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Wrap Oracle’s tnsping

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If you’ve worked with the Oracle database a while, you probably noticed that some utilities write to stdout for both standard output and what should be standard error (stderr). One of those commands is the tnsping utility.

You can wrap the tnsping command to send the TNS-03505 error to stdout with the following code. I put Bash functions like these in a library.sh script, which I can source when automating tasks.

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#!/usr/bin/bash
 
tnsping()
{
  if [ ! -z ${1} ]; then
    # Set default return value.
    stdout=`$ORACLE_HOME/bin/tnsping ${1} | tail -1`
 
    # Check stdout to return 0 for success and 1 for failure.
    if [[ `echo ${stdout} | cut -c1-9` = 'TNS-03505' ]]; then
      python -c 'import os, sys; arg = sys.argv[1]; os.write(2,arg + "\n")' "${stdout}"
    else
      echo "${1}"
    fi
  fi
}

You should notice that the script uses a Python call to redirect the error message to standard out (stdout) but you can redirect in Bash shell with the following:

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#!/usr/bin/bash
 
tnsping()
{
  if [ ! -z ${1} ]; then
    # Set default return value.
    stdout=`$ORACLE_HOME/bin/tnsping ${1} | tail -1`
 
    # Check stdout to return 0 for success and 1 for failure.
    if [[ `echo ${stdout} | cut -c1-9` = 'TNS-03505' ]]; then
      echo ${stdout} 1>&2
    else
      echo "${1}"
    fi
  fi
}

Interactively, we can now test a non-existent service name like wrong with this syntax:

tnsping wrong

It’ll print the standard error to console, like:

TNS-03505: Failed to resolve name

or, you can suppress standard error (stderr) by redirecting it to the traditional black hole, like:

tnsping wrong 2>/dev/null

After redirecting standard error (stderr), you simply receive nothing back. That lets you evaluate in another script whether or not the utility raises an error.

In an automating Bash shell script, you use the source command to put the Bash function in scope, like this:

source library.sh

As always, I hope this helps those looking for a solution.

Written by maclochlainn

September 23rd, 2020 at 11:43 pm

Posted in bash,Fedora,Linux,Oracle,Oracle 11g,Oracle 12c,Oracle 18c,Python,Python 2.7,Python 3.x,Unix

Tagged with ,

Python List & Dictionaries

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The following two sample programs are used in an Python programming course that I teach. I find them useful in qualifying how to work with loops, couple loops, and queues. The first example uses two lists and coupled loops, while the second example uses a single dictionary and FILO queue approach.

The Twelve Days of Christmas lyrics can be printed like so with coupled loops:

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days = ['first','second','third','fourth'       \
       ,'fifth','sixth','seventh','eighth'      \
       ,'nineth','tenth','eleventh','twelveth']
 
verse = ['partridge in a pear tree.'     \
        ,'Two turtle doves,'             \
        ,'Three French hens,'            \
        ,'Four calling birds,'           \
        ,'Five gold rings,'              \
        ,'Six geese a-laying,'           \
        ,'Seven swans a-swimming,'       \
        ,'Eight maids a-milking,'        \
        ,'Nine ladies dancing,'          \
        ,'Tenth lords a-leaping,'        \
        ,'Eleven pipers piping,'         \
        ,'Twelve drummers drumming,']
 
# Loop forward, couple inner loop, and loop backward through list.
for i in range(0,len(days), 1):
  print("On the",str(days[i]),"day of Christmas my true love sent to me")
  for j in range(i, -1, -1):
    if (j > 0):
      print(" ",verse[j])
    elif (i == j):
      print("  A",verse[j])
    else:
      print("  and a",verse[j])

Recreating the problem into a single dictionary, you can solve by approaching it as a FILO queue. Here’s the approach:

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lyric = {'first':'partridge in a pear tree.'
        ,'second':'Two turtle doves,'
        ,'third':'Three French hens,'
        ,'fourth':'Four calling birds,'
        ,'fifth':'Five gold rings,'
        ,'sixth':'Six geese a-laying,'
        ,'seventh':'Seven swans a-swimming,'
        ,'eighth':'maids a-milking,'
        ,'nineth':'Nine ladies dancing,'
        ,'tenth':'Ten lords a-leaping,'
        ,'eleventh':'Eleven pipers piping,'
        ,'twelfth':'Twelve drummers drumming,'}
 
# Intiate a list for collecting stanza.
stanza = list()
 
# Generate a list of keys.
for i in lyric.keys():
  # Append keys to list of stanza.
  stanza.append(i)
 
  # Print the first line of each stanza.
  print("On the",i,"day of Christmas my true love sent to me")
 
  # Print the progressive stanza.
  for j in reversed(stanza):
    if (j not in ['first','twelveth']):
      print(" ",lyric[j])
    elif (i == j):
      print("  A",lyric[j])
    else:
      print("  and a",lyric[j])

As always, I hope this helps for approaches and solutions.

Written by maclochlainn

February 16th, 2020 at 12:42 am

Posted in Python,Python 3.x

Tagged with

Developing Python Libraries

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I put this together to show my students how to simplify writing and testing Python library files. The trick requires that you learn how to set a relative $PYTHONPATH environment file.

export set PYTHONPATH=./lib

After setting the $PYTHONPATH environment variable, connect to Python’s IDLE environment and run the following code:

import os
print(os.environ['PYTHONPATH'])

It prints the following:

./lib

You can also discover all the standard libraries and your $PYTHONPATH value in your environment with the following command:

for i in sys.path:
  print(i)

It prints the following, which lists the one set by the $PYTHONPATH first:

/home/student/Code/python/path/lib
/usr/lib64/python37.zip
/usr/lib64/python3.7
/usr/lib64/python3.7/lib-dynload
/home/student/.local/lib/python3.7/site-packages
/usr/lib64/python3.7/site-packages
/usr/lib/python3.7/site-packages

You create a test my_module.py library file in the relative ./lib directory, like the following:

# Define a hello function that accept a name and prints a salutation.
def hello(whom):
  return "Hello " + whom + "!"

Next, you can create a testlib.py program:

# Import the hello function into the local namesapce from the my_module.
from my_module import hello
 
# Call the module hello, which returns a formatted string.
print(hello("Suzie Q"))

It imports the hello(whom) function into the local namespace and then calls the hello(whom) function with the string literal "Susie". It prints:

Hello Suzie Q!

