How to update values using pymongo?

You can use the $set syntax if you want to set the value of a document to an arbitrary value. This will either update the value if the attribute already exists on the document or create it if it doesn't. If you need to set a single value in a dictionary like you describe, you can use the dot notation to access child values.

If p is the object retrieved:

existing = p['d']['a']

For pymongo versions < 3.0

db.ProductData.update({
  '_id': p['_id']
},{
  '$set': {
    'd.a': existing + 1
  }
}, upsert=False, multi=False)

For pymongo versions >= 3.0

db.ProductData.update_one({
  '_id': p['_id']
},{
  '$set': {
    'd.a': existing + 1
  }
}, upsert=False)

However if you just need to increment the value, this approach could introduce issues when multiple requests could be running concurrently. Instead you should use the $inc syntax:

For pymongo versions < 3.0:

db.ProductData.update({
  '_id': p['_id']
},{
  '$inc': {
    'd.a': 1
  }
}, upsert=False, multi=False)

For pymongo versions >= 3.0:

db.ProductData.update_one({
  '_id': p['_id']
},{
  '$inc': {
    'd.a': 1
  }
}, upsert=False)

This ensures your increments will always happen.

With my pymongo version: 3.2.2 I had do the following

from bson.objectid import ObjectId
import pymongo

client = pymongo.MongoClient("localhost", 27017)
db = client.mydbname

db.ProductData.update_one({
  '_id': ObjectId(p['_id']['$oid'])
},{
  '$set': {
    'd.a': existing + 1
  }
}, upsert=False)