Python reduce in list of dictionaries

Question

I am a Java developer; however, occasionally due to cold starts in aws lambda for Java, I decide to use python for simple lambdas since it doesn't require compilation, eliminating cold starts.

The problem: I have a list of dictionaries, I retrieve this from Dynamo, in short a valid example is the following

[
   {
    "data":"data",
    "Count": 60
   },
   {
    "data":"data",
    "Count": 60
   }
]

I am trying to reduce this by getting the sum of the Count fields by doing the following; however, I don't understand what I am missing.

reduce(lambda x, y: int(x['Count']) +
                  int(y['Count']), query_response)

This fails with "errorMessage": "'int' object is not iterable"

What is the correct approach to doing this ?

Thanks in advance.

Answer 1

You need to give x an initial value of 0:

>>> from functools import reduce
>>> query_reponse = [
...    {
...     "data": "data",
...     "Count": 60
...    },
...    {
...     "data": "data",
...     "Count": 60
...    }
... ]
>>> reduce(lambda x, y: x + y["Count"], query_reponse, 0)
120

Ideally, you would want to use more descriptive variable names, e.g.:

>>> reduce(lambda total, d: total + d["Count"], query_reponse, 0)
120

For more info look at the docs for functools.reduce

As the other answer suggests you probably want to use sum() for small tasks like this. However, I do still think learning how reduce works is important with its prevalence in things like PySpark/Spark or other languages.