Python GroupBy, Map & Reduce

I came across a really interesting data mangling technique while watching this presentation on advanced Python programming techniques

Here is the example from this talk. Suppose you have a list of dictionary objects, sorted by the 'id' key, my_list as defined below.
>>> my_list = [
    {'id':1, 'name':'raymond'},
    {'id':1, 'email':'ray@spkrbar.com'},
    {'id':2, 'name':'sue'},
    {'id':2, 'email':'sue@sally.com'}] #sorted

Using dict, groupby, map and reduce, there is a very elegant way of grouping all of those dictionaries by the 'id' to get the following list:

[
  {'id': 1, 'email': 'ray@spkrbar.com', 'name': 'raymond'}, 
  {'id': 2, 'email': 'sue@sally.com', 'name': 'sue'}
]

And here is how you do it:
>>>from itertools import groupby
>>> [dict (
    reduce(lambda y,z: y + z,
        map(lambda x: x.items(), v)
    )
)
for k, v in groupby(my_list, key=lambda x: x['id'])]

Notice how much this is 'SQL' like. Let us break this statement down to its individual components to understand what is happening under the covers. Like any SQL statement, we have to start deciphering it inside out.

Let us look at the for loop with the groupby operation in it. The groupby operation will return an iterator grouping by the 'key' parameter. In this case, it is an anonymous function that returns the 'id' value. Essentially, we are asking for the grouping to happen using the 'id' values (1, 2 etc.).

>>>print({k:list(v) for k,v in groupby(my_list, key=lambda x: x['id'])})

{
 1: [{'id': 1, 'name': 'raymond'}, 
     {'id': 1, 'email': 'ray@spkrbar.com'}], 
 2: [{'id': 2, 'name': 'sue'}, 
     {'id': 2, 'email': 'sue@sally.com'}]
}

Note that the 'id' values are the keys and they are also repeated as part of the values. This will come in handy for the next step.

Next, we map the anonymous function, which calls the items() method for the parameter passed in, over each of the groups returned from the groupby operation.

>>> for k, v in groupby(my_list, key=lambda x: x['id']):
...     print(map(lambda x: x.items(), v))
...
[[('id', 1), ('name', 'raymond')], [('id', 1), ('email', 'ray@spkrbar.com')]]
[[('id', 2), ('name', 'sue')], [('id', 2), ('email', 'sue@sally.com')]]

This gives us lists of tuples instead of a dictionaries, which makes the reduction step very easy.

Now, we reduce the list that comes out of the mapping step by doing a simple addition of lists. Addition over two lists results in a list with elements from both. We would not have been able to use the '+' operator if these were dictionaries instead. Notice also the duplicate 'id' tuple.
>>>print(reduce(lambda y,z: y + z, [[('id', 1), ('name', 'raymond')], [('id', 1), ('email', 'ray@spkrbar.com')]]))
[('id', 1), ('name', 'raymond'), ('id', 1), ('email', 'ray@spkrbar.com')]

The reduce step will also happen for the list for 'id' 2 in this example.

We are almost at the end now. The last step is to make a dictionary from the list coming out of the reduce step to remove duplicates and conform back to the input which was a list of dictionaries.

>>>print(dict([('id', 1), ('name', 'raymond'), ('id', 1), ('email', 'ray@spkrbar.com')]))
{'id': 1, 'email': 'ray@spkrbar.com', 'name': 'raymond'}

Note that the duplicate 'id' tuple got removed. As before, this step will also happen for the list for 'id' 2.

That is it! Now, we have what we need. As list of dictionary objects, grouped by 'id'.

[
  {'id': 1, 'email': 'ray@spkrbar.com', 'name': 'raymond'}, 
  {'id': 2, 'email': 'sue@sally.com', 'name': 'sue'}
]

What are some of the practical uses you see for this technique? Do you have any other slick trick to share?Let me know in the comments below.

