Principled BSDF via Python API

Question

The title is general: how can we use the API to edit Principled BSDF properties?

More specifically, I have a mesh that has vertex colors in it. The colors are fine, and I can use the mouse to have vertex colors as base color using the Shader Editor. Likewise, I can use the Material Properties and select "Vertex Colors | Color" for the base color of the material assigned to the mesh. This is all fine and both are equivalent ways.

However, how to accomplish that programmatically, via the API? I could do that in 2.7x but don't seem to be able to do the same in 2.81. Any idea?

EDIT: here a minimal example that works with 2.7x:

import bpy
mat = "MyMaterial"
bpy.ops.material.new()
setattr(bpy.data.materials["Material"],"name",mat)
setattr(bpy.data.materials[mat],"use_vertex_color_paint",True)

Traceback (most recent call last):
  File "<blender_console>", line 1, in <module>
AttributeError: 'Material' object has no attribute 'use_vertex_color_paint'
```

Answer 1

Add and link the vertex colour node.

Adds a new material using mat = bpy.data.materials.new()

Recommend: almost never use bpy.ops.materials.new() The material bpy.data.materials[mat] in script in question is confusing and varies dependant on pre existing materials "Material" and "MyMaterial".

Notice I used the index of the io when making links. Could use vcol.outputs["Color"] for vcol.outputs[0], however if the user is using a different language, then "Color" will be in that language.

The principled bsdf base color is input 0. A simple test in python console to save counting by eye.

>>> for i, o in enumerate(bsdf.inputs):
...     i, o.name
...     
(0, 'Base Color')
(1, 'Subsurface')
(2, 'Subsurface Radius')
(3, 'Subsurface Color')
(4, 'Metallic')
(5, 'Specular')
(6, 'Specular Tint')
(7, 'Roughness')
(8, 'Anisotropic')
(9, 'Anisotropic Rotation')
(10, 'Sheen')
(11, 'Sheen Tint')
(12, 'Clearcoat')
(13, 'Clearcoat Roughness')
(14, 'IOR')
(15, 'Transmission')
(16, 'Transmission Roughness')
(17, 'Emission')
(18, 'Alpha')
(19, 'Normal')
(20, 'Clearcoat Normal')
(21, 'Tangent')

Script to add material, add vertex colour node, link it to base color of bsdf.

import bpy

newmat = bpy.data.materials.new("VertCol")
newmat.use_nodes = True
node_tree = newmat.node_tree
nodes = node_tree.nodes

bsdf = nodes.get("Principled BSDF") 
'''
# alternatively

output, bsdf = nodes
'''


assert(bsdf) # make sure it exists to continue

vcol = nodes.new(type="ShaderNodeVertexColor")
vcol.layer_name = "VColor" # the vertex color layer name

# make links

node_tree.links.new(vcol.outputs[0], bsdf.inputs[0])

Answer 2

For people coming after Blender 4.0 update, here's the updated version -

(0, 'Base Color')
(1, 'Metallic')
(2, 'Roughness')
(3, 'IOR')
(4, 'Alpha')
(5, 'Normal')
(6, 'Weight')
(7, 'Subsurface Weight')
(8, 'Subsurface Radius')
(9, 'Subsurface Scale')
(10, 'Subsurface IOR')
(11, 'Subsurface Anisotropy')
(12, 'Specular IOR Level')
(13, 'Specular Tint')
(14, 'Anisotropic')
(15, 'Anisotropic Rotation')
(16, 'Tangent')
(17, 'Transmission Weight')
(18, 'Coat Weight')
(19, 'Coat Roughness')
(20, 'Coat IOR')
(21, 'Coat Tint')
(22, 'Coat Normal')
(23, 'Sheen Weight')
(24, 'Sheen Roughness')
(25, 'Sheen Tint')
(26, 'Emission Color')
(27, 'Emission Strength')

You're welcome!