Sunday, 24 July 2011

Introduction To Python From Byte Of Python.

Introduction
Python is one of those rare languages which can claim to be both simple and powerful.
You will find that you will be pleasantly surprised on how easy it is to concentrate on the
solution to the problem rather than the syntax and structure of the language you are
programming in.
The official introduction to Python is:
      Python is an easy to learn, powerful programming language. It has efficient
      high-level data structures and a simple but effective approach to object-oriented
      programming. Python's elegant syntax and dynamic typing, together with its
      interpreted nature, make it an ideal language for scripting and rapid application
      development in many areas on most platforms.
I will discuss most of these features in more detail in the next section.
Note
      Guido van Rossum, the creator of the Python language, named the language after the
      BBC show "Monty Python's Flying Circus". He doesn't particularly like snakes that kill
      animals for food by winding their long bodies around them and crushing them.
Features of Python
Simple
      Python is a simple and minimalistic language. Reading a good Python program feels
      almost like reading English, although very strict English! This pseudo-code nature of
      Python is one of its greatest strengths. It allows you to concentrate on the solution to
      the problem rather than the language itself.
Easy to Learn
      As you will see, Python is extremely easy to get started with. Python has an
      extraordinarily simple syntax, as already mentioned.
Free and Open Source
      Python is an example of a FLOSS (Free/Libré and Open Source Software). In simple
      terms, you can freely distribute copies of this software, read its source code, make
      changes to it, and use pieces of it in new free programs. FLOSS is based on the
      concept of a community which shares knowledge. This is one of the reasons why
      Python is so good - it has been created and is constantly improved by a community who
      just want to see a better Python.
High-level Language
      When you write programs in Python, you never need to bother about the low-level
      details such as managing the memory used by your program, etc.
Portable
      Due to its open-source nature, Python has been ported to (i.e. changed to make it work
      on) many platforms. All your Python programs can work on any of these platforms
      without requiring any changes at all if you are careful enough to avoid any
system-dependent features.
You can use Python on Linux, Windows, FreeBSD, Macintosh, Solaris, OS/2, Amiga,
AROS, AS/400, BeOS, OS/390, z/OS, Palm OS, QNX, VMS, Psion, Acorn RISC OS,
VxWorks, PlayStation, Sharp Zaurus, Windows CE and even PocketPC !

Interpreted
    This requires a bit of explanation.
    A program written in a compiled language like C or C++ is converted from the source
    language i.e. C or C++ into a language that is spoken by your computer (binary code
    i.e. 0s and 1s) using a compiler with various flags and options. When you run the
    program, the linker/loader software copies the program from hard disk to memory and
    starts running it.
    Python, on the other hand, does not need compilation to binary. You just run the
    program directly from the source code. Internally, Python converts the source code
    into an intermediate form called bytecodes and then translates this into the native
    language of your computer and then runs it. All this, actually, makes using Python
    much easier since you don't have to worry about compiling the program, making sure
    that the proper libraries are linked and loaded, etc, etc. This also makes your Python
    programs much more portable, since you can just copy your Python program onto
    another computer and it just works!
Object Oriented
    Python supports procedure-oriented programming as well as object-oriented
    programming. In procedure-oriented languages, the program is built around
    procedures or functions which are nothing but reusable pieces of programs. In
    object-oriented languages, the program is built around objects which combine data
    and functionality. Python has a very powerful but simplistic way of doing OOP,
    especially when compared to big languages like C++ or Java.
Extensible
    If you need a critical piece of code to run very fast or want to have some piece of
    algorithm not to be open, you can code that part of your program in C or C++ and
    then use it from your Python program.
Embeddable
    You can embed Python within your C/C++ programs to give 'scripting' capabilities for
    your program's users.
Extensive Libraries
    The Python Standard Library is huge indeed. It can help you do various things
    involving regular expressions, documentation generation, unit testing, threading,
    databases, web browsers, CGI, FTP, email, XML, XML-RPC, HTML, WAV files,
    cryptography, GUI (graphical user interfaces), Tk, and other system-dependent stuff.
    Remember, all this is always available wherever Python is installed. This is called the
    'Batteries Included' philosophy of Python.
    Besides, the standard library, there are various other high-quality libraries such as
    wxPython (http:/ / www. wxpython. org) , Twisted (http:/ / www. twistedmatrix. com/
    products/ twisted), Python Imaging Library (http:/ / www. pythonware. com/ products/
    pil/ index. htm) and many more.