If you import the my_module module, you must refer to the hello(whom) function by prefacing it with my_module., like the following example:

# Import the hello function into the local namesapce from the my_module.
import my_module
 
# Call the module hello, which returns a formatted string.
print(my_module.hello("Suzie Q"))

A direct import doesn’t add the method to the local namespace. It remains in the my_module‘s namespace.

It’s probably important to note where my_module.pyc files are written for the those migrating from Python 2.7 to Python 3. In Python 2.7 they would be written to the ./lib directory, but in Python 3 they’re written to the ./lib/__pycache__ directory.

As always, I hope this helps those who find it and read it.

Written by maclochlainn

February 13th, 2020 at 12:23 am

Posted in Python,Python 2.7,Python 3.x

Tagged with

Python-Postgres Query

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As I committed to a student, here are sample programs for writing a Python query against the Postgres 11 database. The first one returns rows or tuples. The latter formats the text returned as columns.

If you’re one of many looking for the key psycopg2 driver library, you can find it in most distro repositories as: python3-psycopg2. You can use dnf or yum to install it separately or you can install pgadmin4, which includes the psycopg2 library.

The first example returns the entire row from a new_hire table with two rows:

import psycopg2
 
try:
  # Open a connection to the database.
  connection = psycopg2.connect( user="student"
                               , password="student"
                               , port="5432"
                               , dbname="videodb")
 
  # Open a cursor.
  cursor = connection.cursor()
 
  # Assign a static query.
  query = "SELECT * FROM new_hire"
 
  # Parse and execute the query.
  cursor.execute(query)
 
  # Fetch all rows from a table.
  records = cursor.fetchall()
 
  # Read through and print the rows as tuples.
  for row in range(0, len(records)):
    print(records[row]) 
 
except (Exception, psycopg2.Error) as error :
  print("Error while fetching data from PostgreSQL", error)
 
finally:
  # Close the database connection.
  if (connection):
    cursor.close()
    connection.close()

The first example returns the rows as tuples, which is probably desired if you want to consume the result in another Python program. Here’s the output retrieved:

(1001, 'Malcolm', 'Jacob', 'Lewis', datetime.date(2018, 2, 14))
(1002, 'Henry', None, 'Chabot', datetime.date(1990, 7, 31))

The second one returns the rows and formats the columns into output for a csv style file:

import psycopg2
 
try:
  # Open a connection to the database.
  connection = psycopg2.connect( user="student"
                               , password="student"
                               , port="5432"
                               , dbname="videodb")
 
  # Open a cursor.
  cursor = connection.cursor()
 
  # Assign a static query.
  query = "SELECT * FROM new_hire"
 
  # Parse and execute the query.
  cursor.execute(query)
 
  # Read through and print the formatted columns of each row.
  for (new_hire_id, first_name, middle_name, last_name, hire_date) in cursor:
    if (isinstance(middle_name,type(None))):
      print("{},'{} {}','{:%d-%b-%Y}'".format(new_hire_id, first_name, last_name, hire_date))
    else:
      print("{},'{} {} {}','{:%d-%b-%Y}'".format(new_hire_id, first_name, middle_name, last_name, hire_date))
 
except (Exception, psycopg2.Error) as error :
  print("Error while fetching data from PostgreSQL", error)
 
finally:
  # Close the database connection.
  if (connection):
    cursor.close()
    connection.close()

The second one returns the rows and formatted columns for a csv style file:

1001,'Malcolm Jacob Lewis','14-Feb-2018'
1002,'Henry Chabot','31-Jul-1990'

As always, I hope these help those looking for a starting place with Python and Postgres.

Written by maclochlainn

September 23rd, 2019 at 1:23 am

Posted in Fedora,Linux,Postgres,PostgreSQL,Python,Python 3.x

Tagged with

pgAdmin4 on Fedora 30

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While attempting an install of pgAdmin and updating a Fedora 30 environment, I encountered a conflict on the upgrade of MySQL 8.0.17-1 to 8.0.17.2. The community-mysql-8.0.17-2.fc30.x86_64 had conflicts with:

  • mysql-community-client-8.0.17-1.fc30.x86_64 package
  • mysql-community-server-8.0.17-1.fc30.x86_64 package

I tried to update the system before install pgadmin4 with the following syntax:

dnf -y update && dnf -y install pgadmin4

The dnf utility raise the following MySQL package errors during transaction checking:

