Load a yaml stream (src) and merge it with another yaml stream (dst) allowing *dst* to use aliases defined in *src*.

merge_load.py

import sys
import yaml

def merge_load(src, dst, loadfunc=yaml.load, **kwargs):
    '''\
    Load a yaml stream (src) and merge it with another yaml stream
    (dst) allowing *dst* to use aliases defined in *src*.

    Other Parameters
    ----------------
    **kwargs
        Keyword arguments to pass to loadfunc
    '''
    # For convenience, allow dst to be None
    if dst is None: return loadfunc(src, **kwargs)

    srcanchors = { x.anchor: x for x in yaml.parse(src)
                   if getattr(x, 'anchor', None) is not None}

    dstanchors = { x.value for x in yaml.scan(dst)
                   if isinstance(x, yaml.tokens.AnchorToken) }

    dstevents = []
    for event in yaml.parse(dst):
        if not isinstance(event, yaml.events.AliasEvent) \
                or event.anchor in dstanchors \
                or event.anchor not in srcanchors:
            dstevents.append(event)
            continue
        dstevents.append(srcanchors[event.anchor])
        dstevents[-1].anchor = None

    override = loadfunc(yaml.emit(dstevents), **kwargs)
    dst = loadfunc(src, **kwargs)
    if override:
        dst.update(override)
    return dst

print merge_load(sys.argv[1], sys.argv[2])