def merge_dicts(d1, d2):
    """Safely merges the contents of the dictionary d2 into dictionary d1.

    Unlike dict.update(), this is a non-destructive operation: a KeyError is
    raised if one of d2's keys already exists in d1.
    """
    # Based on timing tests, it's faster to scan for conflicts upfront and
    # then perform a bulk update.  This presumes that, generally speaking,
    # the destination dictionary d1 will be larger (in item count) than d2.
    for key in d2.iterkeys():
        if key in d1:
            raise KeyError("destination dict already contains key '%s'" % key)
    d1.update(d2)