CODEWITHSUNDEEP
pythondictionarynested-lists
Paul
Paul

Reputation: 558

large nested lists versus dictionaries

Please could I solicit some general advice regarding Python lists. I know I shouldn't answer 'open' questions on here but I am worried about setting off on completely the wrong path.

My problem is that I have .csv files that are approximately 600,000 lines long each. Each row of the .csv has 6 fields, of which the first field is a date-time stamp in the format DD/MM/YYYY HH:MM:SS. The next two fields are blank and the last three fields contain float and integer values, so for example:

23/05/2017 16:42:17,  ,   , 1.25545, 1.74733, 12 
23/05/2017 16:42:20,  ,   , 1.93741, 1.52387, 14 
23/05/2017 16:42:23,  ,   , 1.54875, 1.46258, 11

etc

No two values in column 1 (date-time stamp) will ever be the same.

I need to write a program that will do a few basic operations with the data, such as:

  1. read all of the data into a dictionary, list, set (?) etc as appropriate.
  2. search through the date time stamp column for a particular value.
  3. read through the list and do basic calculations on the floats in columns 4 and 5.
  4. write a new list based on the searches/calculations.

My question is - how should I 'handle' the data and am I likely to run into problems due to the length of the dataset?

For example, should I import all of the data into a list, and each element of the list is a sublist of each rows data? E.g:

[[23/05/2017 16:42:17,'','', 1.25545, 1.74733, 12],[23/05/2017 16:42:20,'','', 1.93741, 1.52387, 14], ...]

Or would it be better to make each date-time stamp the 'key' in a dictionary and make the dictionary 'value' a list with all the other values, e.g:

{'23/05/2017 16:42:17': [ , , 1.25545, 1.74733, 12], ...} etc

If I use the list approach, is there a way to get Python to 'search' in only the first column for a particular time stamp rather than making it search through 600,000 rows times 6 columns when we know that only the first column contains timestamps?

I apologize if my query is a little vague, but would appreciate any guidance that anyone can offer.

Upvotes: 1

Views: 2868

Answers (1)

Eric Duminil
Eric Duminil

Reputation: 54313

600000 lines aren't that many, your script should run fine with either a list or a dict.

As a test, let's use:

data = [["2017-05-02 17:28:24", 0.85260, 1.16218, 7],
["2017-05-04 05:40:07", 0.72118, 0.47710, 15],
["2017-05-07 19:27:53", 1.79476, 0.47496, 14],
["2017-05-09 01:57:10", 0.44123, 0.13711, 16],
["2017-05-11 07:22:57", 0.17481, 0.69468, 0],
["2017-05-12 10:11:01", 0.27553, 0.47834, 4],
["2017-05-15 05:20:36", 0.01719, 0.51249, 7],
["2017-05-17 14:01:13", 0.35977, 0.50052, 7],
["2017-05-17 22:05:33", 1.68628, 1.90881, 13],
["2017-05-18 14:44:14", 0.32217, 0.96715, 14],
["2017-05-18 20:24:23", 0.90819, 0.36773, 5],
["2017-05-21 12:15:20", 0.49456, 1.12508, 5],
["2017-05-22 07:46:18", 0.59015, 1.04352, 6],
["2017-05-26 01:49:38", 0.44455, 0.26669, 13],
["2017-05-26 18:55:24", 1.33678, 1.24181, 7]]

dict

If you're looking for exact timestamps, a lookup will be much faster with a dict than with a list. You have to know exactly what you're looking for though: "23/05/2017 16:42:17" has a completely different hash than "23/05/2017 16:42:18".

data_as_dict = {l[0]: l[1:] for l in data}
print(data_as_dict)
# {'2017-05-21 12:15:20': [0.49456, 1.12508, 5], '2017-05-18 14:44:14': [0.32217, 0.96715, 14], '2017-05-04 05:40:07': [0.72118, 0.4771, 15], '2017-05-26 01:49:38': [0.44455, 0.26669, 13], '2017-05-17 14:01:13': [0.35977, 0.50052, 7], '2017-05-15 05:20:36': [0.01719, 0.51249, 7], '2017-05-26 18:55:24': [1.33678, 1.24181, 7], '2017-05-07 19:27:53': [1.79476, 0.47496, 14], '2017-05-17 22:05:33': [1.68628, 1.90881, 13], '2017-05-02 17:28:24': [0.8526, 1.16218, 7], '2017-05-22 07:46:18': [0.59015, 1.04352, 6], '2017-05-11 07:22:57': [0.17481, 0.69468, 0], '2017-05-18 20:24:23': [0.90819, 0.36773, 5], '2017-05-12 10:11:01': [0.27553, 0.47834, 4], '2017-05-09 01:57:10': [0.44123, 0.13711, 16]}

print(data_as_dict.get('2017-05-17 14:01:13'))
# [0.35977, 0.50052, 7]

print(data_as_dict.get('2017-05-17 14:01:10'))
# None

Note that your DD/MM/YYYY HH:MM:SS format isn't very convenient : sorting the cells lexicographically won't sort them by datetime. You'd need to use datetime.strptime() first:

from datetime import datetime
data_as_dict = {datetime.strptime(l[0], '%Y-%m-%d %H:%M:%S'): l[1:] for l in data}    
print(data_as_dict.get(datetime(2017,5,17,14,1,13)))
# [0.35977, 0.50052, 7]

print(data_as_dict.get(datetime(2017,5,17,14,1,10)))
# None

list with binary search

If you're looking for timestamps ranges, a dict won't help you much. A binary search (e.g. with bisect) on a list of timestamps should be very fast.

import bisect
timestamps = [datetime.strptime(l[0], '%Y-%m-%d %H:%M:%S') for l in data]
i = bisect.bisect(timestamps, datetime(2017,5,17,14,1,10))
print(data[i-1])
# ['2017-05-15 05:20:36', 0.01719, 0.51249, 7]
print(data[i])
# ['2017-05-17 14:01:13', 0.35977, 0.50052, 7]

Database

Before reinventing the wheel, you might want to dump all your CSVs into a small database (sqlite, Postgresql, ...) and use the corresponding queries.

Pandas

If you don't want the added complexity of a database but are ready to invest some time learning a new syntax, you should use pandas.DataFrame. It does exactly what you want, and then some.

Upvotes: 2

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