Passing mongo aggregation to python

Question

I have the following piece of code in python which is trying to pass a mongo query. Details of the connection are below. When I execute this, I get "invalid syntax" and the syntax error points right after the code "total:{"... I think this has something to do with dictionary objects in Python but I am a little confused how to proceed.

import pymongo

from pymongo import MongoClient

client = MongoClient()

db = client.trackers

a = list((db.monthlytracker.aggregate([{$match:   
          {'Country':'Japan','Vendor':'Others'}}, 
          {$group:{_id:"$Fiscal Quarter",total:{$sum:"$Units"}}}])))

oz123 · Accepted Answer · 2015-02-01 22:01:15Z

2

You are using the wrong syntax. The pymongo driver accepts only strings:

a = list((db.monthlytracker.aggregate([{'$match':   
          {'Country':'Japan','Vendor':'Others'}}, 
          {'$group':{'_id':"$Fiscal Quarter",'total':{'$sum':"$Units"}}}])))

Simply quote everything, and you'll do fine.

The problem is that python looks to evaluate $group $match and so on, and it does not know about these variables. So you probably get errors like:

$match
  File "<ipython-input-1-972ca9ab5d06>", line 1
    $match
    ^
SyntaxError: invalid syntax

or:

NameError: name 'total' is not defined

and one more final thing, be careful with turning cursors to list objects, as you do with:

list(db.monthly.tracker.aggregate(...))

This can easily consume your memory. Instead you should iterate over the items in the cursor as with:

for item in db.monthly.tracker.aggregate(...):
    do_your_magic_here(item)

edited Feb 1, 2015 at 22:01

answered Feb 1, 2015 at 21:56

oz123

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2 Comments

thomassantosh Over a year ago

Thanks! Appreciate the quick response. So I tried quoting everything exactly as above but got a new error which said: KeyError: "['Units' 'MS Quarter'] not in index". I think this has more to do with the dictionary object issue. For the original error, I didn't actually get a syntax error on match or the group function, it was when it hit "total" in the code. Thanks for the recommendation on the memory usage.

thomassantosh Over a year ago

Correction: I think quoting match and group is necessary. The syntax error actually was pointing under $match to begin with...

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