## Basic Python

This document is intended to be handed out at the end of the workshop. It has been designed for Engineering students who are Python beginners and have basic programming skills. The focus is on basic numerics and plotting using Python.

The system requirements:

- Python - version 2.5.x or newer.

- IPython

- Text editor - scite, vim, emacs or whatever you are comfortable with.

## 1. Introduction

The Python programming language was created by a dutch named Guido van Rossum. The idea of Python was conceived in December 1989. The name Python has nothing to do with the reptilian, but its been named after the 70s comedy series "Monty Python's Flying Circus", since it happens to be Guido's favourite TV series.

Current stable version of Python is 2.6.x, although Python 3.0 is also the stable version, it is not backwards compatible with the previous versions and is hence not entirely popular at the moment. This material will focus on the 2.6.x series.

Python is licensed under the Python Software Foundation License (PSF License) which is GPL compatible Free Software license (excepting license version 1.6 and 2.0) It is a no strings attached license, which means the source code is free to modify and redistribute.

The Python docs define Python as "Python is an interpreted, object-oriented, high-level programming language with dynamic semantics." A more detailed summary can be found at http://www.python.org/doc/essays/blurb.html. Python is a language that has been designed to help the programmer concentrate on solving the problem at hand and not worry about the programming language idiosyncrasies.

Python is a highly cross platform compatible language on account of it being an interpreted language. It is highly scalable and hence has been adapted to run on the Nokia 60 series phones. Python has been designed to be readable and easy to use

Resources available for reference

- Web: http://www.python.org

- Doc: http://www.python.org/doc

- Free Tutorials: Official Python Tutorial: http://docs.python.org/tut/tut.html Byte of Python: http://www.byteofpython.info/ * Dive into Python: http://diveintopython.org/

Advantages of Python - Why Python??

- Python has been designed for readability and ease of use. Its been designed in such a fashion that it imposes readability on the programmer. Python does away with the braces and the semicolons and instead implements code blocks based on indentation, thus enhancing readability.

- Python is a high level, interpreted, modular and object oriented language. Python performs memory management on its own, thus the programmer need not bother about allocating and deallocating memory to variables. Python provides extensibility by providing modules which can be easily imported similar to headers in C and packages in Java. Python is object oriented and hence provides all the object oriented characteristics such as inheritance, encapsulation and polymorphism.

- Python offers a highly powerful interactive programming interface in the form of the 'Interactive Interpreter' which will be discussed in more detail in the following sections.

- Python provides a rich standard library and an extensive set of modules. The power of Python modules can be seen in this slightly exaggerated cartoon http://xkcd.com/353/

- Python interfaces well with most other programming languages such as C, C++ and FORTRAN.

Although, Python has one setback. Python is not fast as some of the compiled languages like C or C++. Yet, the amount of flexibility and power more than make up for this setback.

## 2. The Python Interpreter

### 2.1. The Interactive Interpreter

Typing python at the shell prompt on any standard Unix/Gnu-Linux system and hitting the enter key fires up the Python 'Interactive Interpreter'. The Python interpreter is one of the most integral features of Python. The prompt obtained when the interactive interpreter is similar to what is shown below. The exact appearance might differ based on the version of Python being used. The `>>>` thing shown is the python prompt. When something is typed at the prompt and the enter key is hit, the python interpreter interprets the command entered and performs the appropriate action. All the examples presented in this document are to be tried hands on, on the interactive interpreter.

    Python 2.5.2 (r252:60911, Oct  5 2008, 19:24:49)
    [GCC 4.3.2] on linux2
    Type "help", "copyright", "credits" or "license" for more information.
    >>>

Lets try with an example, type `print 'Hello, World!'` at the prompt and hit the enter key.

    >>> print 'Hello, World!'
    Hello, World!

This example was quite straight forward, and thus we have written our first line of Python code. Now let us try typing something arbitrary at the prompt. For example:

    >>> arbit word
      File "<stdin>", line 1
        arbit word
                ^
    SyntaxError: invalid syntax
    >>>

The interpreter gave an error message saying that 'arbit word' was invalid syntax which is valid. The interpreter is an amazing tool when learning to program in Python. The interpreter provides a help function that provides the necessary documentation regarding all Python syntax, constructs, modules and objects. Typing help() at the prompt gives the following output:

    >>> help()

    Welcome to Python 2.5!  This is the online help utility.

    If this is your first time using Python, you should definitely check out
    the tutorial on the Internet at http://www.python.org/doc/tut/.

    Enter the name of any module, keyword, or topic to get help on writing
    Python programs and using Python modules.  To quit this help utility and
    return to the interpreter, just type "quit".

    To get a list of available modules, keywords, or topics, type "modules",
    "keywords", or "topics".  Each module also comes with a one-line summary
    of what it does; to list the modules whose summaries contain a given word
    such as "spam", type "modules spam".

    help>

As mentioned in the output, entering the name of any module, keyword or topic will provide the documentation and help regarding the same through the online help utility. Pressing Ctrl+d exits the help prompt and returns to the python prompt.

