To give your objects a more baked-in feel, you can use python’s standard object protocol functions so that native operators start to operate on your object.
By implementing the following items on your custom objects, infix operators start to work executing your custom code as per defined.
General
By overriding __bool__ in your objects, you can define how your object will respond in conditional scenarios. __bool__ effectively allows you to use your object as a condition in an if or while statement.
The method __call__ will allow your object to openly accept function calls:
classCallable:def__call__(self,*args,*kwargs):# implementation here
c=Callable()c()
Array
The following overrides allow you to make your objects appear like containers (arrays, etc.):
Operation
Method
Description
Length
__len__
Allows the len function to operate on the object
Iterator
__iter__
Gets an object to start the iteration process
Next
__next__
Gets an object to continue the iteration process
Reverse
__reversed__
Reverses the internal sequence
Dictionary
The following overrides allow you to make your object respond like a dict:
Operation
Method
Description
Set item
__setitem__
Sets an item in the dictionary
Get item
__getitem__
Retrieves an item from the dictionary
Delete item
__delitem__
Removes an item from the dictionary
Mathematic
The following table lists out all of the methods that you can override on a class that will give you access to arithmetic operators.
Operation
Operator
LHS
RHS
Inline
Addition
+
__add__
__radd__
__iadd__
Subtraction
-
__sub__
__rsub__
__isub__
Multiplication
*
__mul__
__rmul__
__imul__
True Division
/
__truediv__
__rtruediv__
__itruediv__
Floor Division
//
__floordiv__
__rfloordiv__
__ifloordiv__
Modulo
%
__mod__
__rmod__
__imod__
Division and Modulo
divmod
__divmod__
__rdivmod__
Exponentiation
**
__pow__
__rpow__
__ipow__
Shift left
<<
__lshift__
__rlshift__
__ilshift__
Shift right
>>
__rshift__
__rrshift__
__irshift__
Bitwise AND
&
__and__
__rand__
__iand__
Bitwise OR
|
__or__
__ror__
__ior__
Bitwise XOR
^
__xor__
__rxor__
__ixor__
Bitwise NOT
~
__invert__
Function
Method
Floor
__floor__
Ceil
__ceil__
Round
__round__
Negate
__neg__
Positive
__pos__
Absolute
__abs__
Comparison
The following table lists all of the comparison operators
Operation
Operator
Method
Equals
==
__eq__
Not Equal
!=
__ne__
Greater than, equal to
>=
__gte__
Greater than
>
__gt__
Lesser than, equal to
<=
__lte__
Lesser than
<
__lt__
Type conversions
Type
Method
Description
int
__int__
float
__float__
complex
__complex__
index
__index__
Allows python to use your object as an array index
Context
The following override allow your objects to measure contexts:
Operation
Method
Description
Enter
__enter__
Measures when a context enters
Exit
__exit__
Measures when a context exits
These functions are useful when your object is supplied to a with statement.
classContextMeasurement:def__enter__(self):print("Entering context")def__exit__(self,exc_class,exc_instance,traceback):print("Exiting context")withContextMeasurement():print("Inside the context right now")
Akka is a library designed for building applications using the actor model. From their site:
Akka is a toolkit and runtime for building highly concurrent, distributed, and resilient message-driven applications on the JVM.
In today’s post, I’m going to start with some of the primitives to using this framework.
Messages
Actors process messages that you’ll define in your modules. For today’s example, I’m going to implement a very basic logging application. Messages sent into this system are expected to be logged out to the console. To start off, we define the messages for this system:
Using scala’s case classes we can clean up the definition of these log messages. We have a message that will do general logging LogMessage, one that will log a string in LogString and one that will dissect and log out an exception object LogException.
Actor Logic
We now focus on the logic required to log information out from our actor. This is really quite simple; we’re just going to push everything out to the console:
The receive method is just a big pattern matching statement. Each of the message types are handled in here. Note how LogString and LogException send messages to self. self is a built-in, given to us representing this actor. All we’re doing is just on-forwarding the message in the string and exception cases.
Creating a system
We have actors; we have messages to pass between the actors; we now need a system that the actors will participate in.
// create the systemvalsystem=ActorSystem("myLoggingSystem")// create an actorvallogger=system.actorOf(Props[LogActor],"logger")
Using the tell and ask methods, we can send and send/receive messages to/from this actor. We also can create a logic-less actor that just acts as a message sender/receiver:
valinbox=Inbox.create(system)
Mailboxes are an important abstraction; they hold messages for actors. Each actor has its own mailbox, but we’ve created one above attached to a system that we can pipe messages into:
inbox.send(logger,LogString("This is the first line of log"))inbox.send(logger,LogException(newException("DOH!")))
Lots of Actors
A slightly more complex topic is to create a pool of actors. In this next snippet, we’ll create a RoundRobinPool.
The Bourne Again SHell is one of the most widely deployed shell for Linux that I use all the time. In today’s post, I’m going to collate a lot of the gems that I’d discovered in my travels of using this software.
Finding Help
Nothing can substitute the reference manual materials distributed with this software when it’s installed. At the console, you can read documentation in info format on bash using the following:
info bash
You’re able to deduce executing this command by doing some research at the console, by yourself. Using apropos(which searches the manual pages) you can look for key words.
