Pyodbc - "Data source name not found, and no default driver specified"

For me, it was all down to a single whitespace character.

$cat /home/ec2-user/.odbc.ini
[DSNNAME]
Driver =FreeTDS
Description=description
Server =serverpath
Port =1433
Database =dbname

Gave me the “Data source name not found, and no default driver specified” error.

Removing all the whitespaces before the '=' character made it work.

On a secondary note, using osql for DSN connection testing gives you a much more verbose description of any errors. It helped me a lot in the process.

$ osql -S DSNNAME -U username -P password
checking shared odbc libraries linked to isql for default directories...
    trying /txM ... no
    trying /tmp/sql ... no
    trying /tmp/sql ... no
    trying /w}H ... no
    trying /usr/loc ... no
    trying /tmp/sql.log ... no
    trying /home ... no
    trying /.odbc.ini ... no
    trying /usr/local/etc ... OK
checking odbc.ini files
    reading /home/ec2-user/.odbc.ini
[DSNNAME] found in /home/ec2-user/.odbc.ini
found this section:
    [DSNNAME]
    Driver =FreeTDS
    Description=description
    Server =serverpath
    Port =1433
    Database =dbname

looking for driver for DSN [DSNNAME] in /home/ec2-user/.odbc.ini
  no driver mentioned for [DSNNAME] in .odbc.ini
looking for driver for DSN [default] in /home/ec2-user/.odbc.ini
osql: error: no driver found for [DSNNAME] in .odbc.ini

Comparing the error message against my ini file made triaging the issue a lot easier.

For me, the issue was the actual location of my odbc.ini and odbcinst.ini files.

On many systems, the install location of these files is in /etc/

However, in my case, these files were located under /usr/local/etc/

The could be determined by typing
odbcinst -j

Which yielded:

unixODBC 2.3.0
DRIVERS............: /usr/local/etc/odbcinst.ini
SYSTEM DATA SOURCES: /usr/local/etc/odbc.ini
FILE DATA SOURCES..: /usr/local/etc/ODBCDataSources
USER DATA SOURCES..: /usr/local/etc/odbc.ini
SQLULEN Size.......: 8
SQLLEN Size........: 8
SQLSETPOSIROW Size.: 8

My odbc.ini files already exists in /etc, so the solution was to copy them over over from /etc/ to /usr/local/etc/

cp /etc/odbc.ini /etc/odbcinst.ini /usr/local/etc/

Edit: It's also worth noting that the path outputted by the odbcinst -j command can change depending on using sudo or not.

I believe the answer to your problem is that in your ~/.odbc.ini file you are saying to use driver PostgreSQL - but you have not defined that driver in your /etc/odbcinst.ini file. Try changing PostgreSQL to PostgreSQL ANSI or PostgreSQL Unicode (both of which are defined in /etc/odbcinst.ini).