #  equivalent to "from some.sub import module as mod"
        
            # Alternatively the returned reference can be made to point to the
            #  base module:
            some = lazy_import.lazy_module("some.sub.module", level="base")
        
            # This is equivalent to "import some.sub.module" in that only the base
            #  module's name is added to the namespace. All submodules must be accessed
            #  via that:
            some.sub # Returns lazy module 'some.sub' without triggering full loading.
            some.sub.attr # Triggers full loading of 'some' and 'some.sub'.
            some.sub.module.function() # Triggers loading also of 'some.sub.module'.
        
        
        Finally, if you want to mark some modules and submodules your package imports
        as always being lazy, it is as simple as lazily importing them at the root
        `__init__.py` level. Other files can then import all modules normally, and
        those that have already been loaded as lazy in `__init__.py` will remain so:
        
        .. code:: python
        
            # in __init__.py:
        
            import lazy_import
            lazy_import.lazy_module("numpy")
            lazy_import.lazy_module("scipy.stats")
        
        
            # then, in any other file in the package just use the imports normally:
        
            import requests # This one is not lazy.
            import numpy # This one is lazy, as long as no other code caused its
                         #  loading in the meantime.
            import scipy # This one is also lazy. It was lazily loaded as part of the
                         #  lazy loading of scipy.stats.
            import scipy.stats # Also lazy.
            import scipy.linalg # Uh-oh, we didn't lazily import the 'linalg' submodule
                                #  earlier, and importing it like this here will cause
                                #  both scipy and scipy.linalg (but not scipy.stats) to
                                #  immediately become fully loaded.
        
        
        Examples: lazy callable loading
        -------------------------------
        
        To emulate the ``from some.module import function`` syntax ``lazy_module``
        provides ``lazy_callable``. It returns a wrapper function. Only upon being
        called will it trigger the loading of the target module and the calling of the
        target callable (function, class, etc.).
        
        .. code:: python
        
            import lazy_import
            fn = lazy_import.lazy_callable("numpy.arange")
            # 'numpy' is now in sys.modules and is 'Lazily-loaded module numpy'
        
            fn(10)
            # array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
        
        ``lazy_callable`` is only useful when the target callable is going to be called:
        
        .. code:: python
        
            import lazy_import
            cl = lazy_import.lazy_callable("numpy.ndarray") # a class
        
            obj = cl([1, 2]) # This works OK (and also triggers the loading of numpy)
        
            class MySubclass(cl): # This fails because cl is just a wrapper,
                pass              #  not an actual class.
        
        
        Installation
        ------------
        
        .. code:: bash
        
            pip install lazy_import
        
        Or, to include dependencies needed to run regression tests:
        
        .. code:: bash
        
            pip install lazy_import[test]
        
        Tests
        -----
        
        The ``lazy_module`` module comes with a series of tests. If you install with
        test dependencies (see above), just run
        
        .. code:: python
        
            import lazy_import.test_lazy
            lazy_import.test_lazy.run()
            # This will automatically parallelize over the available number of cores
        
        Alternatively, tests can be run from the command line:
        
        .. code:: bash
        
            pytest -n 4 --boxed -v --pyargs lazy_import
            # (replace '4' with the number of cores in your machine, or set to 1 if
            #  you'd rather test in serial)
        
        Tests depend only on |pytest|_ and |pytest-xdist|_, so if you didn't install
        them along ``lazy_import`` (as described under `Installation`_) just run
        
        .. code:: bash
        
            pip install pytest pytest-xdist
        
        Note that ``pytest-xdist`` is required even for serial testing because of its
        ``--boxed`` functionality.
        
        License
        -------
        
        ``lazy_import`` is released under GPL v3. It was based on code from the
        |importing|_ module from the PEAK_ package. The licenses for both
        ``lazy_import`` and the PEAK package are included in the ``LICENSE`` file. The
        respective license notices are reproduced here:
        
          lazy_import — a module to allow lazy importing of python modules
        
          Copyright (C) 2017-2018 Manuel Nuno Melo 
        
          lazy_import is free software: you can redistribute it and/or modify
          it under the terms of the GNU General Public License as published by
          the Free Software Foundation, either version 3 of the License, or
          (at your option) any later version.
        
          lazy_import is distributed in the hope that it will be useful,
          but WITHOUT ANY WARRANTY; without even the implied warranty of
          MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
          GNU General Public License for more details.
        
          You should have received a copy of the GNU General Public License
          along with lazy_import.  If not, see <http://www.gnu.org/licenses/>.
        
        
        The PEAK ``importing`` code is
        
          Copyright (C) 1996-2004 by Phillip J. Eby and Tyler C. Sarna.
          All rights reserved.  This software may be used under the same terms
          as Zope or Python.  THERE ARE ABSOLUTELY NO WARRANTIES OF ANY KIND.
          Code quality varies between modules, from "beta" to "experimental
          pre-alpha".  :)
          
        Code pertaining to lazy loading from PEAK ``importing`` was included in
        ``lazy_import``, modified in a number of ways. These are detailed in the
        ``CHANGELOG`` file of ``lazy_import``. Changes mainly involved Python 3
        compatibility, extension to allow customizable behavior, and added
        functionality (lazy importing of callable objects).
        
        
        .. |Build Status| image:: https://api.travis-ci.org/mnmelo/lazy_import.svg
           :target: https://travis-ci.org/mnmelo/lazy_import
        
        .. |importing| replace:: ``importing``
        .. |pytest| replace:: ``pytest``
        .. |pytest-xdist| replace:: ``pytest-xdist``
        
        .. _importing: http://peak.telecommunity.com/DevCenter/Importing
        .. _PEAK: http://peak.telecommunity.com/DevCenter/FrontPage
        .. _pytest: https://docs.pytest.org/en/latest/
        .. _pytest-xdist: https://pypi.python.org/pypi/pytest-xdist
        
Platform: any
Classifier: Development Status :: 4 - Beta
Classifier: Intended Audience :: Developers
Classifier: Topic :: Software Development :: Libraries :: Python Modules
Classifier: License :: OSI Approved :: GNU General Public License v3 or later (GPLv3+)
Classifier: Programming Language :: Python :: 2
Classifier: Programming Language :: Python :: 2.7
Classifier: Programming Language :: Python :: 3
Classifier: Programming Language :: Python :: 3.3
Classifier: Programming Language :: Python :: 3.4
Classifier: Programming Language :: Python :: 3.5
Classifier: Programming Language :: Python :: 3.6
Classifier: Operating System :: OS Independent
Provides-Extra: test