How a Sub Registrar Officer does corruption

I have been following the India Against Corruption movement for the past 6 months. This movement inspired me to blog about my experience with the Sub Registrar Office in Pune, India where I witnessed corruption happening in the first hand.

The Right To Information act has made it a little difficult for corruption to go unnoticed. However, one of the biggest contributor to corruption is public ignorance. Let me spell out the details of how the events unfolded and explain this point.

I was trying to give my Father the Power Of Attorney so that he could do transactions on my behalf in Pune. In order to do that, one has to type up a 5-6 page document that states, in legal jargon, that you are giving the other person the right to act on your behalf. There are many templates floating around that are used for this. Since one of my distant relations happens to have a few lawyers on his staff, we approached them for some legal advice. Naturally, they got us the template, we filled in the blanks, and now we had a document ready.

This document really doesn't mean a lot until it has been registered by the State Department of Registration and Stamps.

Now, we knew that we had to get this document registered. So we asked the lawyers at our disposal how to proceed. And this is where our ignorance got the best of us. The lawyers immediately referred us to their "agent" who "knows" the process and will "get things done" for us. Obviously for a nominal fee!

The Department of Registration and Stamps has a website and there they have instructions on what is required to get your documents registered. If the lawyers, would have pointed us to this link, we would have been saved.

Anyway, we contacted the agent and asked him what was a good time to meet him and setup the "terms" of our transaction. It was decided that we should come to the Sub Registrar Office where he "works" the next day and he will have our job done within a few hours. As the ignorant fools we were, we fell for it.

So, this is how it works. This "agent" is sometimes a lawyer by education. We are not talking about the cream of the crop here, definitely. This guy has managed to get the LLB degree and is using a little miscommunication and a little bribery to get an "edge". So what is the "edge" you ask? Let me elaborate.

The department website allows you to make an online appointment for your business at the office. Once you have a token, you are expected to appear, with all documents and personnel, 30 minutes before your time of appointment and the rest will take care of itself. The Sub Registrar Officer, however, has the authority to book time-slots from his terminal. And this is how he can provide an "edge" to the "agents". For some money in cash, he hands out time-slots to the "agents". The "agents" now sell those time-slots to their customers by telling them when they should come to the office and be gone once complete. The reason the "agent" has an LLB degree is that you can substitute one lawyer in lieu of two witnesses for the registration. Nowhere on the website does it say that it is okay to bring in one lawyer instead of two witnesses. This "one lawyer" clause is written on a board in the Sub Registrar Office.

So, by selling appointments, that are available online for free, the Sub Registrar Officer is able to make a few bucks on the side.

It is not often that people go to the Sub Registrar Office. So, the attitude they have is that might as well pay a few extra rupees and be done with it. However, from my experience, I did not get any "edge" and it would have been much better if I would have used the website.

The lesson(s) I learnt from this experience is/are:

• Check the government websites on what is needed for your work.
• Don't follow the crowd by approaching an "agent".
• Right To Information Act is a very powerful tool, use it to your advantage

What experiences have you had that would allow other's to not fall into such ignorance traps?

Umass Boston CS Alumni Speech 2012

I was invited as the speaker at the 2012 Alumni Party held by the CS department at UMass Boston.
It was an honor and a privilege to speak to the wonderful audience.
Here is the presentation.

Peter Norvig's Spelling Corrector in VB

Some Background: The idea behind this implementation was not to build the shortest or the fastest version of the spelling corrector. I looked at the list of languages that this was implemented in and found VB .NET missing. So I decided to fill that void. I also thought of it as a way to unite the cult of VB .NET programmers with the others.
(!!!Noble Peace Prize nomination here please!!!)

More importantly, I wanted to spell out each of the steps to make it easier to understand the concept. I also wanted to use native libraries so that you can dive into the spelling corrector concepts quickly without first having to learn other technologies like LINQ etc.

Please feel free to post any comments, suggestions for improvement and any bugs you find.

Copy the text below into a VB class file and get the BIG.txt file (http://norvig.com/big.txt).

