I found solution like that:
database.collection('collection_name').document('document_id').update({'array_name': firestore.ArrayUnion([val])})
where 'array_name' is name of array-type field in document with 'document_id' id in collection 'collection_name'.
That code used:
import firebase_admin
from firebase_admin import firestore
and database is:
database = firestore.client()
But PyCharm says:
Cannot find reference 'ArrayUnion' in 'firestore.py'
The problem was in PyCharm Indexes, something like that. Code could run and ArrayUnion worked, but IDE said that there are no function ArrayUnion.