Display detailed console log →

Error: Transaction check error:
  file /usr/bin/mysql conflicts between attempted installs of community-mysql-8.0.17-2.fc30.x86_64 and mysql-community-client-8.0.17-1.fc30.x86_64
  file /usr/bin/mysql_config_editor conflicts between attempted installs of community-mysql-8.0.17-2.fc30.x86_64 and mysql-community-client-8.0.17-1.fc30.x86_64
  file /usr/bin/mysqladmin conflicts between attempted installs of community-mysql-8.0.17-2.fc30.x86_64 and mysql-community-client-8.0.17-1.fc30.x86_64
  file /usr/bin/mysqlbinlog conflicts between attempted installs of community-mysql-8.0.17-2.fc30.x86_64 and mysql-community-client-8.0.17-1.fc30.x86_64
  file /usr/bin/mysqlcheck conflicts between attempted installs of community-mysql-8.0.17-2.fc30.x86_64 and mysql-community-client-8.0.17-1.fc30.x86_64
  file /usr/bin/mysqldump conflicts between attempted installs of community-mysql-8.0.17-2.fc30.x86_64 and mysql-community-client-8.0.17-1.fc30.x86_64
  file /usr/bin/mysqlimport conflicts between attempted installs of community-mysql-8.0.17-2.fc30.x86_64 and mysql-community-client-8.0.17-1.fc30.x86_64
  file /usr/bin/mysqlpump conflicts between attempted installs of community-mysql-8.0.17-2.fc30.x86_64 and mysql-community-client-8.0.17-1.fc30.x86_64
  file /usr/bin/mysqlshow conflicts between attempted installs of community-mysql-8.0.17-2.fc30.x86_64 and mysql-community-client-8.0.17-1.fc30.x86_64
  file /usr/bin/mysqlslap conflicts between attempted installs of community-mysql-8.0.17-2.fc30.x86_64 and mysql-community-client-8.0.17-1.fc30.x86_64
  file /usr/bin/ibd2sdi conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/bin/innochecksum conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/bin/my_print_defaults conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/bin/myisam_ftdump conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/bin/myisamchk conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/bin/myisamlog conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/bin/myisampack conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/bin/mysql_secure_installation conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/bin/mysql_ssl_rsa_setup conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/bin/mysql_tzinfo_to_sql conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/bin/mysql_upgrade conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/bin/mysqld_pre_systemd conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/bin/perror conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib/systemd/system/mysqld.service conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib/systemd/system/mysqld@.service conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/adt_null.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/auth_socket.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/component_audit_api_message_emit.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/component_log_filter_dragnet.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/component_log_sink_json.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/component_log_sink_syseventlog.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/component_validate_password.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/connection_control.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/ddl_rewriter.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/group_replication.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/ha_example.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/ha_mock.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/innodb_engine.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/keyring_file.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/keyring_udf.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/libmemcached.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/locking_service.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/mypluglib.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/mysql_clone.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/mysql_no_login.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/rewrite_example.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/rewriter.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/semisync_master.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/semisync_slave.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/validate_password.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/plugin/version_token.so conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/sbin/mysqld conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /var/lib/mysql conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /var/lib/mysql-keyring conflicts between attempted installs of community-mysql-server-8.0.17-2.fc30.x86_64 and mysql-community-server-8.0.17-1.fc30.x86_64
  file /usr/lib64/mysql/libmysqlclient.so.21.1.17 conflicts between attempted installs of community-mysql-libs-8.0.17-2.fc30.x86_64 and mysql-community-libs-8.0.17-1.fc30.x86_64

Since I’m not sure what’s wrong or how to fix it, I’ve put it in my queue of things to get to later. However, when I figure it out I’ll update this blog page with the solution or work around. If anybody knows the fix and would like to share, please let me know.

I removed the pending update packages with the following command:

dnf clean packages

Then, I simply installed pgadmin4 with the following command:

dnf -y install pgadmin4

Display detailed console log →

 

The pgadmin4 configuration instructions can be found for several Linux versions at Josphat Mutai’s Computing for Geeks web page. On Fedora 30, you need to do the following:

  • Install, start, and enable Apache as the httpd service unless you already have done that.
  • Copy the /etc/httpd/conf.d/pgadmin4.conf.sample file to /etc/httpd/conf.d/pgadmin4.conf, which is a new file.
  • Restart the httpd service to incorporate the pgadmin4 configuration file.

After that, you create the following new directories as the root or sudo user:

  • /var/lib/pgadmin4
  • /var/log/pgadmin4

You can make both directories with a single mkdir command, like:

mkdir -p /var/lib/pgadmin4 /var/log/pgadmin4

As the root or sudo user, change the ownership of these two directories to the apache user with the following syntax:

chown -R apache:apache /var/lib/pgadmin4 /var/log/pgadmin4

You add the following four statements to the config_distro.py file in the /usr/lib/python3.7/site-packages/pgadmin4-web directory as the root or sudo user:

LOG_FILE = '/var/log/pgadmin4/pgadmin4.log'
SQLITE_PATH = '/var/lib/pgadmin4/pgadmin4.db'
SESSION_DB_PATH = '/var/lib/pgadmin4/sessions'
STORAGE_DIR = '/var/lib/pgadmin4/storage'

You need to setup the pgadmin user with the following python3 command:

python3 /usr/lib/python3.7/site-packages/pgadmin4-web/setup.py

Enter the following values, a real email address and a password twice:

NOTE: Configuring authentication for SERVER mode.
 
Enter the email address and password to use for the initial pgAdmin user account:
 
Email address: admin@example.com   
Password: your_password
Retype password: your_password
pgAdmin 4 - Application Initialisation
======================================

Assuming you have an enabled firewall, you need to issue the following two commands as the root or sudo user:

rirewall-cmd --permanent --add-service=http
firewall-cmd --reload

You invoke pgAdmin4 from within a browser window with the following URL for a stand alone workstation (for a workstation on a DNS network you would enter pgadmin.domain.domain_type in lieu of localhost):

pgadmin/localhost/pgadmin4

You most likely will encounter an Internal Server Error, the recommended fix is reputed to be:

ausearch -c 'httpd' --raw | audit2allow -M my-httpd
semodule -X 300 -i my-httpd.pp

It didn’t work for me. At the end of the process, I have an Internal Server Error. It is something that I’ll try to fix next. The actual error message:

Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
 
Please contact the server administrator at root@localhost to inform them of the time this error occurred, and the actions you performed just before this error.
 
More information about this error may be available in the server error log.

If somebody figures out the last step before I do, that’s great. Let me and everybody else know the mystery.

On a positive note, the pgadmin4 package provided the psycopg2 library. I had looked for it as a psycopg2 package but it is in python3-psycopg2 package.

Written by maclochlainn

September 21st, 2019 at 5:29 pm

Posted in Linux,MySQL,MySQL 8,pgAdmin4,Postgres,Python,Python 2.7,Python 3.x

Tagged with , , ,

Django on Fedora 30

without comments

It seemed opportune to add Django to the Fedora 30 instance that I build and maintain for my students. Here are the instructions, which I developed with the prior Fedora 28/29 instructions.