If you wanted to find any command that begins with the characters ‘ls’ in an attempt to find the command ls, you can perform the following search:
apropos ls | grep'^ls.*'
On my system here, this function emits the following result:
ls (1) - list directory contents
lsattr (1) - list file attributes on a Linux second extended file s...
lsb_release (1) - print distribution-specific information
lsblk (8) - list block devices
lscpu (1) - display information about the CPU architecture
lsdiff (1) - show which files are modified by a patch
lsearch (3) - linear search of an array
lseek (2) - reposition read/write file offset
lseek64 (3) - reposition 64-bit read/write file offset
lshw (1) - list hardware
lsinitramfs (8) - list content of an initramfs image
lslocks (8) - list local system locks
lsmod (8) - Show the status of modules in the Linux Kernel
lsof (8) - list open files
lspci (8) - list all PCI devices
lspcmcia (8) - display extended PCMCIA debugging information
lspgpot (1) - extracts the ownertrust values from PGP keyrings and l...
lstat (2) - get file status
lstat64 (2) - get file status
lsusb (8) - list USB devices
We’re only interested in the first item there, but we’re given all of the options. We can now display the manual page with the following:
man ls 1
Variables
Variable creation is fairly straight forward:
# stores the string "John" in var1var1="John"# stores the text output of the command 'ls' into var2var2=`ls-al`# simple string replacementvar3=${var1/h/a}# sub-string (turns "John" into "Jo")var4=${var1:0:2}# default string substitution (where null)var6=${var5:-"Value for var5 was not supplied"}# string interpolation is achieved with $echo"His name is $var1"
Special variables exist to tell the developer a little bit about their environment:
Variable
Description
$?
Return code from the last program that just ran
$$
Currently executing script’s PID
$#
Number of arguments passed to this script (argc)
$@
All arguments passed to this script
$1$2
Each argument passed to the script ($3, $4, etc.)
Functions
# define a functionfunction syntax(){echo"usage: prog.sh [options]"return 0
}function print_name(){echo"Hello $1"return 0
}# call the function
syntax
print_name "John"
Control Flow Constructs
# Conditionalsif[ var1 == 10 ]then
echo"It was 10"else
echo"It was not 10"fi
case"$var1"in
0)echo"Value was zero";;
1)echo"Value was one";;*)echo"Anything but null";;esac# Repetitionfor var1 in{1..10}do
done
for((x=1; x <= 10; x++))do
done
while[ var1 == 10 ]do
done
Redirection
Special file descriptors of 0 as /dev/stdin, 1 as /dev/stdout and 2 as /dev/stderr.
Note that the order of redirections is significant. For example, the command
ls> dirlist 2>&1
directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the file dirlist, while the command
ls 2>&1 > dirlist
directs only the standard output to file dirlist, because the standard error was made a copy of the standard output before the standard output was redirected to dirlist.
JDBC(Java Database Connectivity) is a general purpose data access library baked into the Java development and runtime. This library’s purpose is to lower the level of complexity in connecting to different database vendors providing a consistent interface no matter what database you’re connecting to.
In today’s post, I’ll go through the basics of using this library. This blog post will be in context of connecting to a PostgreSQL database.
Drivers
JDBC is based on the premise of drivers. The driver code itself is what fills in the architecture with an implementation that your applications will use. To enumerate all of the drivers, currently in context of your application you can use the following:
I use the term “in context” because whilst you may have the required JAR installed on your system which will be a particular database vendor’s connection library for JDBC, you’ll need to make sure that it’s available on your class path.
For my example, I only have Postgres available to me:
class org.postgresql.Driver
The driver string that you saw in the section above plays an important role in establishing a connection to your database. Before you can start to work with Connection, Statement and ResultSet objects you first need to load in your vendor’s library implementation.
Class.forName("org.postgresql.Driver");
This will reflect your driver into your application ready for use.
Making a connection
To establish a connection with a database, you’ll need to specify a connection string with all of the attributes required to direct your application to the database.
JDBC has a uniform format for specifying its connections with each vendor. Postgres conncetions are no different.
A connection is established using the DriverManager class like so.
Running retrieves on your database normally comprises of three processes:
Preparing a statement to run
Executing the statement
Enumerating the results
The preparation of the statement is fairly straight forward. The createStatement method on the Connection object will allow you to create an empty statement, whereas prepareStatement will allow you to provide some SQL directly.
// prepare the statement StatementretrieveStatement=connection.createStatement();// execute the statementResultSetstreetTypes=retrieveStatement.executeQuery("SELECT * FROM \"StreetType\"");// enumerate the resultwhile(streetTypes.next()){intid=streetTypes.getInt(streetTypes.findColumn("ID"));Stringname=streetTypes.getString(streetTypes.findColumn("Name"));System.out.println(String.format("ID: %d, Name: %s\n",id,name));}
A slightly more complex example where you’d pass in some parameters into your statement might look like this:
PreparedStatementretrieveStatement=connection.prepareStatement("SELECT * FROM \"StreetType\" WHERE \"ID\" > ?");retrieveStatement.setInt(1,10);ResultSetstreetTypes=retrieveStatement.executeQuery();
Enumerating a ResultSet object can be achieved with a simple while loop:
H2 is a relational database written entirely in Java. It has an extremely small footprint and has an in-memory mode making it an excellent choice for embedded applications.
In today’s post, I’ll take you through using the H2 shell.
Shell
Once you’ve downloaded H2 from their site, you can get a database created and running using the shell. You can invoke the shell with the following command:
I’m using version 1.4.190 here. The -url command line directs us to the file of the database that we’ll create/open.
Once the shell is running, you’re presented with a sql> prompt. You can start creating your table definitions. The documentation on the website is quite extensive with the supported sql grammar, functions and data types.
Further development
Now that you’ve created a database, you can write java applications using JDBC to run queries against your H2 database.