Example of how context managers work for mesh editing in Blender

final_context_advanced.py

import contextlib

import bpy
import bmesh


class Mode:
    Edit = "EDIT_MESH"
    Object = "OBJECT"


@contextlib.contextmanager
def get_bmesh_from_edit_mode(mesh_obj):
    # create a new bmesh and initialize it from mesh data in Edit Mode
    bm = bmesh.from_edit_mesh(mesh_obj.data)

    yield bm

    bm.normal_update()

    # writes the bmesh data into the mesh data in Edit Mode
    bmesh.update_edit_mesh(mesh_obj.data)

    # clean up/free memory that was allocated for the bmesh
    bm.free()


@contextlib.contextmanager
def get_bmesh_from_object_mode(mesh_obj):
    # create a new bmesh
    bm = bmesh.new()

    # initialize the bmesh data using the mesh data
    bm.from_mesh(mesh_obj.data)

    yield bm

    bm.normal_update()

    # writes the bmesh data into the mesh data
    bm.to_mesh(mesh_obj.data)

    # [Optional] update the mesh data (helps with redrawing the mesh in the viewport)
    mesh_obj.data.update()

    # clean up/free memory that was allocated for the bmesh
    bm.free()


@contextlib.contextmanager
def get_bmesh(mesh_obj):
    if bpy.context.mode not in (Mode.Object, Mode.Edit):
        raise RuntimeError(f"{bpy.context.mode} mode is not supported for creating bmesh representation of mesh data")

    if bpy.context.mode == Mode.Object:
        get_bmesh_impl = get_bmesh_from_object_mode
    else:
        get_bmesh_impl = get_bmesh_from_edit_mode

    with get_bmesh_impl(mesh_obj) as bm:
        yield bm


# get a reference to the active object
mesh_obj = bpy.context.active_object

with get_bmesh(mesh_obj) as bm:
    bmesh.ops.bevel(bm, geom=bm.edges, offset=0.2, segments=4, affect="EDGES", profile=0.5)

final_context_advanced_class.py

import bpy
import bmesh


def create_bmesh_from_object_mode(mesh_obj):
    # create a new bmesh
    bm = bmesh.new()

    # initialize the bmesh data using the mesh data
    bm.from_mesh(mesh_obj.data)
    return bm


def apply_bmesh_to_object_mesh(mesh_obj):
    # writes the bmesh data into the mesh data
    bm.to_mesh(mesh_obj.data)

    # [Optional] update the mesh data (helps with redrawing the mesh in the viewport)
    mesh_obj.data.update()


def create_bmesh_from_edit_mode(mesh_data):
    # create a new bmesh and initialize it from mesh data in Edit Mode
    bm = bmesh.from_edit_mesh(mesh_obj.data)

    return bm


def apply_bmesh_to_edit_mesh(mesh_data):
    # writes the bmesh data into the mesh data in Edit Mode
    bmesh.update_edit_mesh(mesh_obj.data)


class BmeshManager:
    EditMode = "EDIT_MESH"
    ObjectMode = "OBJECT"

    def __init__(self, mesh_obj):
        if bpy.context.mode not in (BmeshManager.ObjectMode, BmeshManager.EditMode):
            raise RuntimeError(f"{bpy.context.mode} mode is not supported for creating bmesh representation of mesh data")

        self.mesh_obj = mesh_obj
        if bpy.context.mode == BmeshManager.ObjectMode:
            self.create_bmesh = create_bmesh_from_object_mode
            self.apply_bmesh_to_mesh = apply_bmesh_to_object_mesh
        else:
            self.create_bmesh = create_bmesh_from_edit_mode
            self.apply_bmesh_to_mesh = apply_bmesh_to_edit_mesh

    def __enter__(self):
        self.bm = self.create_bmesh(self.mesh_obj)
        return self.bm

    def __exit__(self, exc_type, exc_value, exc_traceback):
        self.bm.normal_update()

        self.apply_bmesh_to_mesh(self.mesh_obj)

        # clean up/free memory that was allocated for the bmesh
        self.bm.free()


# get a reference to the active object
mesh_obj = bpy.context.active_object

with BmeshManager(mesh_obj) as bm:
    bmesh.ops.bevel(bm, geom=bm.edges, offset=0.2, segments=4, affect="EDGES", profile=0.5)