  1. Check your Python3 installation with the following command: 

    python3 -V

    It should return this but if it doesn’t you should install python3:

    Python 3.7.4

  2. Check whether pip3 is installation by installing it when its not: 

    sudo def -y install python3-php

    It should return:

    Last metadata expiration check: 0:44:52 ago on Tue 10 Sep 2019 11:02:33 AM MDT.
Package python3-pip-19.0.3-3.fc30.noarch is already installed.
Dependencies resolved.
Nothing to do.
Complete!

  3. Check whether Django is installation by installing it when its not with pip3 installation utility: 

    sudo pip3 install --user Django

    It should return the following if installed:

    Requirement already satisfied: Django in /usr/lib/python3.7/site-packages (2.1.10)
Requirement already satisfied: pytz in /usr/lib/python3.7/site-packages (from Django) (2018.5)

  4. Check your django-admin account location with the which utility: 

    which django-admin

    It should return the following on Fedora 30 when installed:

    /usr/bin/django-admin

  5. Create a Django test application with the django-admin utility by creating a project directory. My directory is a bit deep. For reference, it is: 

    /home/student/Code/python/django/projects

    Change to that projects directory, and run the following command:

    django-admin startproject test_app

    After that command change directory with the cd command into the test_app subdirectory in your projects directory. Run the manage.py program with the following command:

    python3 manage.py migrate

    You should see the following:

    Operations to perform:
  Apply all migrations: admin, auth, contenttypes, sessions
Running migrations:
  Applying contenttypes.0001_initial... OK
  Applying auth.0001_initial... OK
  Applying admin.0001_initial... OK
  Applying admin.0002_logentry_remove_auto_add... OK
  Applying admin.0003_logentry_add_action_flag_choices... OK
  Applying contenttypes.0002_remove_content_type_name... OK
  Applying auth.0002_alter_permission_name_max_length... OK
  Applying auth.0003_alter_user_email_max_length... OK
  Applying auth.0004_alter_user_username_opts... OK
  Applying auth.0005_alter_user_last_login_null... OK
  Applying auth.0006_require_contenttypes_0002... OK
  Applying auth.0007_alter_validators_add_error_messages... OK
  Applying auth.0008_alter_user_username_max_length... OK
  Applying auth.0009_alter_user_last_name_max_length... OK
  Applying sessions.0001_initial... OK

Next, your would create an admin account. You’re done.

Written by maclochlainn

September 10th, 2019 at 12:47 pm

Posted in Django,Fedora,LAMP,Linux,Python,Python 3.x

Tagged with

Python MySQL Query

without comments

Somebody asked me how to expand a prior example with the static variables so that it took arguments at the command line for the variables. This example uses Python 3 new features in the datetime package.

There’s a small trick converting the string arguments to date data types. Here’s a quick example that shows you how to convert the argument list into individual date data type variables:

#!/usr/bin/python3
 
# include standard modules
import sys
from datetime import datetime
 
# Capture argument list.
fullCmdArguments = sys.argv
 
# Assignable variables.
beginDate = ""
endDate = ""
 
# Assign argument list to variable.
argumentList = fullCmdArguments[1:]
 
# Enumerate through the argument list where beginDate precedes endDate as strings.
try:
  for i, s in enumerate(argumentList):
    if (i == 0):
      beginDate = datetime.date(datetime.fromisoformat(s))
    elif (i == 1):
      endDate = datetime.date(datetime.fromisoformat(s))
except ValueError:
  print("One of the first two arguments is not a valid date (YYYY-MM-DD).")
 
# Print the processed values and types.
print("Begin Date: [",beginDate,"][",type(beginDate),"]")
print("End Date:   [",endDate,"][",type(endDate),"]")

Assume you call this arguments.py. Then, you call it with valid conforming date format value like the following command-line example:

./arguments.py 2001-01-01 2003-12-31

It returns the arguments after they have been converted to date data types. The results should look like this:

Begin Date:  1991-01-01 [ <class 'datetime.date'> ]
End Date:    2004-12-31 [ <class 'datetime.date'> ]

The next Python example accepts dynamic arguments at the command line to query the MySQL database:

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#!/usr/bin/python3
 
# Import the library.
import sys
import mysql.connector
from datetime import datetime
from datetime import date
from mysql.connector import errorcode
 
# Capture argument list.
fullCmdArguments = sys.argv
 
# Assignable variables.
start_date = ""
end_date = ""
 
# Assign argument list to variable.
argumentList = fullCmdArguments[1:]
 
#  Check and process argument list.
# ============================================================
#  If there are less than two arguments provide default values.
#  Else enumerate and convert strings to dates.
# ============================================================
if (len(argumentList) < 2):
  # Set a default start date.
  if (isinstance(start_date,str)):
    start_date = date(1980, 1, 1)
 
  # Set the default end date.
  if (isinstance(end_date,str)):
    end_date = datetime.date(datetime.today())
else:
  # Enumerate through the argument list where beginDate precedes endDate as strings.
  try:
    for i, s in enumerate(argumentList):
      if (i == 0):
        start_date = datetime.date(datetime.fromisoformat(s))
      elif (i == 1):
        end_date = datetime.date(datetime.fromisoformat(s))
  except ValueError:
    print("One of the first two arguments is not a valid date (YYYY-MM-DD).")
 