' VB .NET Implementation of Peter Norvig's Spelling Corrector.
' VERSION 1.0, last updated 03 Mar 10.
'
' Peter Norvig's original article located at
' http://norvig.com/spell-correct.html
'
' Copyright (c) 2010 Shantanu Inamdar
'
' Permission is hereby granted, free of charge, to any person obtaining a copy
' of this software and associated documentation files (the "Software"), to deal
' in the Software without restriction, including without limitation the rights
' to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
' copies of the Software, and to permit persons to whom the Software is
' furnished to do so, subject to the following conditions:

' The above copyright notice and this permission notice shall be included in
' all copies or substantial portions of the Software.

' THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
' EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
' MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
' IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR
' ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF
' CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
' WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.


' Some Background: The idea behind this implementation was not build the shortest
' or the fastest version of the spelling corrector.
' I looked at the list of languages that this was implemented in and found VB .NET
' missing. So I decided to fill that void. I also thought of it as a way to unite
' the VB .NET programmers with the others.
' (!!!Noble Peace Prize nomination here please!!!)
'
' More importantly, I wanted to spell out each of the steps to make it easier to
' understand the concept. I also wanted to use native libraries
' so that you can dive into the spelling corrector concepts quickly
' without first having to learn other technologies like LINQ etc.
'
' Please feel free to post any comments, suggestions for improvement and any bugs
' you find.

Imports System.Collections.Generic

Public Class SimpleSpellingCorrector

Private Const COMMAASCII As Integer = 44
Private Const ALPHABET As String = "abcdefghijklmnopqrstuvwxyz"
Private nWords As New Dictionary(Of String, Integer)


Public Function getCorrection(ByVal word As String) As String
Dim c As String = String.Empty
Dim maxc As Integer = -1
Dim wc As Integer = 0
Dim candidates As System.Collections.Generic.List(Of String)

'Train the model with word occurences in our "dictionary"
nWords = getModel()

'Choose the most probable word with the shortest edit distance
For ed As Integer = 0 To 2
'If we have found a correction, exit loop
If String.Empty <> c Then
Exit For
End If

'Otherwise, start over
c = String.Empty
wc = 0
maxc = -1
candidates = getCandidates(word, ed)
For Each cd As String In candidates
wc = getWordCount(cd)
If wc > maxc Then
maxc = wc
c = cd
End If
Next cd
Next ed

'If no match is found, just send the same word back!
If String.Empty = c Then
c = word
End If

Return c
End Function

'Get the count of how often the word is found in our "dictionary"
'Return 1 for a "new" word
Private Function getWordCount(ByVal word As String) As Integer
If nWords.ContainsKey(word) Then
Return nWords.Item(word)
Else
Return 1
End If
End Function


'Get the big.txt file from http://norvig.com/big.txt
Private Function getModel() As Dictionary(Of String, Integer)
Dim model As New Dictionary(Of String, Integer)
For Each f As System.Text.RegularExpressions.Match In System.Text.RegularExpressions.Regex.Matches(System.IO.File.ReadAllText("big.txt").ToLower(), "[a-z]+", System.Text.RegularExpressions.RegexOptions.Compiled)
If model.ContainsKey(f.Value) Then
model.Item(f.Value) += 1
Else
model.Add(f.Value, 1)
End If
Next f

Return model
End Function

'Get Candidate words that are at the given edit distance
Private Function getCandidates(ByVal word As String, ByVal edits As Integer) As List(Of String)
Dim c As New List(Of String)
Select Case edits
Case 0
Dim wl As New List(Of String)
wl.Add(word)
c.AddRange(getKnown(wl))
Case 1
c.AddRange(getKnown(getEdits1(word)))
Case 2
c.AddRange(getKnownEdits2(word))
Case Else
c.Add(word)
End Select
Return c
End Function

'Get words that are at an edit distance of 1
Private Function getEdits1(ByVal word As String) As List(Of String)

Dim e1 As New List(Of String)
Dim splits As New List(Of String)

