Stages and layers are what make USD work, it is our entry point to accessing our hierarchies.

There are also utility methods available, that are not in the Sdf.Layer/Usd.Stage namespace. We cover these in our advanced concepts in production section.

• Layers are managed via a singleton pattern: Each layer is only opened once in memory and is identified by the layer identifier.
• Layers identifiers can have two different formats:
  • Standard identifiers: Sdf.Layer.CreateNew("/file/path/or/URI/identifier.<ext(.usd/.usdc/.usda)>")
  • Anonymous identifiers: Sdf.Find('anon:<someHash(MemoryLocation)>:<customName>'
• Layers store our prim and property specs, they are the data container for all USD data that gets persistently written to file. When we want to edit layers directly, we have to use the low-level API, the high level API edits the stage, which in return forwards the edits to the layer that is set by the active edit target.
• The Sdf.FileFormat plugin interface allows us to implement plugins that convert the content of (custom) file format's to the USD's prim/property/metadata data model. This is how USD manages the USD crate (binary), alembic and vdb formats.
• USD's crate (binary) format allows layers to be lazily read and written to. Calling layer.Save() multiple times, flushes the in-memory content to disk by appending it to the .usd file, which allows us to efficiently write large layer files. This format can also read in hierarchy data without loading property value data. This way we have low IO when opening files, as the property data gets lazy loaded on demand. This is similar to how we can parse image metadata without reading the image content.

for layer in Sdf.Layer.GetLoadedLayers():
    print(layer.identifier)
# Skip anonymous layers
for layer in Sdf.Layer.GetLoadedLayers():
    if layer.anonymous:
        continue
    print(layer.identifier)

from pxr import Sdf
flippy_layer = Sdf.Layer.FindOrOpen("/opt/hfs19.5/houdini/usd/assets/rubbertoy/geo.usdc")
pig_layer = Sdf.Layer.FindOrOpen("/opt/hfs19.5/houdini/usd/assets/pig/geo.usdc")
flippy_layer.TransferContent(pig_layer)

# Add code to modify the stage.
# Use drop down menu to select examples.
#### Low Level ####
# Get: 'identifier', 'resolvedPath', 'realPath', 'fileExtension' 
# Set: 'identifier'
## Helper functions:
# Get: 'GetFileFormat', 'GetFileFormatArguments', 'ComputeAbsolutePath'
# Create: 'CreateIdentifier', 'SplitIdentifier'
### Anoymous identifiers
# Get: 'anonymous'
## Helper functions:
# Get: 'IsAnonymousLayerIdentifier'
import os
from pxr import Sdf
## Anonymous layers
layer = Sdf.Layer.CreateAnonymous() 
print(layer.identifier) # Returns: anon:0x7f8a1040ba80
layer = Sdf.Layer.CreateAnonymous("myCustomAnonLayer")
print(layer.identifier) # Returns: anon:0x7f8a10498500:myCustomAnonLayer
print(layer.anonymous, layer.resolvedPath or "-", layer.realPath or "-", layer.fileExtension) # Returns: True, "-", "-", "sdf"
print(Sdf.Layer.IsAnonymousLayerIdentifier(layer.identifier)) # Returns True
## Standard layers
layer.identifier = "/my/cool/file/path/example.usd"
print(layer.anonymous, layer.resolvedPath or "-", layer.realPath or "-", layer.fileExtension)
# Returns: False, "/my/cool/file/path/example.usd", "/my/cool/file/path/example.usd", "usd"
# When accesing an identifier string, we should always split it for args to get the URI
layer_uri, layer_args = layer.SplitIdentifier(layer.identifier)
print(layer_uri, layer_args) # Returns: "/my/cool/file/path/example.usd", {}
layer_identifier = layer.CreateIdentifier("/dir/file.usd", {"argA": "1", "argB": "test"})
print(layer_identifier) # Returns: "/dir/file.usd:SDF_FORMAT_ARGS:argA=1&argB=test"
layer_uri, layer_args = layer.SplitIdentifier(layer_identifier)
print(layer_uri, layer_args) # Returns: "/dir/file.usd", {'argA': '1', 'argB': 'test'}
# CreateNew requires the file path to be writable
layer_file_path = os.path.expanduser("~/Desktop/layer_identifier_example.usd")
layer = Sdf.Layer.CreateNew(layer_file_path, {"myCoolArg": "test"})
print(layer.GetFileFormat()) # Returns: Instance of Sdf.FileFormat
print(layer.GetFileFormatArguments()) # Returns: {'myCoolArg': 'test'}
# Get the actuall resolved path (from our asset resolver):
print(layer.identifier, layer.realPath, layer.fileExtension) # Returns: "~/Desktop/layer_identifier_example.usd" "~/Desktop/layer_identifier_example.usd" "usd"
# This is the same as:
print(layer.identifier, layer.resolvedPath.GetPathString())
# Compute a file relative to the directory of the layer identifier
print(layer.ComputeAbsolutePath("./some/other/file.usd")) # Returns: ~/Desktop/some/other/file.usd

