"""Tools to assist importing optional external modules."""

import sys

# Override these in the module to change the default warning behavior.
# For example, you might set both to False before running the tests so that
# warnings are not printed to the console, or set both to True for debugging.

WARN_NOT_INSTALLED = None # Default is False
WARN_OLD_VERSION = None # Default is True

def __sympy_debug():
    # helper function from sympy/__init__.py
    # We don't just import SYMPY_DEBUG from that file because we don't want to
    # import all of sympy just to use this module.
    import os
    return eval(os.getenv('SYMPY_DEBUG', 'False'))

if __sympy_debug():
    WARN_OLD_VERSION = True
    WARN_NOT_INSTALLED = True

def import_module(module, min_module_version=None, min_python_version=None,
        warn_not_installed=None, warn_old_version=None,
        module_version_attr='__version__', module_version_attr_call_args=None,
        __import__kwargs={}, catch=()):
    """
    Import and return a module if it is installed.

    If the module is not installed, it returns None.

    A minimum version for the module can be given as the keyword argument
    min_module_version.  This should be comparable against the module version.
    By default, module.__version__ is used to get the module version.  To
    override this, set the module_version_attr keyword argument.  If the
    attribute of the module to get the version should be called (e.g.,
    module.version()), then set module_version_attr_call_args to the args such
    that module.module_version_attr(*module_version_attr_call_args) returns the
    module's version.

    If the module version is less than min_module_version using the Python <
    comparison, None will be returned, even if the module is installed. You can
    use this to keep from importing an incompatible older version of a module.

    You can also specify a minimum Python version by using the
    min_python_version keyword argument.  This should be comparable against
    sys.version_info.

    If the keyword argument warn_not_installed is set to True, the function will
    emit a UserWarning when the module is not installed.

    If the keyword argument warn_old_version is set to True, the function will
    emit a UserWarning when the library is installed, but cannot be imported
    because of the min_module_version or min_python_version options.

    Note that because of the way warnings are handled, a warning will be
    emitted for each module only once.  You can change the default warning
    behavior by overriding the values of WARN_NOT_INSTALLED and WARN_OLD_VERSION
    in sympy.external.importtools.  By default, WARN_NOT_INSTALLED is False and
    WARN_OLD_VERSION is True.

    This function uses __import__() to import the module.  To pass additional
    options to __import__(), use the __import__kwargs keyword argument.  For
    example, to import a submodule A.B, you must pass a nonempty fromlist option
    to __import__.  See the docstring of __import__().

    This catches ImportError to determine if the module is not installed.  To
    catch additional errors, pass them as a tuple to the catch keyword
    argument.

    Examples
    ========

    >>> from sympy.external import import_module

    >>> numpy = import_module('numpy')

    >>> numpy = import_module('numpy', min_python_version=(2, 6),
    ... warn_old_version=False)

    >>> numpy = import_module('numpy', min_module_version='1.5',
    ... warn_old_version=False) # numpy.__version__ is a string

    >>> # gmpy does not have __version__, but it does have gmpy.version()

    >>> gmpy = import_module('gmpy', min_module_version='1.14',
    ... module_version_attr='version', module_version_attr_call_args=(),
    ... warn_old_version=False)

    >>> # To import a submodule, you must pass a nonempty fromlist to
    >>> # __import__().  The values do not matter.
    >>> p3 = import_module('mpl_toolkits.mplot3d',
    ... __import__kwargs={'fromlist':['something']})

    >>> # matplotlib.pyplot can raise RuntimeError when the display cannot be opened
    >>> matplotlib = import_module('matplotlib',
    ... __import__kwargs={'fromlist':['pyplot']}, catch=(RuntimeError,))

    """
    # keyword argument overrides default, and global variable overrides
    # keyword argument.
    warn_old_version = (WARN_OLD_VERSION if WARN_OLD_VERSION is not None
        else warn_old_version or True)
    warn_not_installed = (WARN_NOT_INSTALLED if WARN_NOT_INSTALLED is not None
        else warn_not_installed or False)

    import warnings

    # Check Python first so we don't waste time importing a module we can't use
    if min_python_version:
        if sys.version_info < min_python_version:
            if warn_old_version:
                warnings.warn("Python version is too old to use %s "
                    "(%s or newer required)" % (module, '.'.join(map(str, min_python_version))),
                    UserWarning)
            return

    # PyPy 1.6 has rudimentary NumPy support and importing it produces errors, so skip it
    if module == 'numpy' and '__pypy__' in sys.builtin_module_names:
        return

    try:
        mod = __import__(module, **__import__kwargs)
    except ImportError:
        if warn_not_installed:
            warnings.warn("%s module is not installed" % module, UserWarning)
        return
    # TODO: After 2.5 is dropped, use new 'as' keyword
    #except catch as e:
    except catch, e:
        if warn_not_installed:
            warnings.warn("%s module could not be used (%s)" % (module, repr(e)))
        return

    if min_module_version:
        modversion = getattr(mod, module_version_attr)
        if module_version_attr_call_args is not None:
            modversion = modversion(*module_version_attr_call_args)
        if modversion < min_module_version:
            if warn_old_version:
                # Attempt to create a pretty string version of the version
                if isinstance(min_module_version, basestring):
                    verstr = min_module_version
                elif isinstance(min_module_version, (tuple, list)):
                    verstr = '.'.join(map(str, min_module_version))
                else:
                    # Either don't know what this is.  Hopefully
                    # it's something that has a nice str version, like an int.
                    verstr = str(min_module_version)
                warnings.warn("%s version is too old to use "
                    "(%s or newer required)" % (module, verstr),
                    UserWarning)
            return

    return mod
