add blendshape proc because the maya ui for this is obtuse and i always fuck it up like an idiot

blendshape.py

import pymel.core as pm

def get_blendShape(target):
    """Get blendshape of target."""
    if (pm.nodeType(target.getShape()) in ["mesh", "nurbsSurface", "nurbsCurve"]):
        blendShape = pm.listHistory(target, type="blendShape")[0]
    return blendShape 

def add_blendShape(new_shape, blend_mesh):
    """
    Add blendshape target to mesh with existing blendShape deformer

    Parameters
    ----------
    new_shape : mesh, nurbsSurface, nurbsCurve
    blend_mesh : mesh, nurbsSurface, nurbsCurve

    """
    #pm.select(blend_mesh)
    blendShape_node = get_blendShape(blend_mesh)
    new_index = blendShape_node.numWeights()
    print(new_index)
    try:
        pm.blendShape(blendShape_node, edit=True, target=(blend_mesh, new_index, new_shape, 1.0))
    except:
        pm.warning('not working')
    return


####====    USAGE   ====####

#first selected is the new shape, second is the target mesh with a blendshape to add to
new_shape, blend_mesh = pm.ls(sl=1)[0:2]
add_blendShape(new_shape, blend_mesh)

Happy to help and entertain :)