Python is indeed an exciting and powerful language. It has the right combination of
performance and features that make writing programs in Python both fun and easy.
Why not Perl?
If you didn't know already, Perl is another extremely popular open source interpreted
programming language.
If you have ever tried writing a large program in Perl, you would have answered this
question yourself! In other words, Perl programs are easy when they are small and it excels
at small hacks and scripts to 'get work done'. However, they quickly become unwieldy once
you start writing bigger programs and I am speaking this out of my experience writing
large Perl programs at Yahoo!
When compared to Perl, Python programs are definitely simpler, clearer, easier to write
and hence more understandable and maintainable. I do admire Perl and I do use it on a
daily basis for various things but whenever I write a program, I always start thinking in
terms of Python because it has become so natural for me. Perl has undergone so many
hacks and changes, that it feels like it is one big (but one hell of a) hack. Sadly, the
upcoming Perl 6 does not seem to be making any improvements regarding this.
The only and very significant advantage that I feel Perl has, is its huge CPAN (http:/ / cpan.
perl. org) library - the Comprehensive Perl Archive Network. As the name suggests, this is a
humongous collection of Perl modules and it is simply mind-boggling because of its sheer
size and depth - you can do virtually anything you can do with a computer using these
modules. One of the reasons that Perl has more libraries than Python is that it has been
around for a much longer time than Python. However this seems to be changing with the
growing Python Package Index (http:/ / pypi. python. org/ pypi).
Why not Ruby?
If you didn't know already, Ruby is another popular open source interpreted programming
language.
If you already like and use Ruby, then I would definitely recommend you to continue using
it.
For other people who have not used it and are trying to judge whether to learn Python or to
learn Ruby, then I would recommend Python, purely from an ease-of-learning perspective. I
personally found it hard to grok the Ruby language, but for people who understand Ruby,
they all praise the beauty of the language. Unfortunately, I am not as lucky.
What Programmers Say
You may find it interesting to read what great hackers like ESR have to say about Python:
•  Eric S. Raymond is the author of "The Cathedral and the Bazaar" and is also the person
    who coined the term Open Source. He says that Python has become his favorite
    programming language (http:/ / www. linuxjournal. com/ article. php?sid=3882). This
    article was the real inspiration for my first brush with Python.
•  Bruce Eckel is the author of the famous Thinking in Java and Thinking in C++ books.
    He says that no language has made him more productive than Python. He says that
    Python is perhaps the only language that focuses on making things easier for the
   programmer. Read the complete interview (http:/ / www. artima. com/ intv/ aboutme.
   html) for more details.
•  Peter Norvig is a well-known Lisp author and Director of Search Quality at Google
   (thanks to Guido van Rossum for pointing that out). He says that Python has always been
   an integral part of Google. You can actually verify this statement by looking at the Google
   Jobs (http:/ / www. google. com/ jobs/ index. html) page which lists Python knowledge as a
   requirement for software engineers.
About Python 3.0
Python 3.0 is the new version of the language. It is sometimes referred to as Python 3000 or
Py3K.
The main reason for a major new version of Python is to remove all the small problems and
nitpicks that have accumulated over the years and to make the language even more clean.
If you already have a lot of Python 2.x code, then there is a utility to assist you to convert
2.x to 3.x source (http:/ / docs. python. org/ dev/ 3. 0/ library/ 2to3. html).
More details are at:
•  Guido van Rossum's introduction (http:/ / www. artima. com/ weblogs/ viewpost.
   jsp?thread=208549)
•  What's New in Python 2.6 (http:/ / docs. python. org/ dev/ whatsnew/ 2. 6. html) (features significantly different from previous Python 2.x versions and most likely will be included in Python 3.0)
•  What's New in Python 3.0 (http:/ / docs. python. org/ dev/ 3. 0/ whatsnew/ 3. 0. html)
•  Python 2.6 and 3.0 Release Schedule (http:/ / www. python. org/ dev/ peps/ pep-0361/ )
•  Python 3000 (the official authoritative list of proposed changes) (http:/ / www. python.
   org/ dev/ peps/ pep-3000/ )
•  Miscellaneous Python 3.0 Plans (http:/ / www. python. org/ dev/ peps/ pep-3100/ )
•  Python News (detailed list of changes) (http:/ / www. python. org/ download/ releases/ 3.0/ NEWS. txt)

