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Matt Rocklin

Introducing Blaze - Expressions

tl;dr Blaze abstracts tabular computation, providing uniform access to a variety of database technologies

Introduction

NumPy and Pandas couple a high level interface with fast low-level computation. They allow us to manipulate data intuitively and efficiently.

Occasionally we run across a dataset that is too big to fit in our computer’s memory. In this case NumPy and Pandas don’t fit our needs and we look to other tools to manage and analyze our data. Popular choices include databases like Postgres and MongoDB, out-of-disk storage systems like PyTables and BColz and the menagerie of tools on top of the Hadoop File System (Hadoop, Spark, Impala and derivatives.) Each of these systems has their own strengths and weaknesses and an experienced data analyst will choose the right tool for the problem at hand. Unfortunately learning how each system works and pushing data into the proper form often takes most of the data scientist’s time.

The startup costs of learning to munge and migrate data between new technologies often dominate biggish-data analytics.

Blaze strives to reduce this friction. Blaze provides a uniform interface to a variety of database technologies and abstractions for migrating data.

Expressions

At its core, Blaze is a way to express data and computations.

In the following example we build an abstract table for accounts in a simple bank. We then describe a query, deadbeats, to find the names of the account holders with a negative balance.

from blaze import TableSymbol, compute
accounts = TableSymbol('accounts', '{id: int, name: string, amount: int}')
 
# The names of account holders with negative balance
deadbeats = accounts[accounts['amount'] < 0]['name']

Python

Programmers familiar with Pandas should find the syntax to create deadbeats familiar. Note that we haven’t actually done any work yet. The table accounts is purely imaginary and so the deadbeats expression is just an expression of intent. The Pandas-like syntax builds up a graph of operations to perform later.

However, if we happen to have some similarly shaped data lying around

L = [[1, 'Alice',   100],
[2, 'Bob', -200],
[3, 'Charlie', 300],
[4, 'Dennis', 400],
[5, 'Edith', -500]]

Python

We can combine our expression, deadbeats with our data L to compute an actual result

>>> compute(deadbeats, L) # an iterator as a result
<itertools.imap at 0x7fdd7fcde450>
 
>>> list(compute(deadbeats, L))
['Bob', 'Edith']

Python

So in its simplest incarnation, Blaze is a way to write down computations abstractly which can later be applied to real data.

Multiple Backends - Pandas

Fortunately the deadbeats expression can run against many different kinds of data. We just computed deadbeats against Python lists, here we compute it against a Pandas DataFrame

from pandas import DataFrame
df = DataFrame([[1, 'Alice', 100],
[2, 'Bob', -200],
[3, 'Charlie', 300],
[4, 'Dennis', 400],
[5, 'Edith', -500]],
columns=['id', 'name', 'amount'])
 
>>> compute(deadbeats, df)
1 Bob
4 Edith
Name: name, dtype: object

Python

Note that Blaze didn’t perform the computation here, Pandas did (it’s good at that), Blaze just told Pandas what to do. Blaze doesn’t compute results; Blaze drives other systems to compute results.

Multiple Backends - MongoDB

To demonstrate some breadth, let’s show Blaze driving a Mongo Database.

$ # We install and run MongoDB locally
$ sudo apt-get install mongodb-server
$ mongod &
$ pip install pymongo
import pymongo
db = pymongo.MongoClient().db
db.mycollection.insert([{'id': 1, 'name': 'Alice', 'amount': 100},
{'id': 2, 'name': 'Bob', 'amount': -200},
{'id': 3, 'name': 'Charlie', 'amount': 300},
{'id': 4, 'name': 'Dennis', 'amount': 400},
{'id': 5, 'name': 'Edith', 'amount': -500}])
 
>>> compute(deadbeats, db.mycollection)
[u'Bob', u'Edith']

Python

Recap

To remind you we created a single Blaze query

accounts = TableSymbol('accounts', '{id: int, name: string, amount: int}')
deadbeats = accounts[accounts['amount'] < 0]['name']

Python

And then executed that same query against multiple backends

>>> list(compute(deadbeats, L))          # Python
['Bob', 'Edith']
 
>>> compute(deadbeats, df) # Pandas
1 Bob
4 Edith
Name: name, dtype: object
 
>>> compute(deadbeats, db.mycollection) # MongoDB
[u'Bob', u'Edith']

Python

At the time of this writing Blaze supports the following backends

  • Pure Python
  • Pandas
  • MongoDB
  • SQL
  • PySpark
  • PyTables
  • BColz

Interactivity

The separation of expressions and computation is core to Blaze. It’s also confusing for new Blaze users. NumPy and Pandas demonstrated the value of immediate data interaction and having to explicitly call compute is a step backward from that goal.

To this end we create the Table abstraction, a TableSymbol that knows about a particular data resource. Operations on this Table object produce abstract expressions just like normal, but statements that would normally print results to the screen initiate calls to compute and then print those results, giving an interactive feel in a console or notebook

>>> from blaze import Table
>>> t = Table(db.mycollection) # give MongoDB resource to Table
>>> t
amount id name
0 100 1 Alice
1 -200 2 Bob
2 300 3 Charlie
3 400 4 Dennis
4 -500 5 Edith
 
>>> t[t.amount < 0]
amount id name
0 -200 2 Bob
1 -500 5 Edith
 
>>> type(t[t.amount < 0]) # Still just an expression
blaze.expr.table.Selection
 
>>> print repr(t[t.amount < 0]) # repr triggers call to compute
amount id name
0 -200 2 Bob
1 -500 5 Edith

Python

These expressions generate the appropriate MongoDB queries, call compute only when we print a result to the screen, and then push the result into a DataFrame to use Pandas’ excellent tabular printing. For large datasets we always append a .head(10) call to the expression to only retrieve the sample of the output necessary to print to the screen; this avoids large data transfers when not necessary.

Using the interactive Table object we can interact with a variety of computational backends with the familiarity of a local DataFrame.

More Information

Sign up for our Wednesday, October 8th webinar on Getting Started with Blaze.

Tags: blaze scipy Python Programming
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