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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
|
Cluster
|
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]:
['data/0.json',
'data/1.json',
'data/2.json',
'data/3.json',
'data/4.json',
'data/5.json',
'data/6.json',
'data/7.json',
'data/8.json',
'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": 31, "name": ["Arianna", "Perry"], "occupation": "Tour Agent", "telephone": "868.239.4777", "address": {"address": "492 Bennington Avenue", "city": "Seaside"}, "credit-card": {"number": "5115 3723 4545 6580", "expiration-date": "03/19"}}
{"age": 58, "name": ["Cheree", "Sheppard"], "occupation": "Optical Assistant", "telephone": "(668) 440-4007", "address": {"address": "1047 Mark Twain Heights", "city": "Warren"}, "credit-card": {"number": "5255 1996 7376 1178", "expiration-date": "01/22"}}
[4]:
import dask.bag as db
import json
b = db.read_text('data/*.json').map(json.loads)
b
[4]:
dask.bag<loads-0..., npartitions=10>
[5]:
b.take(2)
[5]:
({'age': 31,
'name': ['Arianna', 'Perry'],
'occupation': 'Tour Agent',
'telephone': '868.239.4777',
'address': {'address': '492 Bennington Avenue', 'city': 'Seaside'},
'credit-card': {'number': '5115 3723 4545 6580',
'expiration-date': '03/19'}},
{'age': 58,
'name': ['Cheree', 'Sheppard'],
'occupation': 'Optical Assistant',
'telephone': '(668) 440-4007',
'address': {'address': '1047 Mark Twain Heights', 'city': 'Warren'},
'credit-card': {'number': '5255 1996 7376 1178',
'expiration-date': '01/22'}})
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.
[6]:
b.filter(lambda record: record['age'] > 30).take(2) # Select only people over 30
[6]:
({'age': 31,
'name': ['Arianna', 'Perry'],
'occupation': 'Tour Agent',
'telephone': '868.239.4777',
'address': {'address': '492 Bennington Avenue', 'city': 'Seaside'},
'credit-card': {'number': '5115 3723 4545 6580',
'expiration-date': '03/19'}},
{'age': 58,
'name': ['Cheree', 'Sheppard'],
'occupation': 'Optical Assistant',
'telephone': '(668) 440-4007',
'address': {'address': '1047 Mark Twain Heights', 'city': 'Warren'},
'credit-card': {'number': '5255 1996 7376 1178',
'expiration-date': '01/22'}})
[7]:
b.map(lambda record: record['occupation']).take(2) # Select the occupation field
[7]:
('Tour Agent', 'Optical Assistant')
[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-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.
[10]:
result.compute()
[10]:
[('Tax Manager', 16),
('Gaming Board Inspector', 15),
('Tyre Technician', 15),
('Ornamental', 15),
('Optical', 14),
('Rug Maker', 13),
('Auctioneer', 13),
('Safety Officer', 13),
('Museum Attendant', 13),
('Induction Moulder', 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]:
['data/processed.0.json',
'data/processed.1.json',
'data/processed.2.json',
'data/processed.3.json',
'data/processed.4.json',
'data/processed.5.json',
'data/processed.6.json',
'data/processed.7.json',
'data/processed.8.json',
'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': 31,
'name': ['Arianna', 'Perry'],
'occupation': 'Tour Agent',
'telephone': '868.239.4777',
'address': {'address': '492 Bennington Avenue', 'city': 'Seaside'},
'credit-card': {'number': '5115 3723 4545 6580',
'expiration-date': '03/19'}},)
[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': 31,
'occupation': 'Tour Agent',
'telephone': '868.239.4777',
'credit-card-number': '5115 3723 4545 6580',
'credit-card-expiration': '03/19',
'name': 'Arianna Perry',
'street-address': '492 Bennington Avenue',
'city': 'Seaside'},)
[14]:
df = b.map(flatten).to_dataframe()
df.head()
[14]:
| age | city | credit-card-expiration | credit-card-number | name | occupation | street-address | telephone | |
|---|---|---|---|---|---|---|---|---|
| 0 | 31 | Seaside | 03/19 | 5115 3723 4545 6580 | Arianna Perry | Tour Agent | 492 Bennington Avenue | 868.239.4777 |
| 1 | 58 | Warren | 01/22 | 5255 1996 7376 1178 | Cheree Sheppard | Optical Assistant | 1047 Mark Twain Heights | (668) 440-4007 |
| 2 | 42 | Glen Cove | 12/21 | 5480 1974 4516 2938 | Serafina Snider | Musician | 917 Exposition High Street | (969) 861-2381 |
| 3 | 30 | Lebanon | 02/17 | 2666 3976 3692 2230 | Nicky Sanchez | Lithographer | 728 Le Conte Parkway | 1-810-221-5666 |
| 4 | 53 | Sacramento | 11/22 | 4059 3818 6949 2922 | Rosy William | Archivist | 673 Winfield Drung | (589) 568-4228 |
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]:
Tax Manager 16
Tyre Technician 15
Gaming Board Inspector 15
Ornamental 15
Optical 14
Induction Moulder 13
Marine Geologist 13
Auctioneer 13
Ship Builder 13
Museum Attendant 13
Name: occupation, dtype: int64