#  Attempt the query.
# ============================================================
#  Use a try-catch block to manage the connection.
# ============================================================
try:
  # Open connection.
  cnx = mysql.connector.connect(user='student', password='student',
                                host='127.0.0.1',
                                database='studentdb')
  # Create cursor.
  cursor = cnx.cursor()
 
  # Set the query statement.
  query = ("SELECT CASE "
           "         WHEN item_subtitle IS NULL THEN item_title "
           "         ELSE CONCAT(item_title,': ',item_subtitle) "
           "         END AS title, "
           "release_date "
           "FROM item "
           "WHERE release_date BETWEEN %s AND %s "
           "ORDER BY item_title")
 
  # Execute cursor.
  cursor.execute(query, (start_date, end_date))
 
  # Display the rows returned by the query.
  for (item_name, release_date) in cursor:
    print("{}, {:%d-%b-%Y}".format(item_name, release_date))
 
#  Handle exception and close connection.
# ============================================================
except mysql.connector.Error as e:
  if e.errno == errorcode.ER_ACCESS_DENIED_ERROR:
    print("Something is wrong with your user name or password")
  elif e.errno == errorcode.ER_BAD_DB_ERROR:
    print("Database does not exist")
  else:
    print("Error code:", e.errno)        # error number
    print("SQLSTATE value:", e.sqlstate) # SQLSTATE value
    print("Error message:", e.msg)       # error message
 
# Close the connection when the try block completes.
finally:
  cnx.close()

You can call the python-mysql-query.py program with the following syntax:

./python-mysql-query.py 2001-01-01 2003-12-31

It returns the films between 1 Jan 2001 and 31 Dec 2003, like this:

Clear and Present Danger: Special Collector's Edition, 06-May-2003
Die Another Day: 2-Disc Ultimate Version, 03-Jun-2003
Die Another Day, 03-Jun-2003
Die Another Day, 03-Jun-2003
Golden Eye, 03-Jun-2003
Golden Eye: Special Edition, 03-Jun-2003
Harry Potter and the Chamber of Secrets, 28-May-2002
Harry Potter and the Chamber of Secrets: Two-Disc Special Edition, 28-May-2002
Harry Potter and the Sorcerer's Stone, 28-May-2002
Harry Potter and the Sorcerer's Stone: Two-Disc Special Edition, 28-May-2002
Harry Potter and the Sorcerer's Stone: Full Screen Edition, 28-May-2002
MarioKart: Double Dash, 17-Nov-2003
Pirates of the Caribbean, 30-Jun-2003
RoboCop, 24-Jul-2003
Splinter Cell: Chaos Theory, 08-Apr-2003
Star Wars II: Attack of the Clones, 16-May-2002
Star Wars II: Attack of the Clones, 16-May-2002
The Chronicles of Narnia: The Lion, the Witch and the Wardrobe, 30-Jun-2003
The Chronicles of Narnia: The Lion, the Witch and the Wardrobe, 16-May-2002

As always, I hope this helps somebody who wants to learn how to use Python with the MySQL database.

Written by maclochlainn

September 6th, 2019 at 10:31 pm

Posted in Fedora,Linux,MySQL,MySQL 8,Python,Python 3.x

Tagged with ,

MySQL Python Connector

with one comment

While building my student image on Fedora 30, I installed the MySQL PHP Connector (php-mysqlndrp) but neglected to install the Python Connector. This adds the installation and basic test of the Python Connector to the original blog post.

You use the following command with a wildcard as a privileged user. The wildcard is necessary because you need to load two libraries to support Python 2.7 and 3.7, which are installed on Fedora 30. You also need to be the root user or a user that is found in the sudoer’s list:

yum install -y mysql-connector-python*

Display detailed console log →

Last metadata expiration check: 0:35:46 ago on Tue 20 Aug 2019 05:36:29 PM MDT.
Dependencies resolved.
=====================================================================================================================================
 Package                                  Architecture       Version                    Repository                              Size
=====================================================================================================================================
Installing:
 mysql-connector-python                   x86_64             8.0.17-1.fc30              mysql-connectors-community             435 k
 mysql-connector-python-cext              x86_64             8.0.17-1.fc30              mysql-connectors-community             7.7 M
 mysql-connector-python3                  x86_64             8.0.17-1.fc30              mysql-connectors-community             429 k
 mysql-connector-python3-cext             x86_64             8.0.17-1.fc30              mysql-connectors-community             7.7 M
Installing dependencies:
 python2-protobuf                         noarch             3.6.1-3.fc30               fedora                                 563 k
 python3-protobuf                         noarch             3.6.1-3.fc30               fedora                                 568 k
 
Transaction Summary
=====================================================================================================================================
Install  6 Packages
 
Total download size: 17 M
Installed size: 89 M
Downloading Packages:
(1/6): python3-protobuf-3.6.1-3.fc30.noarch.rpm                                                      1.0 MB/s | 568 kB     00:00    
(2/6): python2-protobuf-3.6.1-3.fc30.noarch.rpm                                                      994 kB/s | 563 kB     00:00    
(3/6): mysql-connector-python-8.0.17-1.fc30.x86_64.rpm                                               481 kB/s | 435 kB     00:00    
(4/6): mysql-connector-python3-8.0.17-1.fc30.x86_64.rpm                                              612 kB/s | 429 kB     00:00    
(5/6): mysql-connector-python-cext-8.0.17-1.fc30.x86_64.rpm                                          3.8 MB/s | 7.7 MB     00:02    
(6/6): mysql-connector-python3-cext-8.0.17-1.fc30.x86_64.rpm                                         4.2 MB/s | 7.7 MB     00:01    
-------------------------------------------------------------------------------------------------------------------------------------
Total                                                                                                5.4 MB/s |  17 MB     00:03     
warning: /var/cache/dnf/mysql-connectors-community-8bcc2bd350b53f70/packages/mysql-connector-python-8.0.17-1.fc30.x86_64.rpm: Header V3 DSA/SHA1 Signature, key ID 5072e1f5: NOKEY
MySQL Connectors Community                                                                           7.0 MB/s |  27 kB     00:00    
Importing GPG key 0x5072E1F5:
 Userid     : "MySQL Release Engineering <mysql-build@oss.oracle.com>"
 Fingerprint: A4A9 4068 76FC BD3C 4567 70C8 8C71 8D3B 5072 E1F5
 From       : /etc/pki/rpm-gpg/RPM-GPG-KEY-mysql
Key imported successfully
Running transaction check
Transaction check succeeded.
Running transaction test
Transaction test succeeded.
Running transaction
  Preparing        :                                                                                                             1/1 
  Installing       : python3-protobuf-3.6.1-3.fc30.noarch                                                                        1/6 
  Installing       : python2-protobuf-3.6.1-3.fc30.noarch                                                                        2/6 
  Installing       : mysql-connector-python-8.0.17-1.fc30.x86_64                                                                 3/6 
  Installing       : mysql-connector-python3-8.0.17-1.fc30.x86_64                                                                4/6 
  Installing       : mysql-connector-python3-cext-8.0.17-1.fc30.x86_64                                                           5/6 
  Installing       : mysql-connector-python-cext-8.0.17-1.fc30.x86_64                                                            6/6 
  Running scriptlet: mysql-connector-python-cext-8.0.17-1.fc30.x86_64                                                            6/6 
  Verifying        : python2-protobuf-3.6.1-3.fc30.noarch                                                                        1/6 
  Verifying        : python3-protobuf-3.6.1-3.fc30.noarch                                                                        2/6 
  Verifying        : mysql-connector-python-8.0.17-1.fc30.x86_64                                                                 3/6 
  Verifying        : mysql-connector-python-cext-8.0.17-1.fc30.x86_64                                                            4/6 
  Verifying        : mysql-connector-python3-8.0.17-1.fc30.x86_64                                                                5/6 
  Verifying        : mysql-connector-python3-cext-8.0.17-1.fc30.x86_64                                                           6/6 
 
