Question

Hello I am a new user to Python and I am having problem doing what I imagined was a fairly basic task.

I have several (>50) csv files containing daily snow depth data. I would like to iterate through the csv files and calculate monthly means for the snow depth. Data example:

Date,SD
1/1/2000,36
1/2/2000,36
1/3/2000,38
1/4/2000,40
2/1/2000,48
2/2/2000,48

In other words I would like to calculate monthly snow depth averages and write the output to a new csv file. I was able to modify a different example of code for my data, but I am receiving Key Errors for using Date as the key value in my Dictionary.

Any advice?

Code so far:

from __future__ import division
import csv
from collections  import defaultdict

def default_factory():
    return [0, None, None, 0]

reader = csv.DictReader(open(r'C:\SandBox\VALIDATION\TestTable.csv'))

dates = defaultdict(default_factory)
for row in reader:
    sd = int(row["SD"])
    dates[row["Dates"]][0] += sd
    max = dates[row["Dates"]][1]
    dates[row["Dates"]][1] = amount if max is None else amount if amount > max else max
    min = dates[row["Date"]][2]
    dates[row["Dates"]][2] = amount if min is None else amount if amount < min else min
    dates[row["Dates"]][3] += 1

for date in dates:
    dates[date][3] = dates[date][0]/dates[date][3]

writer = csv.writer(open(r'C:\SandBox\VALIDATION\TestAvg.csv', 'w', newline = ''))
writer.writerow(["Date", "SD", "max", "min", "mean"])
writer.writerows([date] + dates[date] for date in dates)

EDIT: Just to clarify, I am trying to achieve total monthly mean, i.e. January mean, February mean, etc… not calculate a mean for a single date.

Answer 1

You might want to use a dictionary to make the code a little more readable.

from __future__ import division
import csv
from collections  import defaultdict

def default_factory():
   return { "sum": 0, "max": None, "min": None, "count": 0}

reader = csv.DictReader(open(r'sd.csv'))

dates = defaultdict(default_factory)
rows = []
for row in reader:
    date = row["Date"]
    sd = int(row["Snowdepth"])
    rows.append([date, sd])
    month = date.split("/")[0]
    r = dates[month]
    r["sum"] += sd
    max = r["max"]
    r["max"] = sd if max is None else sd if sd > max else max
    min = r["min"]
    r["min"] = sd if min is None else sd if sd < min else min
    r["count"] += 1

for date in dates:
    r = dates[date]
    r["avg"] = r["sum"]/r["count"]

writer = csv.writer(open(r'TestAvg.csv', 'w'))
writer.writerow(["Date", "SD", "max", "min", "mean"])
for row in rows:
    r = dates[row[0].split("/")[0]]
    writer.writerow(row + [r["max"], r["min"], r["avg"]])