### Low Level ###
# Create: 'New', 'CreateNew', 'CreateAnonymous', 
# Get: 'Find','FindOrOpen', 'OpenAsAnonymous',  'FindOrOpenRelativeToLayer', 'FindRelativeToLayer',
# Set: 'Save', 'TransferContent', 'Import', 'ImportFromString', 'Export', 'ExportToString'
# Clear: 'Clear', 'Reload', 'ReloadLayers'
# See all open layers: 'GetLoadedLayers'
from pxr import Sdf
layer = Sdf.Layer.CreateAnonymous()
## The .CreateNew command will check if the layer is saveable at the file location and create an empty file.
layer_file_path = os.path.expanduser("~/Desktop/layer_identifier_example.usd")
layer = Sdf.Layer.CreateNew(layer_file_path)
print(layer.dirty) # Returns: False
## Our layers are marked as "dirty" (edited) as soon as we make an edit.
prin_spec = Sdf.CreatePrimInLayer(layer, Sdf.Path("/pig"))
print(layer.dirty) # Returns: True
layer.Save()
# Only edited (dirty) layers are saved, when layer.Save() is called
# Only edited (dirty) layers are reloaded, when layer.Reload(force=False) is called.
# Our layer.Save() and layer.Reload() methods also take an optional "force" arg.
# This forces the layer to be saved. We can also call layer.Save() multiple times,
# with the USD binary format (.usd/.usdc). This will then dump the content from memory
# to disk in "append" mode. This avoids building up huge memory footprints when
# creating large layers.
## We can also transfer layer contents:
other_layer = Sdf.Layer.CreateAnonymous()
layer.TransferContent(other_layer)
# Or we import the content from another layer
layer.Import(layer_file_path)
# This is the same as:
layer.TransferContent(layer.FindOrOpen((layer_file_path)))
# We can also import/export to USD ascii representations,
# this is quite usefull for debugging and inspecting the active layer.
# layer.ImportFromString(other_layer.ExportAsString())
layer = Sdf.Layer.CreateAnonymous()
prin_spec = Sdf.CreatePrimInLayer(layer, Sdf.Path("/pig"))
print(layer.ExportToString())
# Returns:
"""
#sdf 1.4.32
over "pig"
{
}
"""
# We can also remove all prims that don't have properties/metadata
# layer.RemoveInertSceneDescription()

The most important methods are:

• layer.GetCompositionAssetDependencies(): This gets layer identifiers of sublayer/reference/payload composition arcs. This is only for the active layer, it does not run recursively.
• layer.UpdateCompositionAssetDependency("oldIdentifier", "newIdentifier"): The allows us to remap any sublayer/reference/payload identifier in the active layer, without having to edit the list-editable ops ourselves. Calling layer.UpdateCompositionAssetDependency("oldIdentifier", "") removes a layer.

