blender-compositing

Enable compositor

scene = bpy.context.scene
scene.use_nodes = True                # harmless on all versions
scene.render.use_compositing = True   # authoritative current toggle; set both for broad compat

tree = scene.node_tree
# Clean existing nodes (start fresh)
for n in list(tree.nodes):
    tree.nodes.remove(n)

Create and connect nodes

# Create
rl   = tree.nodes.new("CompositorNodeRLayers")    # render layer source
blur = tree.nodes.new("CompositorNodeBlur")

# Connect sockets by name
tree.links.new(rl.outputs["Image"], blur.inputs["Image"])

NodeLinks.new(from_socket, to_socket) — both sockets required. handle_dynamic_sockets=False optional param for dynamic socket types.

Node reference

Purpose	bl_idname	Key properties / inputs
Render layer source	CompositorNodeRLayers	outputs: "Image", "Alpha"
External image source	CompositorNodeImage	node.image = img_datablock
Blur	CompositorNodeBlur	size_x, size_y, filter_type (enum); inputs: "Image", "Size"
Rotate	CompositorNodeRotate	`filter_type: 'NEAREST'
Translate	CompositorNodeTranslate	input sockets "X", "Y" (pixels)
Scale	CompositorNodeScale	`frame_method: 'STRETCH'
Alpha Over	CompositorNodeAlphaOver	use_premultiply; input "Fac" for factor
File Output	CompositorNodeOutputFile	see version compat below
Composite output	CompositorNodeComposite	final output — pairs with render(write_still=True)

Set node properties and socket inputs

# Properties on the node
if hasattr(blur, "size_x"):  blur.size_x = 8
if hasattr(blur, "size_y"):  blur.size_y = 8
if hasattr(rot, "filter_type"):  rot.filter_type = 'BILINEAR'

# Socket-driven values
if "Angle" in rot.inputs:   rot.inputs["Angle"].default_value = math.radians(4.0)
if "X" in trn.inputs:       trn.inputs["X"].default_value = 24.0
if "Y" in trn.inputs:       trn.inputs["Y"].default_value = -18.0

File Output node — version compat

fout = tree.nodes.new("CompositorNodeOutputFile")

# Directory field changed between versions:
if hasattr(fout, "base_path"):
    fout.base_path = outdir      # older Blender
elif hasattr(fout, "directory"):
    fout.directory = outdir      # current Blender

# Connect to first input slot (safe cross-version)
tree.links.new(last_node.outputs["Image"], fout.inputs[0])

Avoid multi-output socket creation unless needed — the default slot is stable across versions.

Minimal post-process pipeline

import math, bpy

scene = bpy.context.scene
scene.use_nodes = True
scene.render.use_compositing = True
tree = scene.node_tree
for n in list(tree.nodes): tree.nodes.remove(n)

rl   = tree.nodes.new("CompositorNodeRLayers")
blur = tree.nodes.new("CompositorNodeBlur")
comp = tree.nodes.new("CompositorNodeComposite")

if hasattr(blur, "size_x"): blur.size_x = 4
if hasattr(blur, "size_y"): blur.size_y = 4

tree.links.new(rl.outputs["Image"],   blur.inputs["Image"])
tree.links.new(blur.outputs["Image"], comp.inputs["Image"])

bpy.ops.render.render(write_still=True)   # compositor runs automatically