Installed:
  mysql-connector-python-8.0.17-1.fc30.x86_64                    mysql-connector-python-cext-8.0.17-1.fc30.x86_64                   
  mysql-connector-python3-8.0.17-1.fc30.x86_64                   mysql-connector-python3-cext-8.0.17-1.fc30.x86_64                  
  python2-protobuf-3.6.1-3.fc30.noarch                           python3-protobuf-3.6.1-3.fc30.noarch                               
 
Complete!

Leveraging the MySQL Connector/Python Coding Examples documentation, Section 5.1 Connecting to MySQL Using Connector/Python here’s a test of the connection to MySQL 8.

# Import the library.
import mysql.connector
from mysql.connector import errorcode
 
try:
  # Open connection.
  cnx = mysql.connector.connect(user='student', password='student',
                                host='127.0.0.1',
                                database='studentdb')
 
  # Print the value.
  print("Database connection resolved.")
 
# Handle exception and close connection.
except mysql.connector.Error as e:
  if e.errno == errorcode.ER_ACCESS_DENIED_ERROR:
    print("Something is wrong with your user name or password")
  elif e.errno == errorcode.ER_BAD_DB_ERROR:
    print("Database does not exist")
  else:
    print(e)
 
# Close the connection when the try block completes.
else:
  cnx.close()

Leveraging the MySQL Connector/Python Coding Examples documentation, Section 5.4 Querying Data Using Connector/Python here’s a test of the connection to MySQL 8.

# Import the library.
import datetime
import mysql.connector
from mysql.connector import errorcode
 
try:
  # Open connection.
  cnx = mysql.connector.connect(user='student', password='student',
                                host='127.0.0.1',
                                database='studentdb')
  # Create cursor.
  cursor = cnx.cursor()
 
  # Set the query statement.
  query = ("SELECT "
           "CASE "
           "  WHEN item_subtitle IS NULL THEN item_title "
           "  ELSE CONCAT(item_title,': ',item_subtitle) "
           "END AS title, "
           "release_date "
           "FROM item "
           "WHERE release_date BETWEEN %s AND %s "
           "ORDER BY item_title")
 
  # Set the start and end date.
  start_date = datetime.date(1991, 1, 1)
  end_date = datetime.date(2004, 12, 31)
 
  # Execute cursor.
  cursor.execute(query, (start_date, end_date))
 
  # Display the rows returned by the query.
  for (item_name, release_date) in cursor:
    print("{}, {:%d %b %Y}".format(item_name, release_date))
 
  # Close cursor.
  cursor.close()
 
# ------------------------------------------------------------
# Handle exception and close connection.
except mysql.connector.Error as e:
  if e.errno == errorcode.ER_ACCESS_DENIED_ERROR:
    print("Something is wrong with your user name or password")
  elif e.errno == errorcode.ER_BAD_DB_ERROR:
    print("Database does not exist")
  else:
    print "Error code:", e.errno        # error number
    print "SQLSTATE value:", e.sqlstate # SQLSTATE value
    print "Error message:", e.msg       # error message
 
# Close the connection when the try block completes.
else:
  cnx.close()

If you run the above in Python 2.7 it works fine. It fails to parse successfully in Python 3.x because the print() function requires the parentheses all the time. You would need to re-write the except block, like this with the parentheses:

# Handle exception and close connection.
except mysql.connector.Error as e:
  if e.errno == errorcode.ER_ACCESS_DENIED_ERROR:
    print("Something is wrong with your user name or password")
  elif e.errno == errorcode.ER_BAD_DB_ERROR:
    print("Database does not exist")
  else:
    print("Error code:", e.errno)        # error number
    print("SQLSTATE value:", e.sqlstate) # SQLSTATE value
    print("Error message:", e.msg)       # error message

While it works without the parentheses in Python 2.7, it also works with the parentheses. That means the best practice is to write cross compatible code by always using the parentheses with the print() function.

As always, I hope this helps somebody.j

Written by maclochlainn

August 21st, 2019 at 1:44 am

Posted in MySQL,MySQL 8,MySQL Connect,MySQL Connect/Python,Python,Python 2.7,Python 3.x

Tagged with ,

Python & Oracle 1

without comments

While Python is an interpreted language, Python is a very popular programming language. You may ask yourself why it is so popular? The consensus answers to why it’s so popular points to several factors. For example, Python is a robust high-level programming language that lets you:

  • Get complex things done quickly
  • Automate system and data integration tasks
  • Solve complex analytical problems

You find Python developers throughout the enterprise. Development, release engineering, IT operations, and support teams all use Python to solve problems. Business intelligence and data scientists use Python because it’s easy to use.

Developers don’t generally write end-user applications in Python. They mostly use Python as scripting language. Python provides a simple syntax that lets developers get complex things done quickly. Python also provides you with a vast set of libraries that let you can leverage to solve problems. Those libraries often simplify how you analyze complex data or automate repetitive tasks.