### Low Level ###
# Get: 'GetCompositionAssetDependencies', 'GetAssetInfo', 'GetAssetName', 'GetExternalAssetDependencies',
# Set: 'UpdateCompositionAssetDependency', 'UpdateExternalReference', 'UpdateAssetInfo'
import os
from pxr import Sdf
HFS_env = os.environ["HFS"]
layer = Sdf.Layer.FindOrOpen(os.path.join(HFS_env, "houdini","usd","assets","pig","payload.usdc"))
# Get all sublayer, reference and payload files (These are the only arcs that can load files)
print(layer.GetCompositionAssetDependencies()) # Returns: ['./geo.usdc', './mtl.usdc']
# print(layer.GetExternalReferences(), layer.externalReferences) # The same thing, deprecated method.
# Get external dependencies for non USD file formats. We don't use this with USD files.
print(layer.GetExternalAssetDependencies()) # Returns: [] 
# Get layer asset info. Our asset resolver has to custom implement this.
# A common use case might be to return database related side car data.
print(layer.GetAssetName()) # Returns: None
print(layer.GetAssetInfo()) # Returns: None
layer.UpdateAssetInfo() # Re-resolve/refresh the asset info. This just force requeries our asset resolver query.
# The perhaps most powerful method for dependencies is:
layer.UpdateCompositionAssetDependency("oldIdentifier", "newIdentifier")
# This allows us to repath any composition arc (sublayer/reference/payload) to a new file in the active layer.
# Calling layer.UpdateCompositionAssetDependency("oldIdentifier", ""), will remove the identifier from the
# list-editable composition arc ops.

### Low Level ###
# Get: 'permissionToEdit', 'SetPermissionToEdit'
# Set: 'permissionToSave', 'SetPermissionToSave'
import os
from pxr import Sdf
layer = Sdf.Layer.CreateAnonymous()
print("Can edit layer", layer.permissionToEdit) # Returns: True
Sdf.CreatePrimInLayer(layer, Sdf.Path("/bicycle"))
# Edit permission
layer.SetPermissionToEdit(False)
try: 
    # This will now raise an error
    Sdf.CreatePrimInLayer(layer, Sdf.Path("/car"))
except Exception as e:
    print(e)
layer.SetPermissionToEdit(True)
# Save permission
print("Can save layer", layer.permissionToSave) # Returns: False
try:
    # This fails as we can't save anoymous layers
    layer.Save()
except Exception as e:
    print(e)
# Changing this on anoymous layers doesn't work
layer.SetPermissionToSave(True)
print("Can save layer", layer.permissionToSave) # Returns: False
# If we change the identifer to not be an anonymous identifer, we can save it.
layer.identifier = os.path.expanduser("~/Desktop/layerPermission.usd")
print("Can save layer", layer.permissionToSave) # Returns: True

### Low Level ###
# Get: 'IsMuted', 'GetMutedLayers'
# Set: 'AddToMutedLayers', 'RemoveFromMutedLayers'
from pxr import Sdf
layer = Sdf.Layer.CreateAnonymous()
print(Sdf.Layer.IsMuted(layer)) # Returns: False
Sdf.Layer.AddToMutedLayers(layer.identifier)
print(Sdf.Layer.GetMutedLayers()) # Returns: ['anon:0x7f8a1098f100']
print(Sdf.Layer.IsMuted(layer)) # Returns: True
Sdf.Layer.RemoveFromMutedLayers(layer.identifier)
print(Sdf.Layer.IsMuted(layer)) # Returns: False

# For sublayering we modify the .subLayerPaths attribute on a layer.
# This is the same for both the high and low level API.
### High Level & Low Level ###
from pxr import Sdf, Usd
stage = Usd.Stage.CreateInMemory()
# Layer onto root layer
layer_a = Sdf.Layer.CreateAnonymous()
layer_b = Sdf.Layer.CreateAnonymous()
root_layer = stage.GetRootLayer()
# Here we pass in the file paths (=layer identifiers).
root_layer.subLayerPaths.append(layer_a.identifier)
root_layer.subLayerPaths.append(layer_b.identifier)
# Once we have added the sublayers, we can also access their layer offsets:
print(root_layer.subLayerOffsets) # Returns: [Sdf.LayerOffset(), Sdf.LayerOffset()]
# Since layer offsets are ready only copies, we need to assign a newly created 
# layer offset if we want to modify them. We also can't replace the whole list, as
# it needs to keep a pointer to the array.
layer_offset_a = root_layer.subLayerOffsets[0]
root_layer.subLayerOffsets[0] = Sdf.LayerOffset(offset=layer_offset_a.offset + 10, 
                                                scale=layer_offset_a.scale * 2)
layer_offset_b = root_layer.subLayerOffsets[1]
root_layer.subLayerOffsets[1] = Sdf.LayerOffset(offset=layer_offset_b.offset - 10, 
                                                scale=layer_offset_b.scale * 0.5)
print(root_layer.subLayerOffsets) # Returns: [Sdf.LayerOffset(10, 2), Sdf.LayerOffset(-10, 0.5)]