Python en:Installation
If you have Python 2.x installed already, you do not have to remove it to install Python 3.0.
You can have both installed at the same time.
For Linux and BSD users
If you are using a Linux distribution such as Ubuntu, Fedora, OpenSUSE or {put your
choice here}, or a BSD system such as FreeBSD, then it is most likely you already have
Python installed on your system.
To test if you have Python already installed on your Linux box, open a shell program (like
konsole or gnome-terminal) and enter the command python -V as shown below.
      $ python -V
      Python 3.0b1
Note
      $ is the prompt of the shell. It will be different for you depending on the settings of
      your OS, hence I will indicate the prompt by just the $ symbol.
If you see some version information like the one shown above, then you have Python
installed already.
However, if you get a message like this one:
      $ python -V
      bash: Python: command not found
Then you don't have Python installed. This is highly unlikely but possible.
Note
      If you have Python 2.x already installed, then try python3 -V.
In this case, you have two ways of installing Python on your system.
•  You can compile Python from the source code (http:/ / www. python. org/ download/
   releases/ 3. 0/ ) and install it. The compilation instructions are provided at the website.
•  [This option will be available after the final release of Python 3.0] Install the binary
   packages using the package management software that comes with your OS, such as
   apt-get in Ubuntu/Debian and other Debian-based Linux, yum in Fedora Linux, pkg_add
   in FreeBSD, etc. Note that you will need an internet connection to use this method.
   Alternatively, you can download the binaries from somewhere else and then copy to your
   PC and install it.

For Windows Users
Visit http:/ / www. python. org/ download/ releases/ 3. 0/ and download the latest version
from this website, which was 3.0 beta 1 (http:/ / www. python. org/ ftp/ python/ 3. 0/
python-3. 0b1. msi) as of this writing. This is just 12.8 MB which is very compact compared
to most other languages or software. The installation is just like any other Windows-based
software.
Caution
     When you are given the option of unchecking any "optional" components, don't
     uncheck any! Some of these components can be useful for you, especially IDLE.
An interesting fact is that majority of Python downloads are by Windows users. Of course,
this doesn't give the complete picture since almost all Linux users will have Python installed
already on their systems by default.
DOS Prompt
If you want to be able to use Python from the Windows command line i.e. the DOS prompt,
then you need to set the PATH variable appropriately.
For Windows 2000, XP, 2003 , click on Control Panel -> System -> Advanced ->
Environment Variables. Click on the variable named PATH in the 'System Variables'
section, then select Edit and add ;C:\Python30 to the end of what is already there. Of
course, use the appropriate directory name.
For older versions of Windows, add the following line to the file C:\AUTOEXEC.BAT :
'PATH=%PATH%;C:\Python30' (without the quotes) and restart the system. For Windows NT,
use the AUTOEXEC.NT file.
For Mac OS X Users
Mac OS X Users will find Python already installed on their system. Open the Terminal.app
and run python -V and follow the advice in the above Linux section.
Summary
For a Linux system, you most probably already have Python installed on your system.
Otherwise, you can install it using the package management software that comes with your
distribution. For a Windows system, installing Python is as easy as downloading the
installer and double-clicking on it. From now on, we will assume that you have Python
installed on your system.
Next, we will write our first Python program.