'Create a list of comma separated tuples of all possible ways to split the word
For i As Integer = 0 To word.Length
splits.Add(word.Substring(0, i) & "," & word.Substring(i))
Next i

For Each s As String In splits
'Deletes
If String.Empty <> s.Split(Chr(COMMAASCII))(1) Then
e1.Add(s.Split(Chr(COMMAASCII))(0) & s.Split(Chr(COMMAASCII))(1).Substring(1))
End If

'Transposes
If 1 <> s.Split(Chr(COMMAASCII))(1) Then
e1.Add(s.Split(Chr(COMMAASCII))(0) & c & s.Split(Chr(COMMAASCII))(1).Substring(1))
End If


'Replaces
For Each c As Char In ALPHABET.ToCharArray
If String.Empty <> s.Split(Chr(COMMAASCII))(1) Then
e1.Add(s.Split(Chr(COMMAASCII))(0) & c & s.Split(Chr(COMMAASCII)(1).Substring(1))

End If
Next

'Inserts
For Each c As Char In ALPHABET.ToCharArray
e1.Add(s.Split(Chr(COMMAASCII))(0) & c & s.Split(Chr(COMMAASCII))(1))
Next
Next s

Return e1
End Function

'Get Known words that have edit distance of 2
Private Function getKnownEdits2(ByVal word As String) As List(Of String)
Dim ke2 As New List(Of String)
For Each e1 As String In getEdits1(word)
For Each e2 As String In getEdits1(e1)
If nWords.ContainsKey(e2) Then
ke2.Add(e2)
End If
Next e2
Next e1
Return ke2
End Function

'Get Known words; get rid of unknown words
Private Function getKnown(ByRef words As List(Of String)) As List(Of String)
Dim k As New List(Of String)
For Each w As String In words
If nWords.ContainsKey(w) Then
k.Add(w)
End If
Next w
Return k
End Function

End Class

If it's not personal, it's just business!

As Solozo is explaining the attack on Vito Corleone to Michael, he says that it is not personal, it's business. What needs to be clear here is that it is in regards to the reaction. But the action that caused the reaction was very personal to Solozo.

But I find it often used in an incorrect context of the actual action.

What does this have to do with software development? We had a situation a few weeks back where a project had two people managing/leading it. And it didn't seem to make progress as was expected. I asked one of the leaders, who is also my colleague, if it was imperative for her to have this project done. And the reply was a reluctant no. That's when I knew that the project was headed nowhere.

A few years back, I was working on a project and discussing a complex scenario with my customer. After discussing possible solutions, she asked how in the world was this going to get done, given the resources and time. Then she said something that moved me to the guts. She said that she was so dependent on this project that she would be completely stranded if it didn't happen. I was so motivated to get the project done that I did everything that I could to take it to successful completion.

That's what I mean by that it has to be personal. If a project is not personal for at least one of the involved parties, it is never going to reach completion. That is what the difference is in the commitment of Founders vs Employees.

So, if it is not personal, it is just business. And if it is not personal, good luck getting it to successful completion. (But of course, the definition of 'successful completion' is subjective!)

Simple Database Tuning for SQL Server

Simple Database Tuning for SQL Server 2000:
When you Google for database slowness issues, you get plenty of hits to give you grey hair reading through all of them. After going through a number of them, and reading through some great SQL Server books, here is another addition to that lock of grey hairs!
Most of the articles that I have read online assume that you have taken care of the basic stuff, and now hunting to further fine tune your installation. But I am not going to focus on “fine” tuning; rather on the basics.

