bpy.ops.mesh.fill() Warning: No edges selected

Question

I'm attempting to write a background script to process an obj file. It's a single mesh with a hole at both ends; I want to fill in those holes with triangles for rendering purposes.

The shape is along the Y axis, with holes at X=0 and X=[largest X]. I appear to be able to find and select those edges, but bpy.ops.mesh.fill() continues to give a "No edges selected" error. I've tried using bmesh, and also bpy.ops.mesh.loop_multi_select(ring=False) (in every combination!) but nothing seems to make any difference. I'm about to give up and write the fill manually. My code so far is as follows:

import sys
import bpy
import bmesh

argv = sys.argv
argv = argv[argv.index("--") + 1:]  # get all args after "--"
filename = argv[0]
filenameOut = argv[1]

#Delete the cube mesh
bpy.ops.object.mode_set(mode='EDIT')
bpy.ops.mesh.select_all(action='SELECT')
bpy.ops.mesh.delete(type='VERT')

#Import object:
status = bpy.ops.import_scene.obj(filepath=filename)

#Deselect everything
bpy.ops.object.mode_set(mode='OBJECT')
bpy.ops.object.select_all(action='DESELECT')
bpy.ops.object.mode_set(mode='EDIT')
bpy.ops.mesh.select_all(action="DESELECT")
bpy.ops.object.mode_set(mode='OBJECT')

mesh=bpy.data.objects[0].data
EPSILON = 1.0e-5

#Find the highest X coordinate
maxX=0
for vert in mesh.vertices:
    if vert.co[0]>maxX:
        maxX=vert.co[0]

print(maxX)

i=0
for edge in mesh.edges:
    v1=mesh.vertices[edge.vertices[0]]
    v2=mesh.vertices[edge.vertices[1]]
    if -EPSILON <= v1.co[0] <= EPSILON and -EPSILON <= v2.co[0] <= EPSILON:
        i=i+1
        edge.select = True
        v1.select = True
        v2.select = True
print(i)

i=0;
for edge in mesh.edges:
    v1=mesh.vertices[edge.vertices[0]]
    v2=mesh.vertices[edge.vertices[1]]
    if maxX-EPSILON <= v1.co[0] <= maxX+EPSILON and maxX-EPSILON <= v2.co[0] <= maxX+EPSILON:
        i=i+1
        edge.select = True
        v1.select = True
        v2.select = True
print(i)

bpy.ops.object.mode_set(mode='EDIT')

#Just checking...
i=0
for edge in mesh.edges:
    if edge.select:
        i=i+1
print(i)

print("a")
bpy.ops.mesh.fill()
print("b")

bpy.ops.mesh.select_all(action='SELECT')
bpy.ops.mesh.merge()

bpy.ops.export_scene.obj(filepath=filenameOut)

Output is as follows:

[importing file OK]
1.587500810623169
164
164
328
a
Warning: No edges selected
Warning: No edges selected
b
[exporting file]

Uh. What? I could give the bmesh code as well, but it too seems to successfully select edges... only for the fill() to give the same error.

Answer 1

A bmesh approach.

Used a purely object mode bmesh approach. Selects all boundary edges, then uses the bmesh "F" tool, the operator result = bmesh.ops.contextual_create(...) to fill the holes, and poked the resultant faces, bmesh.ops.poke(bm, faces=result\["faces"\]) to give a triangular fan from the edges. Tested script below on selected endless cylinder(s).

import bpy
import bmesh
context = bpy.context
scene = context.scene
# completely remove cube if in scene
cube = scene.objects.get("Cube")
if cube:
    bpy.data.objects.remove(cube, do_unlink=True)
bm = bmesh.new()
# import from obj file here, only imported objects will be 
# in context.selected_objects after import.    
for o in context.selected_objects:
    # test here to continue if not mesh, can obj import other???
    # load bmesh
    bm.from_mesh(o.data)
    # find boundaries
    bound_edges = set(e for e in bm.edges if e.is_boundary)
    # used sets, may not be required.
    zero_edges = set(e for e in bound_edges 
        if all(abs(v.co.x) < 0.001 for v in e.verts))
    right_edges = bound_edges - zero_edges
    # use bmesh "F" tool
    result = bmesh.ops.contextual_create(bm, geom=list(bound_edges))
    # poke the resulting faces to make triangular fan.
    bmesh.ops.poke(bm, faces=result["faces"])
    bm.to_mesh(o.data)
    o.data.update() # mainly only need for UI really
    bm.clear()
bm.free()

Answer 2

I believe it's because I hadn't marked the object as active. I added

scn = bpy.context.scene
scn.objects.active = bpy.data.objects[0]

before doing the fill, and that seemed to fix the error. Of course, the fill was completely ... not what I'd expected so I'm persevering with a manual one for now!