util.py

r"""
Utility functions for building Sage
"""

# ****************************************************************************
#       Copyright (C) 2017 Jeroen Demeyer <[email protected]>
#
# 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 2 of the License, or
# (at your option) any later version.
#                  https://www.gnu.org/licenses/
# ****************************************************************************


def stable_uniq(L) -> list:
    """
    Given an iterable L, remove duplicate items from L by keeping only
    the last occurrence of any item.

    The items must be hashable.

    EXAMPLES::

        sage: from sage_setup.util import stable_uniq
        sage: stable_uniq( (1, 2, 3, 4, 5, 6, 3, 7, 5, 1, 5, 9) )
        [2, 4, 6, 3, 7, 1, 5, 9]
    """
    D = {}
    for pos, item in enumerate(L):
        D[item] = pos  # Store the last position where an item appears
    return sorted(D, key=lambda item: D[item])


def have_module(name) -> bool:
    """
    Check whether a Python module named ``name`` can be imported.

    This is done by trying to import that module and returning ``True``
    if that import succeeded. So, as a side effect, the module is
    actually imported if possible.

    EXAMPLES::

        sage: from sage_setup.util import have_module
        sage: have_module("itertools")
        True
        sage: have_module("sage.rings.integer")
        True
        sage: have_module("no.such.module")
        False
    """
    try:
        __import__(name, {}, {}, [], 0)
        return True
    except ImportError:
        return False