Python en:First Steps
Introduction
We will now see how to run a traditional 'Hello World' program in Python. This will teach
you how to write, save and run Python programs.
There are two ways of using Python to run your program - using the interactive interpreter
prompt or using a source file. We will now see how to use both of these methods
Using The Interpreter Prompt
Start the interpreter on the command line by entering python at the shell prompt.
For Windows users, you can run the interpreter in the command line if you have set the
PATH variable appropriately.
If you are using IDLE, click on Start → Programs → Python 3.0 → IDLE (Python GUI).
Now enter print('Hello World') followed by the Enter key. You should see the words
Hello World as output.
      $ python
      Python 3.0b2 (r30b2:65106, Jul 18 2008, 18:44:17) [MSC v.1500 32
bit (Intel)] on win32
      Type "help", "copyright", "credits" or "license" for more
information.
      >>> print('Hello World')
      Hello World
      >>>
Notice that Python gives you the output of the line immediately! What you just entered is a
single Python statement. We use print to (unsurprisingly) print any value that you supply
to it. Here, we are supplying the text Hello World and this is promptly printed to the
screen.
How to Quit the Interpreter Prompt
      To exit the prompt, press ctrl-d if you are using IDLE or are using a Linux/BSD shell.
      In case of the Windows command prompt, press ctrl-z followed by enter key.
Choosing An Editor
Before we move on to writing Python programs in source files, we need an editor to write
the source files. The choice of an editor is crucial indeed. You have to choose an editor as
you would choose a car you would buy. A good editor will help you write Python programs
easily, making your journey more comfortable and helps you reach your destination
(achieve your goal) in a much faster and safer way.
One of the very basic requirements is syntax highlighting where all the different parts of
your Python program are colorized so that you can see your program and visualize its
running.
If you are using Windows, then I suggest that you use IDLE. IDLE does syntax highlighting
and a lot more such as allowing you to run your programs within IDLE among other things.
A special note: Do not use Notepad - it is a bad choice because it does not do syntax
highlighting and also importantly it does not support indentation of the text which is very
important in our case as we will see later. Good editors such as IDLE (and also VIM) will
automatically help you do this.
If you are using Linux/FreeBSD, then you have a lot of choices for an editor. If you are just
beginning to program, you might want to use geany. It has a graphical user interface and
has buttons to compile and run your python program without a fuss.
If you are an experienced programmer, then you must be already using Vim or Emacs.
Needless to say, these are two of the most powerful editors and you will be benefitted by
using them to write your Python programs. I personally use Vim for most of my programs.
If you are a beginner programmer, then you can use Kate which is one of my favorites. In
case you are willing to take the time to learn Vim or Emacs, then I highly recommend that
you do learn to use either of them as it will be very useful for you in the long run.
In this book, we will use IDLE, our IDE and editor of choice. IDLE is installed by default
with the Windows and Mac OS X Python installers. It is also available for installation for
Linux (http:/ / love-python. blogspot. com/ 2008/ 03/ install-idle-in-linux. html) and BSDs in
their respective repositories.
We will explore how to use IDLE in the next section. For further details, please refer the
IDLE documentation (http:/ / www. python. org/ idle/ doc/ idlemain. html).
If you still want to explore other choices of an editor, see the comprehensive list of Python
editors (http:/ / www. python. org/ cgi-bin/ moinmoin/ PythonEditors) and make your choice.
You can also choose an IDE (Integrated Development Environment) for Python. See the
comprehensive list of IDEs that support Python (http:/ / www. python. org/ cgi-bin/
moinmoin/ IntegratedDevelopmentEnvironments) for more details. Once you start writing
large Python programs, IDEs can be very useful indeed.
I repeat once again, please choose a proper editor - it can make writing Python programs
more fun and easy.
For Vim users
     There is a good introduction on how to make Vim a powerful Python IDE by John M
     Anderson (http:/ / blog. sontek. net/ 2008/ 05/ 11/ python-with-a-modular-ide-vim/ ).
For Emacs users
     There is a good introduction on how to make Emacs a powerful Python IDE by Ryan
     McGuire        (http:/    /     www.      enigmacurry.      com/      2008/     05/   09/
     emacs-as-a-powerful-python-ide/ ).

