It should be easier to modify pass managers.

[nonhermitian]

What is the expected enhancement?

Currently, if you want to use a default pass manager with only a single pass changed, e.g. for example the swap mapper changed, you need to go and create a whole new pass manager. It should be possible to simply replace one pass in a pass manger with another. Something like:

pm = default_pass_manager()
pm.replace(6, LookaheadSwap) # where 6 is the index of the pass in the pm

An index is used here since you could have multiple passes of the same type. It would also be nice to be able to slice a pm and get a new pm, e.g. pm2 = pm[:5] which would save people from having to rewrite things like this:

pass_manager = PassManager()
pass_manager.property_set['layout'] = initial_layout
pass_manager.append(Unroller(basis_gates))
pass_manager.append(TrivialLayout(coupling_map),
                        condition=lambda property_set: not property_set['layout'])
pass_manager.append(CheckMap(coupling_map))
pass_manager.append(DenseLayout(coupling_map),
                        condition=lambda property_set: not property_set['is_swap_mapped'])
pass_manager.append(FullAncillaAllocation(coupling_map))
pass_manager.append(EnlargeWithAncilla())

[jaygambetta]

I agree.

I also think I should be able to draw a pass manager

pm.draw()

and it make a flow diagram of all the passes I have in my pass manager with the inputs going into them