Lambda expression in a list comprehension Python

Question

So I have my pymongo cursor and very plain loop to get total sum. I need to fetch that value, also I am very new to Mongo and my approach might be not the best practice.

cursor = db.reportsColl.aggregate([
        {"$unwind": "$conditions"},
        {"$group": {"_id": "$_id", "sum": { "$sum": 1}}}])


x = 0
for result in cursor:
    x+= result['sum']
print(x)

I am trying to use lambda expression here, but I feel that I've kinda lost.

total = [(lambda x: x+result['sum'])(x) for _ in cursor]

Any ideas?

Did you mean something like total = sum(result['sum'] for result in cursor) — AChampion
– AChampion, Commented Aug 27, 2017 at 12:57
@AChampion definitely! How I couldn't do it in the 1st place... Thanks. — Alice Jarmusch
– Alice Jarmusch, Commented Aug 27, 2017 at 12:59
@AliceJarmusch slightly confused as to what you're doing here... can't you modify your query and have mongo do it rather than post-processing in Python? Although, I'm not sure if you're trying to get the number of groups, or something else... — Jon Clements
– Jon Clements, Commented Aug 27, 2017 at 13:08
@JonClements if you can suggest how can I do it in mongo, I will be happy. Had never worked with it before, and I just need to grab some aggregated data for the further calculations. — Alice Jarmusch
– Alice Jarmusch, Commented Aug 27, 2017 at 13:10
@Alice that's what I'm wondering... perhaps the real question you should be asking is "How do I get the following using a mongo query" and describing what the aggregation you want is, your attempt (like above) and some sample data etc... — Jon Clements
– Jon Clements, Commented Aug 27, 2017 at 13:11

Martijn Pieters · Accepted Answer · 2017-08-27 12:58:53Z

2

You are building a list of incomplete sums, and that's a waste. Don't use a list comprehension for the side effects.

You don't even need to do this. Use the sum() function with a generator expression instead:

total = sum(result['sum'] for result in cursor)

answered Aug 27, 2017 at 12:58

Martijn Pieters

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Comments

MiracleYoung · Accepted Answer · 2017-08-28 13:35:14Z

0

You only want to do is SUM.

So, you can try to do follow this:

In [9]: sum([1, 2, 3])
Out[9]: 6`

So, by this way, your code can be like:

total = sum([result['sum'] for result in cursor])

answered Aug 28, 2017 at 13:35

MiracleYoung

213 bronze badges

1 Comment

Martijn Pieters Over a year ago

Drop the [...] brackets, there is no need to create a list first to sum the contents. Use a generator expression instead.

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