def merge_traits(original, to_merge):
    # Add or remove traits in to_merge from original
    # traits are prefixed with + if they must be asserted, or - if
    # they must be removed.
    traits_add = set()
    traits_rem = set()

    for trait in to_merge:
        if trait.startswith('+'):
            traits_add.add(trait[1:])
        else:
            traits_rem.add(trait[1:])

    return (set(original) | traits_add) - traits_rem



# Zero day, no traits are set
traits = set()

# Virt driver exposes these traits
# We support VMX and LM cpu flags, but we're not new enough to
# have AVX2
virt_traits = ['+TRAIT_CPU_VMX', '+TRAIT_CPU_LM', '-TRAIT_CPU_AVX2']

# Compute exposes these traits
# We have determined that we have shared storage, but we don't have
# volume support
compute_traits = ['+TRAIT_SHARED_STORAGE', '-TRAIT_SUPPORTS_VOLUMES']

# Compute update periodic runs
traits = merge_traits(traits, virt_traits)
traits = merge_traits(traits, compute_traits)
print(traits)

# Operator decides to set some traits on the provider
operator_traits = ['+TRAIT_TRUSTED', '+TRAIT_CUSTOM_MYTHING']

traits = merge_traits(traits, operator_traits)
print(traits)

# Compute update periodic runs
traits = merge_traits(traits, virt_traits)
traits = merge_traits(traits, compute_traits)
print(traits)

# Operator thinks he can manipulate CPU flag traits
traits -= set(['TRAIT_CPU_VMX'])
traits |= set(['TRAIT_CPU_AVX2'])
print('%s (bad operator)' % traits)

# Compute update periodic runs
# Virt will overwrite what the operator did for those
# traits only, because it knows better
traits = merge_traits(traits, virt_traits)
traits = merge_traits(traits, compute_traits)
print(traits)

# Virt driver determines it now supports AVX2 because nova.conf changed
virt_traits += '+TRAIT_CPU_AVX2'

# Compute update periodic runs
traits = merge_traits(traits, virt_traits)
traits = merge_traits(traits, compute_traits)
print(traits)

# Compute learns it no longer supports shared storage
compute_traits[0] = '-TRAIT_SHARED_STORAGE'

# Compute update periodic runs
traits = merge_traits(traits, virt_traits)
traits = merge_traits(traits, compute_traits)
print(traits)