Blender Python select object

Question

I am running Blender 3.0 on Ubuntu 21.10 with Python, trying to select an object I have just added but it keeps selecting the Cylinder instead of the Cube. Is that due to the order in which they are drawn?

Why does this script not select the Cube.

import bpy
bpy.ops.mesh.primitive_cube_add(enter_editmode=False, align='WORLD', location=(0, 0, 1), scale=(1, 1, 1))
bpy.ops.mesh.primitive_cylinder_add(enter_editmode=False, align='WORLD', location=(0, 0, 0), scale=(1, 1, 1))
obj = bpy.context.scene.objects["Cube"]
obj.location.z +=1

Here is updated code which as you suggested deselects everything and select activates the object I want.

import bpy
bpy.ops.mesh.primitive_cube_add(enter_editmode=False, align='WORLD', location=(0, 0, 1), scale=(1, 1, 1))
bpy.ops.mesh.primitive_cylinder_add(enter_editmode=False, align='WORLD', location=(0, 0, 0), scale=(1, 1, 1))
bpy.data.objects['Cylinder'].select_set(False); 
obj.select_set(True); 
bpy.context.view_layer.objects.active = obj

However now is doesn't seem to be highlight the cube. Thanks

By changing False to True only Cylinder is highlighted as expected.

When I change Cylinder to Cube the cube is highlighted as expected. And so I should be able to use the boolean modifier or any modifier. Thanks for your help.

Strange it is selecting both objects now. Why is this?

Answer 1

The last added object with a primitive_add operator will be selected. Invert your two operator calls and you can have access to the cube :

import bpy
bpy.ops.mesh.primitive_cylinder_add(enter_editmode=False, align='WORLD', location=(0, 0, 0), scale=(1, 1, 1))
cylinder = bpy.context.active_object
bpy.ops.mesh.primitive_cube_add(enter_editmode=False, align='WORLD', location=(0, 0, 1), scale=(1, 1, 1))
cube = bpy.context.active_object
bpy.context.active_object.location.z += 1 # same as cube.location.z += 1
print(cylinder)
print(cube)

<bpy_struct, Object("Cylinder") at 0x00000206999BE408>

<bpy_struct, Object("Cube") at 0x000002069A0A0108>

Edit : To select one or the other :

def select_one_object(obj):
    bpy.ops.object.select_all(action='DESELECT')
    bpy.context.view_layer.objects.active = obj
    obj.select_set(True)
select_one_object(cylinder)  # This will select the cylinder and set it as active
select_one_object(cube)  # This will select the cube and set it as active
bpy.context.view_layer.objects.active = None # After that you won't have an active object in the scene. Note it doesn't deselect the active object though.

Note there can be zero, one or more selected objects (yellow outline or orange outline), but there can only be zero or one active object (yellow outline). The active object can be unselected but still remain active. eg. when you click in the 3D viewport on an empty space, objects are unselected but the active object still has a yellow dot.

Answer 2

This code does what you want:

import bpy
bpy.ops.mesh.primitive_cube_add(enter_editmode=False, align='WORLD', location=(0, 0, 1), scale=(1, 1, 1)) 
bpy.ops.mesh.primitive_cylinder_add(enter_editmode=False, align='WORLD', location=(0, 0, 0), scale=(1, 1, 1)) 
obj = bpy.context.scene.objects["Cube"]
bpy.data.objects['Cylinder'].select_set(False)
obj.select_set(True)
bpy.context.view_layer.objects.active = obj 

NOTE: But...you have to delete the two objects (if they exist) with the names "Cylinder" and "Cube" before starting that script because it hard coded the name "Cube" and "Cylinder" which is...pretty bad coding.

Answer 3

If you create an object with bpy.ops, it will become "active" after creation.

If you register the object in a variable when it is active, the next time you want to access the object, you can directly access the variable (data block) to manipulate the object and make it active again.

If you want to use "bpy.ops・・・・", you will need to make the object active once.

import bpy
If you create an object with bpy.ops, it will become "active" after creation.
bpy.ops.mesh.primitive_cube_add(enter_editmode=False, align='WORLD', location=(0, 0, 1), scale=(1, 1, 1))
"bpy.context.object" stores the object in the active state in a variable.
add_obj1 = bpy.context.object
Similarly, "bpy.context.object" stores the object in the active state in a variable.
bpy.ops.mesh.primitive_cylinder_add(enter_editmode=False, align='WORLD', location=(0, 0, 0), scale=(1, 1, 1)) 
add_obj2 = bpy.context.object
If you want to change the location of the Cube.
add_obj1.location = (5.0, 5.0, 5.0)
If you want to change the location of the cylinder.
add_obj2.location = (-5.0, 5.0, 5.0)

---

How to select it after the script

Sometimes you will want to select a specific object again after the script. In this case, you can select the object from the name of the object in the scene. In this case, you can select the object by its name, or you can use the RNA and ID properties to identify it.

Here's a point. If you select an object later by name If you change the name while you are working on it, you will not know how to set the object again. This is quite common.

With "Custom Properties", you can set the object in a way that does not depend on its name, With custom properties, the setting method is independent of the name. You may want to keep this method in mind if you want to select a particular object again later. You may want to keep this in mind.

Example

1. 1 is selected
2. 2 is selected and active state
3. 0 is unselected
4. And so on...

I have written four scripts.

Unselected script

bool_False.py

import bpy
def bool_False():
        bpy.types.Object.my_rna_bool = bpy.props.IntProperty(name='RNA bool')
        bpy.context.active_object.my_rna_bool = False
        layer = bpy.context.view_layer
        layer.update()
bool_False()

The script to make it selected.

bool_true.py

import bpy
def bool_true():
    bpy.types.Object.my_rna_bool = bpy.props.IntProperty(name='RNA bool')
    bpy.context.active_object.my_rna_bool = True
    layer = bpy.context.view_layer
    layer.update()
bool_true()

Script to select and make active

bool_active_true.py

import bpy
def bool_active_true():
    bpy.types.Object.my_rna_bool = bpy.props.IntProperty(name='RNA bool')
    bpy.context.active_object.my_rna_bool = 2
    layer = bpy.context.view_layer
    layer.update()
bool_active_true()

Script to output the result of the above configuration

select_set_active.py

import bpy
def select_set_active():
    for obj in bpy.context.scene.objects:
        print(obj.name, obj.get("my_rna_bool"))
        obj_bool = obj.get("my_rna_bool")
        if obj_bool == 2:
            # If there is more than one 2 in the value of the Int, the last one in the array will be active.
            bpy.context.view_layer.objects.active = obj 
            obj.select_set(True)
    elif obj_bool == 1:
        # select only.
        obj.select_set(True)

    else:

        obj.select_set(False)
        pass 


select_set_active()

Script results