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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:

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.

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



  • Workers: 4
  • Cores: 4
  • Memory: 15.69 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.

In [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'data/*.json')   # Encode as JSON, write to disk

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.

In [3]:
!head data/0.json -n 2
{"age": 36, "name": ["Boris", "May"], "occupation": "Travel Courier", "telephone": "(967) 473-1076", "address": {"address": "659 Minna Boulevard", "city": "Ontario"}, "credit-card": {"number": "4358 0881 1653 6165", "expiration-date": "02/18"}}
{"age": 29, "name": ["Shawana", "Ramos"], "occupation": "Milklady", "telephone": "+1-(446)-472-1888", "address": {"address": "1081 Clearview Close", "city": "Macomb"}, "credit-card": {"number": "3441 390740 87769", "expiration-date": "10/22"}}
In [4]:
import dask.bag as db
import json

b = db.read_text('data/*.json').map(json.loads)
dask.bag<map-loa..., npartitions=10>
In [5]:
({'age': 36,
  'name': ['Boris', 'May'],
  'occupation': 'Travel Courier',
  'telephone': '(967) 473-1076',
  'address': {'address': '659 Minna Boulevard', 'city': 'Ontario'},
  'credit-card': {'number': '4358 0881 1653 6165',
   'expiration-date': '02/18'}},
 {'age': 29,
  'name': ['Shawana', 'Ramos'],
  'occupation': 'Milklady',
  'telephone': '+1-(446)-472-1888',
  'address': {'address': '1081 Clearview Close', 'city': 'Macomb'},
  'credit-card': {'number': '3441 390740 87769', 'expiration-date': '10/22'}})

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.

In [6]:
b.filter(lambda record: record['age'] > 30).take(2)  # Select only people over 30
({'age': 36,
  'name': ['Boris', 'May'],
  'occupation': 'Travel Courier',
  'telephone': '(967) 473-1076',
  'address': {'address': '659 Minna Boulevard', 'city': 'Ontario'},
  'credit-card': {'number': '4358 0881 1653 6165',
   'expiration-date': '02/18'}},
 {'age': 43,
  'name': ['Daryl', 'Blevins'],
  'occupation': 'Housing Assistant',
  'telephone': '(684) 161-0146',
  'address': {'address': '1383 Mirabel Highway', 'city': 'Lenoir'},
  'credit-card': {'number': '5245 9269 7219 2314',
   'expiration-date': '10/22'}})
In [7]: record: record['occupation']).take(2)  # Select the occupation field
('Travel Courier', 'Milklady')
In [8]:
b.count().compute()  # Count total number of records

Chain computations

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

In [9]:
result = (b.filter(lambda record: record['age'] > 30)
           .map(lambda record: record['occupation'])
           .topk(10, key=1))
dask.bag<topk-ag..., 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.

In [10]:
[('Magistrates Clerk', 16),
 ('Furniture Remover', 16),
 ('Riding Instructor', 15),
 ('Surgeon', 14),
 ('Meat Wholesaler', 14),
 ('Welder', 14),
 ('Care Assistant', 14),
 ('Health Nurse', 14),
 ('Tutor', 13),
 ('Tree Feller', 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.

In [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

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.

In [12]:
({'age': 36,
  'name': ['Boris', 'May'],
  'occupation': 'Travel Courier',
  'telephone': '(967) 473-1076',
  'address': {'address': '659 Minna Boulevard', 'city': 'Ontario'},
  'credit-card': {'number': '4358 0881 1653 6165',
   'expiration-date': '02/18'}},)
In [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']
({'age': 36,
  'occupation': 'Travel Courier',
  'telephone': '(967) 473-1076',
  'credit-card-number': '4358 0881 1653 6165',
  'credit-card-expiration': '02/18',
  'name': 'Boris May',
  'street-address': '659 Minna Boulevard',
  'city': 'Ontario'},)
In [14]:
df =
age city credit-card-expiration credit-card-number name occupation street-address telephone
0 36 Ontario 02/18 4358 0881 1653 6165 Boris May Travel Courier 659 Minna Boulevard (967) 473-1076
1 29 Macomb 10/22 3441 390740 87769 Shawana Ramos Milklady 1081 Clearview Close +1-(446)-472-1888
2 43 Lenoir 10/22 5245 9269 7219 2314 Daryl Blevins Housing Assistant 1383 Mirabel Highway (684) 161-0146
3 42 Bowie 01/22 4418 2528 1924 7862 Nelia Craft Beautician 1384 Constanso Promenade 906-597-6730
4 30 Upper Arlington 10/19 2246 1119 9192 6614 Kristofer Rojas Cellarman 335 Zoo Close 859.122.0468

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

In [15]:
df[df.age > 30].occupation.value_counts().nlargest(10).compute()
Magistrates Clerk    16
Furniture Remover    16
Riding Instructor    15
Health Nurse         14
Care Assistant       14
Meat Wholesaler      14
Surgeon              14
Welder               14
Tunneller            13
Tutor                13
Name: occupation, dtype: int64

Learn More

You may be interested in the following links: