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Dask Bags

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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

Client-db1fb37f-0ddf-11ed-9823-000d3a8f7959

Connection method: Cluster object Cluster type: distributed.LocalCluster
Dashboard: http://127.0.0.1:8787/status

Cluster Info

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/runner/work/dask-examples/dask-examples/data/0.json',
 '/home/runner/work/dask-examples/dask-examples/data/1.json',
 '/home/runner/work/dask-examples/dask-examples/data/2.json',
 '/home/runner/work/dask-examples/dask-examples/data/3.json',
 '/home/runner/work/dask-examples/dask-examples/data/4.json',
 '/home/runner/work/dask-examples/dask-examples/data/5.json',
 '/home/runner/work/dask-examples/dask-examples/data/6.json',
 '/home/runner/work/dask-examples/dask-examples/data/7.json',
 '/home/runner/work/dask-examples/dask-examples/data/8.json',
 '/home/runner/work/dask-examples/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": 61, "name": ["Emiko", "Oliver"], "occupation": "Medical Student", "telephone": "166.814.5565", "address": {"address": "645 Drumm Line", "city": "Kennewick"}, "credit-card": {"number": "3792 459318 98518", "expiration-date": "12/23"}}
{"age": 54, "name": ["Wendolyn", "Ortega"], "occupation": "Tractor Driver", "telephone": "1-975-090-1672", "address": {"address": "1274 Harbor Court", "city": "Mustang"}, "credit-card": {"number": "4600 5899 6829 6887", "expiration-date": "11/25"}}

[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': 61,
  'name': ['Emiko', 'Oliver'],
  'occupation': 'Medical Student',
  'telephone': '166.814.5565',
  'address': {'address': '645 Drumm Line', 'city': 'Kennewick'},
  'credit-card': {'number': '3792 459318 98518', 'expiration-date': '12/23'}},
 {'age': 54,
  'name': ['Wendolyn', 'Ortega'],
  'occupation': 'Tractor Driver',
  'telephone': '1-975-090-1672',
  'address': {'address': '1274 Harbor Court', 'city': 'Mustang'},
  'credit-card': {'number': '4600 5899 6829 6887',
   'expiration-date': '11/25'}})

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': 61,
  'name': ['Emiko', 'Oliver'],
  'occupation': 'Medical Student',
  'telephone': '166.814.5565',
  'address': {'address': '645 Drumm Line', 'city': 'Kennewick'},
  'credit-card': {'number': '3792 459318 98518', 'expiration-date': '12/23'}},
 {'age': 54,
  'name': ['Wendolyn', 'Ortega'],
  'occupation': 'Tractor Driver',
  'telephone': '1-975-090-1672',
  'address': {'address': '1274 Harbor Court', 'city': 'Mustang'},
  'credit-card': {'number': '4600 5899 6829 6887',
   'expiration-date': '11/25'}})

[7]:

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

[7]:

('Medical Student', 'Tractor Driver')

[8]:

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

[8]:

10000

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]:

[('Merchant', 16),
 ('Coroner', 14),
 ('Book Binder', 13),
 ('Medical Practitioner', 13),
 ('Payroll Supervisor', 13),
 ('Telecommunications', 13),
 ('Thermal Insulator', 13),
 ('Pattern Maker', 12),
 ('Advertising Executive', 12),
 ('Insurance Staff', 12)]

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/runner/work/dask-examples/dask-examples/data/processed.0.json',
 '/home/runner/work/dask-examples/dask-examples/data/processed.1.json',
 '/home/runner/work/dask-examples/dask-examples/data/processed.2.json',
 '/home/runner/work/dask-examples/dask-examples/data/processed.3.json',
 '/home/runner/work/dask-examples/dask-examples/data/processed.4.json',
 '/home/runner/work/dask-examples/dask-examples/data/processed.5.json',
 '/home/runner/work/dask-examples/dask-examples/data/processed.6.json',
 '/home/runner/work/dask-examples/dask-examples/data/processed.7.json',
 '/home/runner/work/dask-examples/dask-examples/data/processed.8.json',
 '/home/runner/work/dask-examples/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': 61,
  'name': ['Emiko', 'Oliver'],
  'occupation': 'Medical Student',
  'telephone': '166.814.5565',
  'address': {'address': '645 Drumm Line', 'city': 'Kennewick'},
  'credit-card': {'number': '3792 459318 98518', 'expiration-date': '12/23'}},)

[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': 61,
  'occupation': 'Medical Student',
  'telephone': '166.814.5565',
  'credit-card-number': '3792 459318 98518',
  'credit-card-expiration': '12/23',
  'name': 'Emiko Oliver',
  'street-address': '645 Drumm Line',
  'city': 'Kennewick'},)

[14]:

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

[14]:
age occupation telephone credit-card-number credit-card-expiration name street-address city
0 61 Medical Student 166.814.5565 3792 459318 98518 12/23 Emiko Oliver 645 Drumm Line Kennewick
1 54 Tractor Driver 1-975-090-1672 4600 5899 6829 6887 11/25 Wendolyn Ortega 1274 Harbor Court Mustang
2 33 Doctor 107-044-4885 3464 081512 23342 03/20 Alvin Rich 1242 Vidal Plantation Wyandotte
3 34 Counsellor 219-748-6795 4018 1801 8111 7757 08/23 Toccara Rogers 252 Sampson Drive Parma Heights
4 34 Graphic Designer 1-509-313-7125 4886 7380 4681 0434 05/18 Randal Roberts 767 Telegraph Side road New York

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]:

Merchant                 16
Coroner                  14
Thermal Insulator        13
Book Binder              13
Payroll Supervisor       13
Medical Practitioner     13
Telecommunications       13
Optometrist              12
Advertising Assistant    12
Care Manager             12
Name: occupation, dtype: int64

Learn More

You may be interested in the following links:

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Dask Arrays

next

Dask DataFrames