USD Python API import layer into an existing layer at given path

usd_import_layer.py

from collections.abc import Callable
from pxr import Sdf


def import_layer(
    path: str, 
    target_layer: Sdf.Layer, 
    target_path: Sdf.Path,
    copy_spec_fn: Callable[[Sdf.Layer, Sdf.Path, Sdf.Layer, Sdf.Path], bool] = Sdf.CopySpec
):
    """Merge layer into target layer at path using Sdf.CopySpec.
    
    You can pass in a custom `copy_spec_fn` to allow different copying
    behavior where e.g. existing child prims are not removed but merged
    into instead.
    
    """
    layer = Sdf.Layer.FindOrOpen(path)
    
    if not target_layer.GetPrimAtPath(target_path):
        Sdf.CreatePrimInLayer(target_layer, target_path)
    
    for root_prim in layer.rootPrims:
        root_prim_path = Sdf.Path(f"/{root_prim.name}")
        target_prim_path = target_path.AppendChild(root_prim.name)
        copy_spec_fn(layer, root_prim_path, target_layer, target_prim_path)


# Example usage
target = Sdf.Layer.CreateAnonymous()
path = Sdf.Path("/hello/world")
import_layer(r"C:\Users\User\Desktop\asset.usd", target, path)

To perform more like a merge instead of a full "replace" of target prims you might need to specify the custom copy_spec_fn:

import_layer("file.usd", target_layer, Sdf.Path("/preset"), copy_spec_fn=copy_spec_merge)

For example using copy_spec_merge from usd_qtpy