Question

I just started using Python as hobby, so forgive me if this is a dumb question.

I recently installed a power meter on my computer, and I’m using it to track its power consumption. It gives me real-time minute readings exported to a csv file.

I have parsed and read the file, and now I have a dictionary where the key is a timestamp, stored as a struct_time from the time library. Currently, the dictionary has readings which were taken every minute. I’d like to create a new dictionary whose keys are hour timestamps, and whose values are the sum of the individual minute readings.

I thought about looping through the dictionary with a counter mod 60, but I’m told that isn’t very Pythonic. Also, there’s no guarantee that there exists 60 readings for every hour. How should I do this?

Answer 1

First, I’d suggest using the datetime.datetime class instead of time.time_struct, since the latter is really just a weak wrapper around a 9-tuple but the former is a fully-featured class with operator overloads etc. Next, you can use a collections.defaultdict to easily construct your desired total readings:

# power_reading is the data type corresponding to one set of power readings
# (it could be a simple float, or a tuple, or a class if you want; just make
# sure that you can add them together)
hourly_readings = collections.defaultdict(power_reading)
minutely_readings = { ... };  # dict of time.struct_time ==> power_reading

for timestamp, reading in minutely_readings.iteritems():
    hour = datetime.datetime(timestamp.tm_year, timestamp.tm_mon,
                             timestamp.tm_mday, timestamp.tm_hour);
    hourly_readings[hour] += reading

If power_reading is a tuple, you can’t use += (since that concatenates the tuples instead of doing an element-wise addition), so you’ll have to do something like this instead:

hourly_readings[hour] = tuple(map(sum, zip(hourly_readings[hour], reading)))