# If we want to sublayer on the active layer, we just add it there.
layer_c = Sdf.Layer.CreateAnonymous()
active_layer = stage.GetEditTarget().GetLayer()
root_layer.subLayerPaths.append(layer_c.identifier)

### Low Level ###
# Properties: 'pseudoRoot', 'rootPrims', 'empty'
# Get: 'GetObjectAtPath', 'GetPrimAtPath', 'GetPropertyAtPath', 'GetAttributeAtPath', 'GetRelationshipAtPath',
# Traversal: 'Traverse',
from pxr import Sdf
layer = Sdf.Layer.CreateAnonymous()
# Check if a layer actually has any content:
print(layer.empty) # Returns: True
print(layer.pseudoRoot) # The same as layer.GetPrimAtPath("/")
# Define prims
bicycle_prim_spec = Sdf.CreatePrimInLayer(layer, Sdf.Path("/set/yard/bicycle"))
person_prim_spec = Sdf.CreatePrimInLayer(layer, Sdf.Path("/characters/mike"))
print(layer.rootPrims) # Returns: {'set': Sdf.Find('anon:0x7ff9f8ad7980', '/set'),
                       #           'characters': Sdf.Find('anon:0x7ff9f8ad7980', '/characters')}
# The GetObjectAtPath method gives us prim/attribute/relationship specs, based on what is at the path
attr_spec = Sdf.AttributeSpec(bicycle_prim_spec, "tire:size", Sdf.ValueTypeNames.Float)
attr_spec.default = 10
rel_sec = Sdf.RelationshipSpec(person_prim_spec, "vehicle")
rel_sec.targetPathList.Append(Sdf.Path(bicycle_prim_spec.path))
print(type(layer.GetObjectAtPath(attr_spec.path))) # Returns: <class 'pxr.Sdf.AttributeSpec'>
print(type(layer.GetObjectAtPath(rel_sec.path))) # Returns: <class 'pxr.Sdf.RelationshipSpec'>
# Traversals work differently compared to stages.
def traversal_kernel(path):
    print(path)
layer.Traverse(layer.pseudoRoot.path, traversal_kernel)
print("---")
# Returns:
"""
/set/yard/bicycle.tire:size
/set/yard/bicycle
/set/yard
/set
/characters/mike.vehicle[/set/yard/bicycle]
/characters/mike.vehicle
/characters/mike
/characters
/
"""
# As we can see, it traverses all path related fields, even relationships, as these map Sdf.Paths.
# The Sdf.Path object is used as a "filter", rather than the Usd.Prim object.
def traversal_kernel(path):
    if path.IsPrimPath():
        print(path) 
layer.Traverse(layer.pseudoRoot.path, traversal_kernel)
print("---")
""" Returns:
/set/yard/bicycle
/set/yard
/set
/characters/mike
/characters
"""
def traversal_kernel(path):
    if path.IsPrimPropertyPath():
        print(path) 
layer.Traverse(layer.pseudoRoot.path, traversal_kernel)
print("---")
""" Returns:
/set/yard/bicycle.tire:size
/characters/mike.vehicle
"""
tire_size_attr_spec = attr_spec
tire_diameter_attr_spec = Sdf.AttributeSpec(bicycle_prim_spec, "tire:diameter", Sdf.ValueTypeNames.Float)
tire_diameter_attr_spec.connectionPathList.explicitItems = [tire_size_attr_spec.path]
def traversal_kernel(path):
    if path.IsTargetPath():
        print(">> IsTargetPath", path) 
layer.Traverse(layer.pseudoRoot.path, traversal_kernel)
""" Returns:
IsTargetPath /set/yard/bicycle.tire:diameter[/set/yard/bicycle.tire:size]
IsTargetPath /characters/mike.vehicle[/set/yard/bicycle]
"""

