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Dask Bags¶

Dask Bag implements operations like map, filter, groupby and aggregations on collections of Python objects. It does this in parallel and in small memory using Python iterators. It is similar to a parallel version of itertools or a Pythonic version of the PySpark RDD.

Dask Bags are often used to do simple preprocessing on log files, JSON records, or other user defined Python objects.

Full API documentation is available here: http://docs.dask.org/en/latest/bag-api.html

Start Dask Client for Dashboard¶

Starting the Dask Client is optional. It will provide a dashboard which is useful to gain insight on the computation.

The link to the dashboard will become visible when you create the client below. We recommend having it open on one side of your screen while using your notebook on the other side. This can take some effort to arrange your windows, but seeing them both at the same is very useful when learning.

[1]:

from dask.distributed import Client, progress
client = Client(n_workers=4, threads_per_worker=1)
client

[1]:

Client

Scheduler: tcp://127.0.0.1:37203
Dashboard: http://127.0.0.1:8787/status

Cluster

Workers: 4
Cores: 4
Memory: 8.36 GB

Create Random Data¶

We create a random set of record data and store it to disk as many JSON files. This will serve as our data for this notebook.

[2]:

import dask
import json
import os

os.makedirs('data', exist_ok=True)              # Create data/ directory

b = dask.datasets.make_people()                 # Make records of people
b.map(json.dumps).to_textfiles('data/*.json')   # Encode as JSON, write to disk

[2]:

['/home/travis/build/dask/dask-examples/data/0.json',
 '/home/travis/build/dask/dask-examples/data/1.json',
 '/home/travis/build/dask/dask-examples/data/2.json',
 '/home/travis/build/dask/dask-examples/data/3.json',
 '/home/travis/build/dask/dask-examples/data/4.json',
 '/home/travis/build/dask/dask-examples/data/5.json',
 '/home/travis/build/dask/dask-examples/data/6.json',
 '/home/travis/build/dask/dask-examples/data/7.json',
 '/home/travis/build/dask/dask-examples/data/8.json',
 '/home/travis/build/dask/dask-examples/data/9.json']

Read JSON data¶

Now that we have some JSON data in a file lets take a look at it with Dask Bag and Python JSON module.

[3]:

!head -n 2 data/0.json

{"age": 50, "name": ["Reena", "Rodriguez"], "occupation": "Site Agent", "telephone": "1-829-648-3617", "address": {"address": "1109 Gould Sideline", "city": "Terrell"}, "credit-card": {"number": "3467 024339 73355", "expiration-date": "08/20"}}
{"age": 63, "name": ["Karyl", "Sutton"], "occupation": "Radio Director", "telephone": "(702) 658-6214", "address": {"address": "911 Berkshire Field", "city": "Walnut Creek"}, "credit-card": {"number": "2397 1463 1900 3495", "expiration-date": "07/25"}}

[4]:

import dask.bag as db
import json

b = db.read_text('data/*.json').map(json.loads)
b

[4]:

dask.bag<loads, npartitions=10>

[5]:

b.take(2)

[5]:

({'age': 50,
  'name': ['Reena', 'Rodriguez'],
  'occupation': 'Site Agent',
  'telephone': '1-829-648-3617',
  'address': {'address': '1109 Gould Sideline', 'city': 'Terrell'},
  'credit-card': {'number': '3467 024339 73355', 'expiration-date': '08/20'}},
 {'age': 63,
  'name': ['Karyl', 'Sutton'],
  'occupation': 'Radio Director',
  'telephone': '(702) 658-6214',
  'address': {'address': '911 Berkshire Field', 'city': 'Walnut Creek'},
  'credit-card': {'number': '2397 1463 1900 3495',
   'expiration-date': '07/25'}})

Map, Filter, Aggregate¶

We can process this data by filtering out only certain records of interest, mapping functions over it to process our data, and aggregating those results to a total value.

[6]:

b.filter(lambda record: record['age'] > 30).take(2)  # Select only people over 30

[6]:

({'age': 50,
  'name': ['Reena', 'Rodriguez'],
  'occupation': 'Site Agent',
  'telephone': '1-829-648-3617',
  'address': {'address': '1109 Gould Sideline', 'city': 'Terrell'},
  'credit-card': {'number': '3467 024339 73355', 'expiration-date': '08/20'}},
 {'age': 63,
  'name': ['Karyl', 'Sutton'],
  'occupation': 'Radio Director',
  'telephone': '(702) 658-6214',
  'address': {'address': '911 Berkshire Field', 'city': 'Walnut Creek'},
  'credit-card': {'number': '2397 1463 1900 3495',
   'expiration-date': '07/25'}})

[7]:

b.map(lambda record: record['occupation']).take(2)  # Select the occupation field

[7]:

('Site Agent', 'Radio Director')

[8]:

b.count().compute()  # Count total number of records

[8]:

Chain computations¶

It is common to do many of these steps in one pipeline, only calling compute or take at the end.