Using A Source File
Now let's get back to programming. There is a tradition that whenever you learn a new
programming language, the first program that you write and run is the 'Hello World'
program - all it does is just say 'Hello World' when you run it. As Simon Cozens [1] puts it, it
is the 'traditional incantation to the programming gods to help you learn the language
better' :) .
Start your choice of editor, enter the following program and save it as helloworld.py
If you are using IDLE, click on File → New Window and enter the following program. Then
click on File → Save.
#!/usr/bin/python
#Filename: helloworld.py
print('Hello World')
Run this program by opening a shell (Linux terminal or DOS prompt) and entering the
command python helloworld.py.
If you are using IDLE, use the menu Run → Run Module or the keyboard shortcut F5.
The output is as shown below.
     $ python helloworld.py
     Hello World
If you got the output as shown above, congratulations! - you have successfully run your first
Python program.
In case you got an error, please type the above program exactly as shown and above and
run the program again. Note that Python is case-sensitive i.e. print is not the same as
Print - note the lowercase p in the former and the uppercase P in the latter. Also, ensure
there are no spaces or tabs before the first character in each line - we will see why this is
important later.
How It Works
Let us consider the first two lines of the program. These are called comments - anything to
the right of the # symbol is a comment and is mainly useful as notes for the reader of the
program.
Python does not use comments except for the special case of the first line here. It is called
the shebang line - whenever the first two characters of the source file are #! followed by
the location of a program, this tells your Linux/Unix system that this program should be run
with this interpreter when you execute the program. This is explained in detail in the next
section. Note that you can always run the program on any platform by specifying the
interpreter directly on the command line such as the command python helloworld.py .
Important
     Use comments sensibly in your program to explain some important details of your
     program - this is useful for readers of your program so that they can easily understand
     what the program is doing. Remember, that person can be yourself after six months!
The comments are followed by a Python statement. Here we call the print function this
just prints the text 'Hello World'. We will learn about functions in a → later chapter, what
you should understand now is that whatever you supply in the parentheses will be printed
back to the screen. In this case, we supply 'Hello World' which is referred to as a string -
don't worry, we will explore these terminologies in detail later.
Executable Python Programs
This applies only to Linux/Unix users but Windows users might be curious as well about the
first line of the program. First, we have to give the program executable permission using
the chmod command then run the source program.
      $ chmod a+x helloworld.py
      $ ./helloworld.py
      Hello World
The chmod command is used here to change the mode of the file by giving execute
permission to all users of the system. Then, we execute the program directly by specifying
the location of the source file. We use the ./ to indicate that the program is located in the
current directory.
To make things more fun, you can rename the file to just helloworld and run it as
./helloworld and it will still work since the system knows that it has to run the program
using the interpreter whose location is specified in the first line in the source file.
What if you don't know where Python is located? Then, you can use the special env
program on Linux/Unix systems. Just change the first line of the program to the following:
#!/usr/bin/env python
The env program will in turn look for the Python interpreter which will run the program.
So far, we have been able to run our program as long as we know the exact path. What if
we wanted to be able to run the program from anywhere? You can do this by storing the
program in one of the directories listed in the PATH environment variable. Whenever you
run any program, the system looks for that program in each of the directories listed in the
PATH environment variable and then runs that program. We can make this program
available everywhere by simply copying this source file to one of the directories listed in
PATH.
      $ echo $PATH
      /usr/local/bin:/usr/bin:/bin:/usr/X11R6/bin:/home/swaroop/bin
      $ cp helloworld.py /home/swaroop/bin/helloworld
      $ helloworld
      Hello World
We can display the PATH variable using the echo command and prefixing the variable name
by $ to indicate to the shell that we need the value of this variable. We see that
/home/swaroop/bin is one of the directories in the PATH variable where swaroop is the
username I am using in my system. There will usually be a similar directory for your
username on your system. Alternatively, you can add a directory of your choice to the PATH
variable - this can be done by running PATH=$PATH:/home/swaroop/mydir where
'/home/swaroop/mydir' is the directory I want to add to the PATH variable.
This method is very useful if you want to write useful scripts that you want to run the
program anytime, anywhere. It is like creating your own commands just like cd or any
other commands that you use in the Linux terminal or DOS prompt.
Caution
     W.r.t. Python, a program or a script or software all mean the same thing.
Getting Help
If you need quick information about any function or statement in Python, then you can use
the built-in help functionality. This is very useful especially when using the interpreter
prompt. For example, run help(print) - this displays the help for the print function which
is used to print things to the screen.
Note
     Press q to exit the help.
Similarly, you can obtain information about almost anything in Python. Use help() to learn
more about using help itself!
In case you need to get help for operators like return, then you need to put those inside
quotes such as help('return') so that Python doesn't get confused on what we're trying
to do.
Summary
You should now be able to write, save and run Python programs at ease. Now that you are a
Python user, let's learn some more Python concepts.


Python en:Basics
Just printing 'Hello World' is not enough, is it? You want to do more than that - you want to
take some input, manipulate it and get something out of it. We can achieve this in Python
using constants and variables.
Literal Constants
An example of a literal constant is a number like 5, 1.23, 9.25e-3 or a string like 'This is
a string' or "It's a string!". It is called a literal because it is literal - you use its value
literally. The number 2 always represents itself and nothing else - it is a constant because
its value cannot be changed. Hence, all these are referred to as literal constants.
Numbers
Numbers in Python are of three types - integers, floating point and complex numbers.
•  An examples of an integer is 2 which is just a whole number.
•  Examples of floating point numbers (or floats for short) are 3.23 and 52.3E-4. The E
   notation indicates powers of 10. In this case, 52.3E-4 means 52.3 * 10-4.
•  Examples of complex numbers are (-5+4j) and (2.3 - 4.6j)
Note for Experienced Programmers
      There is no separate 'long int' type. The default integer type can be any large value.
Strings
A string is a sequence of characters. Strings are basically just a bunch of words. The words
can be in English or any other language that is supported in the Unicode standard, which
means almost any language in the world (http:/ / www. unicode. org/ faq/ basic_q. html#16).
Note for Experienced Programmers
      There are no "ASCII-only" strings because Unicode is a superset of ASCII. If a strictly
      ASCII-encoded byte-stream is needed, then use str.encode("ascii"). For more
      details, please see the related discussion at StackOverflow (http:/ / stackoverflow. com/
      questions/                                                                        175240/
      how-do-i-convert-a-files-format-from-unicode-to-ascii-using-python#175270).
      By default, all strings are in Unicode.
I can almost guarantee that you will be using strings in almost every Python program that
you write, so pay attention to the following part on how to use strings in Python. 

