basic.py 文件源码

def _atomic(e):
    """Return atom-like quantities as far as substitution is
    concerned: Derivatives, Functions and Symbols. Don't
    return any 'atoms' that are inside such quantities unless
    they also appear outside, too.

    Examples
    ========

    >>> from sympy import Derivative, Function, cos
    >>> from sympy.abc import x, y
    >>> from sympy.core.basic import _atomic
    >>> f = Function('f')
    >>> _atomic(x + y)
    {x, y}
    >>> _atomic(x + f(y))
    {x, f(y)}
    >>> _atomic(Derivative(f(x), x) + cos(x) + y)
    {y, cos(x), Derivative(f(x), x)}

    """
    from sympy import Derivative, Function, Symbol
    pot = preorder_traversal(e)
    seen = set()
    try:
        free = e.free_symbols
    except AttributeError:
        return {e}
    atoms = set()
    for p in pot:
        if p in seen:
            pot.skip()
            continue
        seen.add(p)
        if isinstance(p, Symbol) and p in free:
            atoms.add(p)
        elif isinstance(p, (Derivative, Function)):
            pot.skip()
            atoms.add(p)
    return atoms