from pxr import Usd
stage = Usd.Stage.CreateInMemory()
print(stage.GetInterpolationType()) # Returns: Usd.InterpolationTypeLinear
stage.SetInterpolationType(Usd.InterpolationTypeHeld)

# Get: 'GetColorConfiguration', 'GetColorManagementSystem', 'GetColorConfigFallbacks'
# Set: 'SetColorConfiguration', 'SetColorManagementSystem', 'SetColorConfigFallbacks'

from pxr import Usd, Sdf
stage = Usd.Stage.CreateInMemory()
bicycle_prim = stage.DefinePrim("/bicycle")
stage.SetMetadata("customLayerData", {"myCustomStageData": 1})
# Is the same as:
layer = stage.GetRootLayer()
metadata = layer.customLayerData
metadata["myCustomRootData"] = 1
layer.metadata = metadata
# As with layers, we can also set the default prim
stage.SetDefaultPrim(bicycle_prim)
# Is the same as:
layer.defaultPrim = "bicycle"

stage.SetEditTarget(layer)
# We can also explicitly create the edit target:
# Or
stage.SetEditTarget(Usd.EditTarget(layer))
# Or
stage.SetEditTarget(stage.GetEditTargetForLocalLayer(layer))
# These all have the same effect.

import os
from pxr import Sdf, Usd, UsdUtils
stage = Usd.Stage.CreateInMemory()
root_layer = stage.GetRootLayer()
# Create sublayers with references
bottom_layer_file_path = os.path.expanduser("~/Desktop/layer_bottom.usda")
bottom_layer = Sdf.Layer.CreateNew(bottom_layer_file_path)
top_layer_file_path = os.path.expanduser("~/Desktop/layer_top.usda")
top_layer = Sdf.Layer.CreateNew(top_layer_file_path)
root_layer.subLayerPaths.append(top_layer_file_path)
top_layer.subLayerPaths.append(bottom_layer_file_path)
stage.SetEditTarget(top_layer)
prim = stage.DefinePrim(Sdf.Path("/pig_1"), "Xform")
prim.GetReferences().AddReference("/opt/hfs19.5/houdini/usd/assets/pig/pig.usd", "/pig")
stage.SetEditTarget(bottom_layer)
prim = stage.DefinePrim(Sdf.Path("/pig_1"), "Xform")
prim.GetReferences().AddReference("/opt/hfs19.5/houdini/usd/assets/rubbertoy/rubbertoy.usd", "/rubbertoy")
# Save
stage.Save()
# Layer stack
print(stage.GetLayerStack(includeSessionLayers=False)) 
""" Returns:
[Sdf.Find('anon:0x7ff9f47b9600:tmp.usda'),
Sdf.Find('/home/lucsch/Desktop/layer_top.usda'),
Sdf.Find('/home/lucsch/Desktop/layer_bottom.usda')]
""" 
layers = set()
for layer in stage.GetLayerStack(includeSessionLayers=False):
    layers.add(layer)
    layers.update([Sdf.Layer.FindOrOpen(i) for i in layer.GetCompositionAssetDependencies()])
print(list(layers))
""" Returns:
[Sdf.Find('/opt/hfs19.5/houdini/usd/assets/rubbertoy/rubbertoy.usd'), 
Sdf.Find('/home/lucsch/Desktop/layer_top.usda'),
Sdf.Find('anon:0x7ff9f5677180:tmp.usda'),
Sdf.Find('/opt/hfs19.5/houdini/usd/assets/pig/pig.usd'),
Sdf.Find('/home/lucsch/Desktop/layer_bottom.usda')]
"""