Single Quotes
You can specify strings using single quotes such as 'Quote me on this'. All white space
i.e. spaces and tabs are preserved as-is.
Double Quotes
Strings in double quotes work exactly the same way as strings in single quotes. An example
is "What's your name?"
Triple Quotes
You can specify multi-line strings using triple quotes - (""" or '''). You can use single quotes
and double quotes freely within the triple quotes. An example is:
      '''This  is a multi-line string. This is the first line.
      This is  the second line.
      "What's  your name?," I asked.
      He said  "Bond, James Bond."
      '''
Escape Sequences
Suppose, you want to have a string which contains a single quote ('), how will you specify
this string? For example, the string is What's your name?. You cannot specify 'What's
your name?' because Python will be confused as to where the string starts and ends. So,
you will have to specify that this single quote does not indicate the end of the string. This
can be done with the help of what is called an escape sequence. You specify the single
quote as \' - notice the backslash. Now, you can specify the string as 'What\'s your
name?'.
Another way of specifying this specific string would be "What's your name?" i.e. using
double quotes. Similarly, you have to use an escape sequence for using a double quote itself
in a double quoted string. Also, you have to indicate the backslash itself using the escape
sequence \\.
What if you wanted to specify a two-line string? One way is to use a triple-quoted string as
shown previously or you can use an escape sequence for the newline character - \n to
indicate the start of a new line. An example is This is the first line\nThis is the
second line. Another useful escape sequence to know is the tab - \t. There are many more
escape sequences but I have mentioned only the most useful ones here.
One thing to note is that in a string, a single backslash at the end of the line indicates that
the string is continued in the next line, but no newline is added. For example:
      "This is the first sentence.\
      This is the second sentence."
is equivalent to "This is the first sentence. This is the second sentence.".

Raw Strings
If you need to specify some strings where no special processing such as escape sequences
are handled, then what you need is to specify a raw string by prefixing r or R to the string.
An example is r"Newlines are indicated by \n".
Strings Are Immutable
This means that once you have created a string, you cannot change it. Although this might
seem like a bad thing, it really isn't. We will see why this is not a limitation in the various
programs that we see later on.
String Literal Concatenation
If you place two string literals side by side, they are automatically concatenated by Python.
For example, 'What\'s ' 'your name?' is automatically converted in to "What's your
name?".
Note for C/C++ Programmers
     There is no separate char data type in Python. There is no real need for it and I am
     sure you won't miss it.
Note for Perl/PHP Programmers
     Remember that single-quoted strings and double-quoted strings are the same - they do
     not differ in any way.
Note for Regular Expression Users
     Always use raw strings when dealing with regular expressions. Otherwise, a lot of
     backwhacking may be required. For example, backreferences can be referred to as
     '\\1' or r'\1'.
The format Method
Sometimes we may want to construct strings from other information. This is where the
format() method is useful.
#!/usr/bin/python
# Filename: str_format.py
age = 25
name = 'Swaroop'
print('{0} is {1} years old'.format(name, age))
print('Why is {0} playing with that python?'.format(name))
Output:
     $ python str_format.py
     Swaroop is 25 years old
     Why is Swaroop playing with that python?
How It Works:
A string can use certain specifications and subsequently, the format method can be called
to substitute those specifications with corresponding arguments to the format method.


Observe the first usage where we use {0} and this corresponds to the variable name which
is the first argument to the format method. Similarly, the second specification is {1}
corresponding to age which is the second argument to the format method.
Notice that we could achieved the same using string concatenation: name + ' is ' +
str(age) + ' years old' but notice how much uglier and error-prone this is. Second, the
conversion to string would be done automatically by the format method instead of the
explicit conversion here. Third, when using the format method, we can change the
message without having to deal with the variables used and vice-versa.
What Python does in the format method is that it substitutes each argument value into the
place of the specification. There can be more detailed specifications such as:
>>> '{0:.3}'.format(1/3) # decimal (.) precision of 3 for float
'0.333'
>>> '{0:_^11}'.format('hello') # fill with underscores (_) with the text
centered (^) to 11 width
'___hello___'
>>> '{name} wrote {book}'.format(name='Swaroop', book='A Byte of Python')
# keyword-based
'Swaroop wrote A Byte of Python'
Details of this formatting specification is explained in the Python Enhancement Proposal
No. 3101 (http:/ / www. python. org/ dev/ peps/ pep-3101/ ).
Variables
Using just literal constants can soon become boring - we need some way of storing any
information and manipulate them as well. This is where variables come into the picture.
Variables are exactly what the name implies - their value can vary, i.e., you can store
anything using a variable. Variables are just parts of your computer's memory where you
store some information. Unlike literal constants, you need some method of accessing these
variables and hence you give them names.
Identifier Naming
Variables are examples of identifiers. Identifiers are names given to identify something.
There are some rules you have to follow for naming identifiers:
•  The first character of the identifier must be a letter of the alphabet (uppercase ASCII or
   lowercase ASCII or Unicode character) or an underscore ('_').
•  The rest of the identifier name can consist of letters (uppercase ASCII or lowercase
   ASCII or Unicode character), underscores ('_') or digits (0-9).
•  Identifier names are case-sensitive. For example, myname and myName are not the same.
   Note the lowercase n in the former and the uppercase N in the latter.
•  Examples of valid identifier names are i, __my_name, name_23, a1b2_c3 and
   resumÃÆâ€TMƒÂ©_count.
•  Examples of invalid identifier names are 2things, this is spaced out, my-name, and
   "this_is_in_quotes".