This article explains how to use the Python programming language with the Oracle database. It shows you how to install and use the cx_Oracle library to query data. Part 2 will cover how you insert, update, and delete data in the Oracle database, and how you call and use PL/SQL stored functions and procedures.

The article has two parts:

  • How you install and use cx_Oracle with the Oracle database
  • How you query data statically or dynamically

This audience for this article should know the basics of writing a Python program. If you’re completely new to Python, you may want to get a copy of Eric Matthes’ Python Crash Course: A Hands-On, Project-Based Introduction to Programming. More experienced developers with shell scripting backgrounds may prefer Al Sweigart’s Automate the Boring Stuff with Python.

This article uses Python 2.7, which appears to be the primary commercial version of Python in most organizations. At least, it’s what most vendors ship with Linux distros. It also happens to be the Python distro on Fedora Linux.

How you install and use cx_Oracle with the Oracle database

The first step requires that you test the current version of Python on your Operating System (OS). For the purpose of this paper, you use the student user account. The student user is in the sudoer list, which gives the account super user privileges.

You can find the Python version by opening a Terminal session and running the following command:

[student@localhost ~]$ python -V

It displays:

Python 2.7.5

You can download the current version of the cx_Oracle library at the Python Software Foundation’s web site. At the time of writing, the current version of the cx_Oracle is the cx_Oracle 5.2.1 version. The cx_Oracle library is available for download as a Red Hat Package Manager (RPM) module.

You download the cx_Oracle-5.2.1-11g-py26-1.x86_64.rpm to the /tmp directory or to a sudoer-enabled user’s downloads directory. Let’s assume you download the RPM into the /tmp directory. After you download the RPM, you can install it with the yum utility with this syntax:

yum install -y /tmp/cx_Oracle-5.2.1-11g-py27-1.x86_64.rpm

However, the most current version is now 7.0. You want the following file on Fedora 64-bit Linux, which can be found at the Python Package Index web site:

cx_Oracle-7.0.0-cp27-cp27mu-manylinux1_x86_64.whl

A wheel file requires that you use the pip utility (make sure to upgrade to the current version), like:

sudo pip install cx_Oracle-7.0.0-cp27-cp27mu*.whl

It should print the following to the console:

Processing ./cx_Oracle-7.0.0-cp27-cp27mu-manylinux1_x86_64.whl                                            
Installing collected packages: cx-Oracle                                                                  
Successfully installed cx-Oracle-7.0.0

The cx_Oracle library depends on the Oracle Client software, which may or may not be installed. It installs without a problem but would raise a runtime error when using the Python software. You can check whether cx_Oracle is installed with the following syntax:

rpm –qa oracle-instantclient11.2-basic

If the oracle-instantclient11.2-basic library isn’t installed, the command returns nothing. If the oracle-instantclient11.2-basic library is installed it returns the following:

oracle-instantclient11.2-basic-11.2.0.4.0-1.x86_64

Assuming you don’t have the Oracle Client software installed, you should download it from Oracle’s Instant Client Downloads web page. After you download the RPM, you install the Oracle 11g Release 2 Client software with the following syntax:

yum install -y /tmp/oracle-instantclient11.2-basic-11.2.0.4.0-1.x86_64.rpm

You now have the necessary software installed and configured to run and test Python programs that work with the Oracle database. Python uses a standard path configuration to look for Python modules or libraries. You can see that set of path values by connecting to the Python IDLE environment, which is the runtime environment. The IDLE environment is very much like the SQL*Plus environment.

You connect to the Python IDLE environment by typing the following:

python

It opens the Python IDLE environment. It should display the following:

Python 2.7.5 (default, Apr 10 2015, 08:09:05) 
[GCC 4.8.3 20140911 (Red Hat 4.8.3-7)] on linux2
Type "help", "copyright", "credits" or "license" for more information.

You import the sys library and then you can print the path elements with the following command:

>>> import sys
print sys.path

It should print the following for Python 2.7 in Fedora Linux:

['', '/usr/lib64/python27.zip', '/usr/lib64/python2.7', '/usr/lib64/python2.7/plat-linux2', '/usr/lib64/python2.7/lib-tk', '/usr/lib64/python2.7/lib-old', '/usr/lib64/python2.7/lib-dynload', '/usr/lib64/python2.7/site-packages', '/usr/lib64/python2.7/site-packages/gtk-2.0', '/usr/lib/python2.7/site-packages']

You can now test whether the environment works by typing the following commands in the IDLE environment:

>>> import cx_Oracle
db = cx_Oracle.connect("student/student@xe")
print db.version

It prints:

11.2.0.2.0

The other two sections require you to test components inside Python files. That means you need to supplement the default Python path variable. You do that by adding values to the Python environment variable, which is $PYTHONPATH.

The following adds the /home/student/Code/python directory to the Python path variable:

export set PYTHONPATH=/home/student/Code/python

Next, we create an connection.py file, which holds the following:

# Import the Oracle library.
import cx_Oracle
 
try:
  # Create a connection.
  db = cx_Oracle.connect("student/student@xe")
 
  # Print a message.
  print "Connected to the Oracle " + db.version + " database."
 
except cx_Oracle.DatabaseError, e:
  error, = e.args
  print >> sys.stderr, "Oracle-Error-Code:", error.code
  print >> sys.stderr, "Oracle-Error-Message:", error.message
 
finally
  # Close connection. 
  db.close()

The import statement adds the cx_Oracle library to the program scope. The cx_Oracle library’s connect function takes either the user name and password, or the user name, password, and TNS alias.

The except block differs from what you typically see. The code value maps to the SQLCODE value and the message value maps to the SQLERRM value.

You can test the connection.py file as follows in the /home/student/Code/python directory:

python connection.py

It prints the following:

Connected to the Oracle 11.2.0.2.0 database.

This section has shown you how to setup the cx_Oracle library, and how you can test the cx_Oracle library with Python programs.

How you query data statically or dynamically

The prior section shows you how to connect to an Oracle instance and how to verify the driver version of the cx_Oracle library. Like most ODBC and JDBC software, Python first creates a connection. Then, you need to create a cursor inside a connection.