[9]:

result = (b.filter(lambda record: record['age'] > 30)
           .map(lambda record: record['occupation'])
           .frequencies(sort=True)
           .topk(10, key=1))
result

[9]:

dask.bag<topk-aggregate, npartitions=1>

As with all lazy Dask collections, we need to call compute to actually evaluate our result. The take method used in earlier examples is also like compute and will also trigger computation.

[10]:

result.compute()

[10]:

[('Audit Clerk', 16),
 ('Instrument Maker', 14),
 ('Plastics Consultant', 14),
 ('Shipyard Worker', 14),
 ('Administration Clerk', 14),
 ('Clerk', 13),
 ('Mineralologist', 13),
 ('Shift Controller', 13),
 ('Roof Tiler', 13),
 ('Machine Operator', 13)]

Transform and Store¶

Sometimes we want to compute aggregations as above, but sometimes we want to store results to disk for future analyses. For that we can use methods like to_textfiles and json.dumps, or we can convert to Dask Dataframes and use their storage systems, which we’ll see more of in the next section.

[11]:

(b.filter(lambda record: record['age'] > 30)  # Select records of interest
  .map(json.dumps)                            # Convert Python objects to text
  .to_textfiles('data/processed.*.json'))     # Write to local disk

[11]:

['/home/travis/build/dask/dask-examples/data/processed.0.json',
 '/home/travis/build/dask/dask-examples/data/processed.1.json',
 '/home/travis/build/dask/dask-examples/data/processed.2.json',
 '/home/travis/build/dask/dask-examples/data/processed.3.json',
 '/home/travis/build/dask/dask-examples/data/processed.4.json',
 '/home/travis/build/dask/dask-examples/data/processed.5.json',
 '/home/travis/build/dask/dask-examples/data/processed.6.json',
 '/home/travis/build/dask/dask-examples/data/processed.7.json',
 '/home/travis/build/dask/dask-examples/data/processed.8.json',
 '/home/travis/build/dask/dask-examples/data/processed.9.json']

Convert to Dask Dataframes¶

Dask Bags are good for reading in initial data, doing a bit of pre-processing, and then handing off to some other more efficient form like Dask Dataframes. Dask Dataframes use Pandas internally, and so can be much faster on numeric data and also have more complex algorithms.

However, Dask Dataframes also expect data that is organized as flat columns. It does not support nested JSON data very well (Bag is better for this).

Here we make a function to flatten down our nested data structure, map that across our records, and then convert that to a Dask Dataframe.

[12]:

b.take(1)

[12]:

({'age': 50,
  'name': ['Reena', 'Rodriguez'],
  'occupation': 'Site Agent',
  'telephone': '1-829-648-3617',
  'address': {'address': '1109 Gould Sideline', 'city': 'Terrell'},
  'credit-card': {'number': '3467 024339 73355', 'expiration-date': '08/20'}},)

[13]:

def flatten(record):
    return {
        'age': record['age'],
        'occupation': record['occupation'],
        'telephone': record['telephone'],
        'credit-card-number': record['credit-card']['number'],
        'credit-card-expiration': record['credit-card']['expiration-date'],
        'name': ' '.join(record['name']),
        'street-address': record['address']['address'],
        'city': record['address']['city']
    }

b.map(flatten).take(1)

[13]:

({'age': 50,
  'occupation': 'Site Agent',
  'telephone': '1-829-648-3617',
  'credit-card-number': '3467 024339 73355',
  'credit-card-expiration': '08/20',
  'name': 'Reena Rodriguez',
  'street-address': '1109 Gould Sideline',
  'city': 'Terrell'},)

[14]:

df = b.map(flatten).to_dataframe()
df.head()

[14]:

	age	occupation	telephone	credit-card-number	credit-card-expiration	name	street-address	city
0	50	Site Agent	1-829-648-3617	3467 024339 73355	08/20	Reena Rodriguez	1109 Gould Sideline	Terrell
1	63	Radio Director	(702) 658-6214	2397 1463 1900 3495	07/25	Karyl Sutton	911 Berkshire Field	Walnut Creek
2	28	Stage Director	233.192.9838	4906 4832 6371 7576	04/20	Claude Santiago	582 Gould Gate	Portage
3	50	Yacht Master	(219) 387-2979	4533 3041 9152 8706	03/24	Armand Castaneda	932 Harriet Gate	Kenmore
4	59	Grave Digger	1-959-176-1685	3795 412775 74402	04/20	Vanda Key	916 Louisiana Gardens	Huntsville

We can now perform the same computation as before, but now using Pandas and Dask dataframe.

[15]:

df[df.age > 30].occupation.value_counts().nlargest(10).compute()

[15]:

Audit Clerk             16
Instrument Maker        14
Plastics Consultant     14
Shipyard Worker         14
Administration Clerk    14
Shift Controller        13
Roof Tiler              13
Flower Arranger         13
Mineralologist          13
Machine Operator        13
Name: occupation, dtype: int64

Learn More¶

You may be interested in the following links:

Dask Bag Documentation
API Documentation
dask tutorial, notebook 02, for a more in-depth introduction.