Let us now try a few examples at the python interpreter.

Eg 1:

    >>> print 'Hello, python!'
    Hello, python!
    >>>

Eg 2:

    >>> print 4321*567890
    2453852690
    >>>

Eg 3:

    >>> 4321*567890
    2453852690L
    >>>

    Note: Notice the 'L' at the end of the output. The 'L' signifies that the
    output of the operation is of type *long*. It was absent in the previous
    example because we used the print statement. This is because *print* formats
    the output before displaying.

Eg 4:

    >>> big = 12345678901234567890 ** 3
    >>> print big
    1881676372353657772490265749424677022198701224860897069000
    >>>

    This example is to show that unlike in C or C++ there is no limit on the
    value of an integer.

Try this on the interactive interpreter: `import this`

Hint: The output gives an idea of Power of Python

### 2.2. ipython - An enhanced interactive Python interpreter

The power and the importance of the interactive interpreter was the highlight of the previous section. This section provides insight into the enhanced interpreter with more advanced set of features called ipython. Entering ipython at the shell prompt fires up the interactive interpreter.

    $ ipython
    Python 2.5.2 (r252:60911, Oct  5 2008, 19:24:49)
    Type "copyright", "credits" or "license" for more information.

    IPython 0.8.4 -- An enhanced Interactive Python.
    ?         -> Introduction and overview of IPython's features.
    %quickref -> Quick reference.
    help      -> Python's own help system.
    object?   -> Details about 'object'. ?object also works, ?? prints more.

    In [1]:

This is the output obtained upon firing ipython. The exact appearance may change based on the Python version installed. The following are some of the various features provided by ipython:

  Suggestions - ipython provides suggestions of the possible methods and operations available for the given python object.

Eg 5:

    In [4]: a = 6

    In [5]: a.
    a.__abs__           a.__divmod__        a.__index__         a.__neg__          a.__rand__          a.__rmod__          a.__rxor__
    a.__add__           a.__doc__           a.__init__          a.__new__          a.__rdiv__          a.__rmul__          a.__setattr__
    a.__and__           a.__float__         a.__int__           a.__nonzero__      a.__rdivmod__       a.__ror__           a.__str__
    a.__class__         a.__floordiv__      a.__invert__        a.__oct__          a.__reduce__        a.__rpow__          a.__sub__
    a.__cmp__           a.__getattribute__  a.__long__          a.__or__           a.__reduce_ex__     a.__rrshift__       a.__truediv__
    a.__coerce__        a.__getnewargs__    a.__lshift__        a.__pos__          a.__repr__          a.__rshift__        a.__xor__
    a.__delattr__       a.__hash__          a.__mod__           a.__pow__          a.__rfloordiv__     a.__rsub__
    a.__div__           a.__hex__           a.__mul__           a.__radd__         a.__rlshift__       a.__rtruediv__

In this example, we initialized 'a' (a variable - a concept that will be discussed in the subsequent sections.) to 6. In the next line when the tab key is pressed after typing ' a.' ipython displays the set of all possible methods that are applicable on the object 'a' (an integer in this context). Ipython provides many such datatype specific features which will be presented in the further sections as and when the datatypes are introduced.

## 3. Editing and running a python file

The previous sections focused on the use of the interpreter to run python code. While the interpeter is an excellent tool to test simple solutions and experiment with small code snippets, its main disadvantage is that everything written in the interpreter is lost once its quit. Most of the times a program is used by people other than the author. So the programs have to be available in some form suitable for distribution, and hence they are written in files. This section will focus on editing and running python files. Start by opening a text editor ( it is recommended you choose one from the list at the top of this page ). In the editor type down python code and save the file with an extension .py (python files have an extension of .py). Once done with the editing, save the file and exit the editor.

Let us look at a simple example of calculating the gcd of 2 numbers using Python:

Creating the first python script(file) :

    $ emacs gcd.py
      def gcd(x,y):
        if x % y == 0:
          return y
        return gcd(y, x%y)

      print gcd(72, 92)

To run the script, open the shell prompt, navigate to the directory that contains the python file and run `python <filename.py>` at the prompt ( in this case filename is gcd.py )

Running the python script :

    $ python gcd.py
    4
    $

Another method to run a python script would be to include the line

`#! /usr/bin/python`

at the beginning of the python file and then make the file executable by

$ chmod a+x filename.py

Once this is done, the script can be run as a standalone program as follows:

$ ./ filename.py

## 4. Basic Datatypes and operators in Python

Python provides the following set of basic datatypes.

> - Numbers: int, float, long, complex

> - Strings

> - Boolean

### 4.1. Numbers

Numbers were introduced in the examples presented in the interactive interpreter section. Numbers include types as mentioned earlier viz., int (integers), float (floating point numbers), long (large integers), complex (complex numbers with real and imaginary parts). Python is not a strongly typed language, which means the type of a variable need not mentioned during its initialization. Let us look at a few examples.

