[Tutor] Collating date data from a csv file

[Cameron Simpson]

On 08May2019 21:04, Dave Hill <dave at the-hills.org.uk> wrote:
>I have a csv file which details the results of equipment tests, I 
>carry out PAT testing as a volunteer at a heriatge railway in N. 
>Wales. I want to extract how many items were tested on each test day. 
>So far I have generated a List of test dates, but I am now stalled at 
>how to efficiently count numbers tested on each date.
>
>Can I have a list of tuples, where one item is the date and the second 
>the count?

Not as such, because you can't modify a tuple (so you can't update the 
count part). But you could use a 2 element list.

>or is there a better construct?

Oh definitely. The easiest thing would be a defaultdict(int). Example:

  from collections import defaultdict
  ...
  by_date = defaultdict(int)
  for row in csvdata:
    timestamp = row[1]  # based on your example data
    # get the date from the timestamp
    date = ...
    by_date[date] += 1

A defaultdict is a dict which magicly makes missing elements when they 
get access, using a factory function you supply. Here we're using "int" 
as that factory, as int() returns zero.

I presume you've got the timestamp => date conversion sorted?

Cheers,
Cameron Simpson <cs at cskk.id.au>