import os
from pxr import Sdf, Usd, UsdUtils
stage = Usd.Stage.CreateInMemory()
root_layer = stage.GetRootLayer()
# Create sublayers with references
bottom_layer_file_path = os.path.expanduser("~/Desktop/layer_bottom.usda")
bottom_layer = Sdf.Layer.CreateNew(bottom_layer_file_path)
top_layer_file_path = os.path.expanduser("~/Desktop/layer_top.usda")
top_layer = Sdf.Layer.CreateNew(top_layer_file_path)
root_layer.subLayerPaths.append(top_layer_file_path)
top_layer.subLayerPaths.append(bottom_layer_file_path)
stage.SetEditTarget(top_layer)
prim = stage.DefinePrim(Sdf.Path("/pig_1"), "Xform")
prim.GetReferences().AddReference("/opt/hfs19.5/houdini/usd/assets/pig/pig.usd", "/pig")
stage.SetEditTarget(bottom_layer)
prim = stage.DefinePrim(Sdf.Path("/pig_1"), "Xform")
prim.GetReferences().AddReference("/opt/hfs19.5/houdini/usd/assets/rubbertoy/rubbertoy.usd", "/rubbertoy")
# Save
stage.Save()
# Layer stack
print(stage.GetLayerStack(includeSessionLayers=False)) 
""" Returns:
[Sdf.Find('anon:0x7ff9f47b9600:tmp.usda'),
Sdf.Find('/home/lucsch/Desktop/layer_top.usda'),
Sdf.Find('/home/lucsch/Desktop/layer_bottom.usda')]
""" 
layers = set()
for layer in stage.GetLayerStack(includeSessionLayers=False):
    layers.add(layer)
    layers.update([Sdf.Layer.FindOrOpen(i) for i in layer.GetCompositionAssetDependencies()])
print(list(layers))
""" Returns:
[Sdf.Find('/opt/hfs19.5/houdini/usd/assets/rubbertoy/rubbertoy.usd'), 
Sdf.Find('/home/lucsch/Desktop/layer_top.usda'),
Sdf.Find('anon:0x7ff9f5677180:tmp.usda'),
Sdf.Find('/opt/hfs19.5/houdini/usd/assets/pig/pig.usd'),
Sdf.Find('/home/lucsch/Desktop/layer_bottom.usda')]
"""

### High Level ###
# Create: 'CreateNew', 'CreateInMemory', 'Open', 'IsSupportedFile', 
# Set: 'Save', 'Export', 'Flatten', 'ExportToString', 'SaveSessionLayers',
# Clear: 'Reload'
import os
from pxr import Sdf, Usd
# The stage.CreateNew has multiple method signatures, these take:
# - stage root layer identifier: The stage.GetRootLayer().identifier, this is where your stage get's saved to.
# - session layer (optional): We can pass in an existing layer to use as an session layer.
# - asset resolver context (optional): We can pass in a resolver context to aid path resolution. If not given, it will call
#                                      ArResolver::CreateDefaultContextForAsset() on our registered resolvers.
# - The initial payload loading mode: Either Usd.Stage.LoadAll or Usd.Stage.LoadNone
stage_file_path = os.path.expanduser("~/Desktop/stage_identifier_example.usda")
stage = Usd.Stage.CreateNew(stage_file_path)
# The stage creation will create an empty USD file at the specified path.
print(stage.GetRootLayer().identifier) # Returns: /home/lucsch/Desktop/stage_identifier_example.usd
prim = stage.DefinePrim(Sdf.Path("/bicycle"), "Xform")
# We can also create a stage in memory, this is the same as Sdf.Layer.CreateAnonymous() and using it as a root layer
# stage = Usd.Stage.CreateInMemory("test")
# Or:
layer = Sdf.Layer.CreateAnonymous()
stage.Open(layer.identifier)
## Saving
# Calling stage.Save(), calls layer.Save() on all dirty layers that contribute to the stage.
stage.Save()
# The same as:
for layer in stage.GetLayerStack(includeSessionLayers=False):
    if not layer.anonymous and layer.dirty:
        layer.Save()
# Calling stage.SaveSessionLayers() will also save all session layers, that are not anonymous
## Flatten
# We can also flatten our layers of the stage. This merges all the data, so it should be used with care,
# as it will likely flood your RAM with large scenes. It removes all composition arcs and returns a single layer
# with the combined result
stage = Usd.Stage.CreateInMemory()
root_layer = stage.GetRootLayer()
prim = stage.DefinePrim(Sdf.Path("/bicycle"), "Xform")
sublayer = Sdf.Layer.CreateAnonymous()
root_layer.subLayerPaths.append(sublayer.identifier)
stage.SetEditTarget(sublayer)
prim = stage.DefinePrim(Sdf.Path("/car"), "Xform")
print(root_layer.ExportToString())
"""Returns:
#usda 1.0
(
    subLayers = [
        @anon:0x7ff9f5fed300@
    ]
)