I agree that SQL Profiler is a very powerful tool, and I am a big fan of it myself. But before you go near it, you should have some basics covered. So here goes nothing:

1. Primary Keys are a must: You will be surprised as to how many “underground” installations are out there that don’t even follow this basic rule. Then these self taught DBAs run the SQL Tuning Advisor tool, which asks you to build a non-clustered index on a column which is supposed to be the primary key! So, please, make sure ALL your tables have primary keys. (FYI: A table that does not have a defined primary key is technically called a heap)

2. 80-20 Rule: 80% of the database problems lie with 20% of the tables! SQL Server is a very robust piece of software and will take abuse from bad design, lack of indexing etc up to a limit. Then, it just grinds to an agonizing halt. To spare it from this fate, run the following set of command:

DBCC UPDATEUSAGE('{your database name here}')

CREATE TABLE #DbTableSizes
(DbTableSizesId INT IDENTITY (1,1)
, TableName varchar(100)
, [Rows] BIGINT
, Reserved varchar(50)
, Data varchar(50)
, IndexSize varchar(50)
, Unused varchar(50))

INSERT INTO #DbTableSizes
EXEC sp_MSforeachtable @command1='EXEC sp_spaceused ''?'''

SELECT * FROM #DbTableSizes
ORDER BY [rows] DESC

DROP TABLE #DbTableSizes

This will give you a list of the tables that are most likely to be in the 20% that are causing problems. Those with the most number of rows are the ones to focus on. If you have tables with large amount of data (meaning large number of columns), but not really a huge number of rows, focus on those as well.

3. Now start targeting tables in this list, one by one. Make sure these have primary keys defined.

4. Using the following query, find out what stored procedures use these tables. You can also use the Action -> All tasks -> Display Dependencies menu option for this.
SELECT OBJECT_NAME(id) AS [Name]
FROM syscomments
WHERE [text] LIKE '%{table name here}%'
AND OBJECTPROPERTY(id, 'IsProcedure') = 1
GROUP BY OBJECT_NAME(id)

5. Now, from within each of the stored procedures and functions, get the queries that are using the table under review. You should create a list of all unique WHERE clauses used on this table.

6. For each of those WHERE clauses, look at the Query Plans, to check none of the following evils are present:
a. Table Scans
b. Clustered Index Scans
c. Parallelism, unless you know you have multiple CPU’s on your server
d. Bookmark Lookups, unless you know exactly why this is happening

7. Modify/Build indexes on the table so that you rid the query plans of the above evils.

This sounds like a very tedious process, and it is. But the benefits are pretty big. You will see a significant improvement in database performance.
If you want to take it a step further, try to build a script that will do all the data collection (Steps 1 through 6) for you and you can review the report periodically. (Maybe I will work on that as a side project)

Advanced Database Tuning for SQL Server 2000:
Once you have taken care of the basics, let us suppose you still find that the database performance is slow. I will point you to a very interesting methodology by Itzik Ben-Gan in the book, Inside SQL Server 2005: T-SQL Querying, chapter 3.
I have used most of the techniques in this chapter and they have helped me to dig much deeper into the slowness/high CPU usage issue that we experienced with one of our production installations.

Stocks and Employees

Just a random thought popped in my mind on how investing in stocks makes you a better employee. The Jim Cramer (http://www.thestreet.com ) style of Buy and Homework instead of Buy and Hold really drives the point through.

When you have money invested in a stock, you should be spending at least one hour per week doing home work on it. Which includes reading all articles published on that stock by management, analysts and media.

I think we should apply the same philosophy for the company that you are employed at. Spend at least an hour or so googling your company every week. Believe me, it will be worth while. Make sure you do it on your own time though.

So why does this make sense to me? Well, if you look at it, your money is invested in your employer in multiple forms. Your future pay checks, benefits, 401(K) etc. So when your livelihood is "invested" in your employer, it only makes sense you do your homework.

Look at your company in the same way you would look at any other that you own stock in.
- Who are the competitors?
- How is it doing?
- How is it doing against the competitors?
- What are the analysts saying?
- What is management saying?
- What is the media saying?

Here are a few additional questions to ask as an employee:
- Is your job aligned with the company strategy?
- Does your job have an impact on the stock price (or the bottom line for private firms)?
- Is management looking at your position/department as an important part of the company strategy?

Unless you are on top of your homework on this big position that you have in your life's portfolio, you leave yourself vulnerable to the bears!