Data Types
Variables can hold values of different types called data types. The basic types are numbers
and strings, which we have already discussed. In later chapters, we will see how to create
our own types using classes.
Objects
Remember, Python refers to anything used in a program as an object. This is meant in the
generic sense. Instead of saying 'the something', we say 'the object'.
Note for Object Oriented Programming users
     Python is strongly object-oriented in the sense that everything is an object including
     numbers, strings and functions.
We will now see how to use variables along with literal constants. Save the following
example and run the program.
How to write Python programs
     Henceforth, the standard procedure to save and run a Python program is as follows:
  1.  Open your favorite editor.
  1.  Enter the program code given in the example.
  1.  Save it as a file with the filename mentioned in the comment. I follow the convention
     of having all Python programs saved with the extension .py.
  1.  Run the interpreter with the command python program.py or use IDLE to run the
     programs. You can also use the executable method as explained earlier.
Example: Using Variables And Literal Constants
# Filename : var.py
i = 5
print(i)
i = i + 1
print(i)
s = '''This is a multi-line string.
This is the second line.'''
print(s)
Output:
     $ python var.py
     5
     6
     This is a multi-line string.
     This is the second line.
How It Works:
Here's how this program works. First, we assign the literal constant value 5 to the variable
i using the assignment operator (=). This line is called a statement because it states that
something should be done and in this case, we connect the variable name i to the value 5.
Next, we print the value of i using the print statement which, unsurprisingly, just prints
the value of the variable to the screen.
Then we add 1 to the value stored in i and store it back. We then print it and expectedly,
we get the value 6.
Similarly, we assign the literal string to the variable s and then print it.
Note for static language programmers
      Variables are used by just assigning them a value. No declaration or data type
      definition is needed/used.
Logical And Physical Lines
A physical line is what you see when you write the program. A logical line is what Python
sees as a single statement. Python implicitly assumes that each physical line corresponds to
a logical line.
An example of a logical line is a statement like print('Hello World') - if this was on a
line by itself (as you see it in an editor), then this also corresponds to a physical line.
Implicitly, Python encourages the use of a single statement per line which makes code more
readable.
If you want to specify more than one logical line on a single physical line, then you have to
explicitly specify this using a semicolon (;) which indicates the end of a logical
line/statement. For example,
      i = 5
      print(i)
is effectively same as
      i = 5;
      print(i);
and the same can be written as
      i = 5; print(i);
or even
      i = 5; print(i)
However, I strongly recommend that you stick to writing a single logical line in              a
single physical line only. Use more than one physical line for a single logical line only     if
the logical line is really long. The idea is to avoid the semicolon as much as possible since it
leads to more readable code. In fact, I have never used or even seen a semicolon in           a
Python program.
An example of writing a logical line spanning many physical lines follows. This is referred to
as explicit line joining.
      s = 'This is a string. \
      This continues the string.'
      print(s)
This gives the output:
      This is a string. This continues the string.
Similarly,
      print\
      (i)
is the same as
      print(i)
Sometimes, there is an implicit assumption where you don't need to use a backslash. This is
the case where the logical line uses parentheses, square brackets or curly braces. This is is
called implicit line joining. You can see this in action when we write programs using lists
in later chapters.
Indentation
Whitespace is important in Python. Actually, whitespace at the beginning of the line is
important. This is called indentation. Leading whitespace (spaces and tabs) at the
beginning of the logical line is used to determine the indentation level of the logical line,
which in turn is used to determine the grouping of statements.
This means that statements which go together must have the same indentation. Each such
set of statements is called a block. We will see examples of how blocks are important in
later chapters.
One thing you should remember is that wrong indentation can give rise to errors. For
example:
i = 5
  print('Value is ', i) # Error! Notice a single space at the start of
the line
print('I repeat, the value is ', i)
When you run this, you get the following error:
        File "whitespace.py", line 4
           print('Value is ', i) # Error! Notice a single space at the
start of the line
           ^
      IndentationError: unexpected indent
Notice that there is a single space at the beginning of the second line. The error indicated
by Python tells us that the syntax of the program is invalid i.e. the program was not
properly written. What this means to you is that you cannot arbitrarily start new blocks of
statements (except for the default main block which you have been using all along, of
course). Cases where you can use new blocks will be detailed in later chapters such as the
control flow chapter.
How to indent
      Do not use a mixture of tabs and spaces for the indentation as it does not work across
      different platforms properly. I strongly recommend that you use a single tab or four
     spaces for each indentation level.
     Choose either of these two indentation styles. More importantly, choose one and use it
     consistently i.e. use that indentation style only.
Note to static language programmers
     Python will always use indentation for blocks and will never use braces. Run from
     __future__ import braces to learn more.
Summary
Now that we have gone through many nitty-gritty details, we can move on to more
interesting stuff such as control flow statements. Be sure to become comfortable with what
you have read in this chapter.
Python en:Operators and Expressions
Introduction
Most statements (logical lines) that you write will contain expressions. A simple example
of an expression is 2 + 3. An expression can be broken down into operators and operands.
Operators are functionality that do something and can be represented by symbols such as +
or by special keywords. Operators require some data to operate on and such data is called
operands. In this case, 2 and 3 are the operands.
Operators
We will briefly take a look at the operators and their usage:
Note that you can evaluate the expressions given in the examples using the interpreter
interactively. For example, to test the expression 2 + 3, use the interactive Python
interpreter prompt:
>>> 2 + 3
5
>>> 3 * 5
15
>>>
 





