write_new_method.py

import csv
import bw2data as bd

bio_db_name = 'ecoinvent-3.10-biosphere'
new_method = ("Something", "Something", "Danger zone")


# Writing
with open(f'{bio_db_name}.csv', 'w', encoding='utf-8') as csvfile:
    fieldnames = ['id', 'name', 'categories', 'unit', 'cf']
    writer = csv.DictWriter(csvfile, fieldnames=fieldnames)

    writer.writeheader()
    
    flows = sorted(bd.Database(bio_db_name), key=lambda x: (x['name'], x['categories'], x['unit']))

    for flow in flows:
        writer.writerow({
            'id': flow.id,
            'name': flow['name'],
            'categories': "::".join(flow['categories']),
            'unit': flow['unit'],
            'cf': 0
        })

# Add your CFs to the CSV. You can also start with your own CSV if that's easier.

# Reading
with open(f'{bio_db_name}.csv', encoding='utf-8') as csvfile:
    data = [(int(obj['id']), obj['cf']) for obj in csv.DictReader(csvfile)]

my_method = bd.Method(new_method)
my_method.register(unit="foobar")
my_method.write(data)

# Check writing was successful
for flow, cf in bd.Method(new_method).load():
    print(bd.get_node(id=flow), cf)