The basicCursor.py program creates a connection and a cursor. The cursor holds a static SQL SELECT statement. The SELECT statement queries a string literal from the pseudo dual table.

# Import the Oracle library.
import sys
import cx_Oracle
 
try:
  # Create a connection.
  db = cx_Oracle.connect("student/student@xe")
 
  # Create a cursor.
  cursor = db.cursor()
 
  # Execute a query.
  cursor.execute("SELECT 'Hello world!' FROM dual")
 
  # Read the contents of the cursor.
  for row in cursor:
    print (row[0]) 
 
except cx_Oracle.DatabaseError, e:
  error, = e.args
  print >> sys.stderr, "Oracle-Error-Code:", error.code
  print >> sys.stderr, "Oracle-Error-Message:", error.message
 
finally:
  # Close cursor and connection. 
  cursor.close()
}  db.close()

The connect function assigns a database connection to the local db variable. The cursor function returns a cursor and assigns it to the local cursor variable. The execute function dispatches the query to Oracle’s SQL*Plus and returns the result set into a row element of the local cursor variable. The for-each loop reads the row element from the cursor variable and prints one row at a time. Since the cursor only returns a string literal, there’s only one row to return.

You test the program with this syntax:

python basicConnection.py

It prints:

Hello world!

The next basicTable.py program queries the item table. The item table holds a number of rows of data. The code returns each row inside a set of parentheses.

# Import the Oracle library.
import cx_Oracle
 
try:
  # Create a connection.
  db = cx_Oracle.connect("student/student@xe")
 
  # Create a cursor.
  cursor = db.cursor()
 
  # Execute a query.
  cursor.execute("SELECT item_title " +
                 ",      item_rating " +
                 "FROM   item " +
                 "WHERE  item_type = "
                 "        (SELECT common_lookup_id " +
                 "         FROM   common_lookup " +
                 "         WHERE  common_lookup_type = 'DVD_WIDE_SCREEN')")
 
  # Read the contents of the cursor.
  for row in cursor:
    print (row[0], row[1]) 
 
except cx_Oracle.DatabaseError, e:
  error, = e.args
  print >> sys.stderr, "Oracle-Error-Code:", error.code
  print >> sys.stderr, "Oracle-Error-Message:", error.message
 
finally:
  # Close cursor and connection. 
  cursor.close()
  db.close()

The SQL query is split across several lines by using the + operator. The + operator concatenates strings, and it lets you format a long query statement. The range for loop returns tuples from the cursor. The tuples are determined by the SELECT-list of the query.

The query returns the following type of results:

('Casino Royale', 'PG-13')
...
('Star Wars - Episode I', 'PG')
('Star Wars - Episode II', 'PG')
('Star Wars - Episode III', 'PG-13')
('Star Wars - Episode IV', 'PG')
('Star Wars - Episode V', 'PG')
('Star Wars - Episode VI', 'PG')

At this point, you know how to work with static queries. The next example shows you how to work with dynamic queries. The difference between a static and dynamic query is that an element of the string changes.

You have two options for creating dynamic strings. The first lets you glue a string inside a query. The second lets you embed one or more bind variables in a string. As a rule, you should use bind variables because they avoid SQL injection risks.

The following is the basicDynamicTable.py script

# Import the Oracle library.
import cx_Oracle
 
sRate = 'PG-13'
 
try:
  # Create a connection.
  db = cx_Oracle.connect("student/student@xe")
 
  # Define a dynamic statment.
  stmt = "SELECT item_title, item_rating FROM item WHERE item_rating = :rating"
 
  # Create a cursor.
  cursor = db.cursor()
 
  # Execute a statement with a bind variable.
  cursor.execute(stmt, rating = sRate)
 
  # Read the contents of the cursor.
  for row in cursor:
    print (row[0], row[1]) 
 
except cx_Oracle.DatabaseError, e:
  error, = e.args
  print >> sys.stderr, "Oracle-Error-Code:", error.code
  print >> sys.stderr, "Oracle-Error-Message:", error.message
 
finally:
  # Close cursor and connection. 
  cursor.close()
  db.close()

You need to assign a dynamic SQL statement to a local string variable. The bind variable is preceded with a colon (:). The execute function takes a string variable with the dynamic SQL statement. Then, you provide a name and value pair. The name needs to match the bind variable in the dynamic SQL statement. The value needs to map to a local Python variable.

The query should return a full list from the item table for the two item_title and item_rating columns:

('Casino Royale', 'PG-13')
...
('Harry Potter and the Goblet of Fire', 'PG-13')
('Harry Potter and the Order of the Phoenix', 'PG-13')
('The Lord of the Rings - Fellowship of the Ring', 'PG-13')
('The Lord of the Rings - Two Towers', 'PG-13')
('The Lord of the Rings - The Return of the King', 'PG-13')
('The Lord of the Rings - The Return of the King', 'PG-13')

This article should have shown you how to effectively work static and dynamic queries. You can find the scripts on the github.com server.

Written by maclochlainn

December 6th, 2018 at 11:40 pm

Posted in Linux,Oracle,Oracle 11g,Oracle 12c,Oracle XE,Python,Python 2.7

Tagged with , ,

Read list of a dictionaries

without comments

My students wanted a quick example of how to read a list of a dictionaries in Python. So, here it is:

#!/usr/bin/python
 
# Declare list of dictionaries.
cakes = [{'cake':"vanilla",'frosting':"chocolate"}
        ,{'cake':"chocolate",'frosting':"vanilla"}]
 
# Read the list of dictionaries.
for lkey, lvalue in enumerate(cakes):
  print lvalue['cake'] + " with " + lvalue['frosting'] + " frosting."

Naturally, a list can contain many things and you should ensure each value you read is a dictionary before trying to read it as a dictionary. At least, I’d suggest you check.

Hope this answers the how.

Written by maclochlainn

June 1st, 2017 at 9:09 pm

Posted in Python,Python 2.7,Python 3.x

Tagged with

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