def Xform "bicycle"
{
}
"""
flattened_result = stage.Flatten()
print(flattened_result.ExportToString())
"""Returns:
#usda 1.0
(
)

def Xform "car"
{
}

def Xform "bicycle"
{
}
"""
## Export:
# The export command calls the same thing we just did
# layer = stage.Flatten()
# layer.Export("/myFilePath.usd")
print(stage.ExportToString()) # Returns: The same thing as above
## Reload:
stage.Reload()
# The same as:
for layer in stage.GetUsedLayers():
    # !IMPORTANT! This does not check if the layer is anonymous,
    # so you will lose all your anon layer content.
    layer.Reload()
    # Here is a saver way:
    if not layer.anonymous:
        layer.Reload()

### High Level ###
# Get: 'GetPseudoRoot', 'GetObjectAtPath',
#      'GetPrimAtPath', 'GetPropertyAtPath','GetAttributeAtPath', 'GetRelationshipAtPath',
# Set: 'DefinePrim', 'OverridePrim', 'CreateClassPrim', 'RemovePrim'
# Traversal: 'Traverse','TraverseAll'
from pxr import Sdf, Usd, UsdUtils
stage = Usd.Stage.CreateInMemory()
# Define and change specifier
stage.DefinePrim("/changedSpecifier/definedCube", "Cube").SetSpecifier(Sdf.SpecifierDef)
stage.DefinePrim("/changedSpecifier/overCube", "Cube").SetSpecifier(Sdf.SpecifierOver)
stage.DefinePrim("/changedSpecifier/classCube", "Cube").SetSpecifier(Sdf.SpecifierClass)
# Or create with specifier
stage.DefinePrim("/createdSpecifier/definedCube", "Cube")
stage.OverridePrim("/createdSpecifier/overCube")
stage.CreateClassPrim("/createdSpecifier/classCube")
# Create attribute
prim = stage.DefinePrim("/bicycle")
prim.CreateAttribute("tire:size", Sdf.ValueTypeNames.Float)
# Get the pseudo root prim at "/"
pseudo_root_prim = stage.GetPseudoRoot()
# Or:
pseudo_root_prim = stage.GetPrimAtPath("/")
# Traverse:
for prim in stage.TraverseAll():
    print(prim)
""" Returns:
Usd.Prim(</changedSpecifier>)
Usd.Prim(</changedSpecifier/definedCube>)
Usd.Prim(</changedSpecifier/overCube>)
Usd.Prim(</changedSpecifier/classCube>)
Usd.Prim(</createdSpecifier>)
Usd.Prim(</createdSpecifier/definedCube>)
Usd.Prim(</createdSpecifier/overCube>)
Usd.Prim(</createdSpecifier/classCube>)
"""
# Get Prims/Properties
# The GetObjectAtPath returns the entity requested by the path (prim/attribute/relationship)
prim = stage.GetObjectAtPath(Sdf.Path("/createdSpecifier"))
prim = stage.GetPrimAtPath(Sdf.Path("/changedSpecifier"))
attr = stage.GetObjectAtPath(Sdf.Path("/bicycle.tire:size"))
attr = stage.GetAttributeAtPath(Sdf.Path("/bicycle.tire:size"))