Shortcut for math operation and assignment
It is common to run a math operation on a variable and then assign the result of the
operation back to the variable, hence there is a shortcut for such expressions:
You can write:
a = 2; a = a * 3
as:
a = 2; a *= 3
Notice that var = var operation expression becomes var operation= expression.
Evaluation Order
If you had an expression such as 2 + 3 * 4, is the addition done first or the multiplication?
Our high school maths tells us that the multiplication should be done first. This means that
the multiplication operator has higher precedence than the addition operator.
The following table gives the precedence table for Python, from the lowest precedence
(least binding) to the highest precedence (most binding). This means that in a given
expression, Python will first evaluate the operators and expressions lower in the table
before the ones listed higher in the table.
The following table, taken from the Python reference manual (http:/ / docs. python. org/
dev/ 3. 0/ reference/ expressions. html#evaluation-order), is provided for the sake of
completeness. It is far better to use parentheses to group operators and operands
appropriately in order to explicitly specify the precedence. This makes the program more
readable. See Changing the Order of Evaluation below for details.



The operators which we have not already come across will be explained in later chapters.
Operators with the same precedence are listed in the same row in the above table. For
example, + and - have the same precedence.
Changing the Order Of Evaluation
To make the expressions more readable, we can use parentheses. For example, 2 + (3 *
4) is definitely easier to understand than 2 + 3 * 4 which requires knowledge of the
operator precedences. As with everything else, the parentheses should be used reasonably
(do not overdo it) and should not be redundant (as in 2 + (3 + 4)).
There is an additional advantage to using parentheses - it helps us to change the order of
evaluation. For example, if you want addition to be evaluated before multiplication in an
expression, then you can write something like (2 + 3) * 4.
Associativity
Operators are usually associated from left to right i.e. operators with same precedence are
evaluated in a left to right manner. For example, 2 + 3 + 4 is evaluated as (2 + 3) + 4.
Some operators like assignment operators have right to left associativity i.e. a = b = c is
treated as a = (b = c).
Expressions
Example:
#!/usr/bin/python
# Filename: expression.py
length = 5
breadth = 2
area = length * breadth
print('Area is', area)
print('Perimeter is', 2 * (length + breadth))
Output:
     $ python expression.py
     Area is 10
     Perimeter is 14
How It Works:
The length and breadth of the rectangle are stored in variables by the same name. We use
these to calculate the area and perimeter of the rectangle with the help of expressions. We
store the result of the expression length * breadth in the variable area and then print it
using the print function. In the second case, we directly use the value of the expression 2
* (length + breadth) in the print function.
Also, notice how Python 'pretty-prints' the output. Even though we have not specified a
space between 'Area is' and the variable area, Python puts it for us so that we get a
clean nice output and the program is much more readable this way (since we don't need to
worry about spacing in the strings we use for output). This is an example of how Python
makes life easy for the programmer.
Summary
We have seen how to use operators, operands and expressions - these are the basic building
blocks of any program. Next, we will see how to make use of these in our programs using
statements.