doImport.py

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# This file is part of utils.
#
# Developed for the LSST Data Management System.
# This product includes software developed by the LSST Project
# (http://www.lsst.org).
# See the COPYRIGHT file at the top-level directory of this distribution
# for details of code ownership.
#
# This program 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.
#
# This program 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 this program.  If not, see <http://www.gnu.org/licenses/>.
 
 
import importlib
 
__all__ = ("doImport",)
 
 
def doImport(importable):
    """Import a python object given an importable string and return it.
 
    Parameters
    ----------
    importable : `str`
        String containing dot-separated path of a Python class, module,
        or member function.
 
    Returns
    -------
    type : `type`
        Type object. Either a module or class or a function.
 
    Raises
    ------
    TypeError
        ``importable`` is not a `str`.
    ModuleNotFoundError
        No module in the supplied import string could be found.
    ImportError
        ``importable`` is found but can not be imported or the requested
        item could not be retrieved from the imported module.
    """
    if not isinstance(importable, str):
        raise TypeError(f"Unhandled type of importable, val: {importable}")
 
    def tryImport(module, fromlist, previousError):
        pytype = importlib.import_module(module)
        # Can have functions inside classes inside modules
        for f in fromlist:
            try:
                pytype = getattr(pytype, f)
            except AttributeError:
                extra = f"({previousError})" if previousError is not None else ""
                raise ImportError(f"Could not get attribute '{f}' from '{module}' {extra}")
        return pytype
 
    # Go through the import path attempting to load the module
    # and retrieve the class or function as an attribute. Shift components
    # from the module list to the attribute list until something works.
    moduleComponents = importable.split(".")
    infileComponents = []
    previousError = None
 
    while moduleComponents:
        try:
            pytype = tryImport(".".join(moduleComponents), infileComponents, previousError)
            if not infileComponents and hasattr(pytype, moduleComponents[-1]):
                # This module has an attribute with the same name as the
                # module itself (like doImport.doImport, actually!).
                # If that attribute was lifted to the package, we should
                # return the attribute, not the module.
                try:
                    return tryImport(".".join(moduleComponents[:-1]), moduleComponents[-1:], previousError)
                except ModuleNotFoundError:
                    pass
            return pytype
        except ModuleNotFoundError as e:
            previousError = str(e)
            # Move element from module to file and try again
            infileComponents.insert(0, moduleComponents.pop())
 
    raise ModuleNotFoundError(f"Unable to import {importable}")