Eg 6:

    >>> a = 1 #here a is an integer variable

Eg 7:

    >>> lng = 122333444455555666666777777788888888999999999 #here lng is a variable of type long
    >>> lng
    122333444455555666666777777788888888999999999L #notice the trailing 'L'
    >>> print lng
    122333444455555666666777777788888888999999999 #notice the absence of the trailing 'L'
    >>> lng+1
    122333444455555666666777777788888889000000000L

Long numbers are the same as integers in almost all aspects. They can be used in operations just like integers and along with integers without any distinction. The only distinction comes during type checking (which is not a healthy practice). Long numbers are tucked with a trailing 'L' just to signify that they are long. Notice that in the example just lng at the prompt displays the value of the variable with the 'L' whereas `print lng` displays without the 'L'. This is because print formats the output before printing. Also in the example, notice that adding an integer to a long does not give any errors and the result is as expected. So for all practical purposes longs can be treated as ints.

Eg 8:

    >>> fl = 3.14159 #fl is a float variable
    >>> e = 1.234e-4 #e is also a float variable, specified in the exponential form
    >>> a = 1
    >>> b = 2
    >>> a/b #integer division
    0
    >>> a/fl #floating point division
    0.31831015504887655
    >>> e/fl
    3.9279473133031364e-05

Floating point numbers, simply called floats are real numbers with a decimal point. The example above shows the initialization of a float variable. Shown also in this example is the difference between integer division and floating point division. 'a' and 'b' here are integer variables and hence the division gives 0 as the quotient. When either of the operands is a float, the operation is a floating point division, and the result is also a float as illustrated.

Eg 9:

    >>> cplx = 3 + 4j #cplx is a complex variable
    >>> cplx
    (3+4j)
    >>> print cplx.real #prints the real part of the complex number
    3.0
    >>> print cplx.imag #prints the imaginary part of the complex number
    4.0
    >>> print cplx*fl  #multiplies the real and imag parts of the complex number with the multiplier
    (9.42477+12.56636j)
    >>> abs(cplx) #returns the absolute value of the complex number
    5.0

Python provides a datatype for complex numbers. Complex numbers are initialized as shown in the example above. The real and imag operators return the real and imaginary parts of the complex number as shown. The abs() returns the absolute value of the complex number.

### 4.2. Variables

Variables are just names that represent a value. Variables have already been introduced in the various examples from the previous sections. Certain rules about using variables:

> - Variables have to be initialized or assigned a value before being used.

> - Variable names can consist of letters, digits and underscores .

> - Variable names cannot begin with digits, but can contain digits in them.

In reference to the previous section examples, 'a', 'b', 'lng', 'fl', 'e' and 'cplx' are all variables of various datatypes.

    Note: Python is not a strongly typed language and hence an integer variable can at a
    later stage be used as a float variable as well.

### 4.3. Strings

Strings are one of the essential data structures of any programming language. The `print "Hello, World!"` program was introduced in the earlier section, and the "Hello, World!" in the print statement is a string. A string is basically a set of characters. Strings can be represented in various ways shown below:

    s = 'this is a string'              # a string variable can be represented using single quotes
    s = 'This one has "quotes" inside!' # The string can have quotes inside it as shown
    s = "I have 'single-quotes' inside!"
    l = "A string spanning many lines\
    one more line\
    yet another"                        # a string can span more than a single line.
    t = """A triple quoted string does  # another way of representing multiline strings.
    not need to be escaped at the end and
    "can have nested quotes" etc."""

Try the following on the interpreter: `s = 'this is a string with 'quotes' of similar kind'`

Exercise: How to use single quotes within single quotes in a string as shown in the above example without getting an error?

#### 4.3.1. String operations

A few basic string operations are presented here.

String concatenation String concatenation is done by simple addition of two strings.

    >>> x = 'Hello'
    >>> y = ' Python'
    >>> print x+y
    Hello Python

Try this yourself:

    >>> somenum = 13
    >>> print x+somenum

The problem with the above example is that here a string variable and an integer variable are trying to be concantenated. To obtain the desired result from the above example the str(), repr() and the `` can be used.

str() simply converts a value to a string in a reasonable form. repr() creates a string that is a representation of the value.

The difference can be seen in the example shown below:

    >>> str(1000000000000000000000000000000000000000000000000L)
    '1000000000000000000000000000000000000000000000000'
    >>> repr(1000000000000000000000000000000000000000000000000L)
    '1000000000000000000000000000000000000000000000000L'

It can be observed that the 'L' in the long value shown was omitted by str(), whereas repr() converted that into a string too. An alternative way of using repr(value) is ` `value``.

A few more examples:

    >>> x = "Let's go \nto Pycon"
    >>> print x
    Let's go
    to Pycon

In the above example, notice that the '\n'(newline) character is formatted and the string is printed on two lines. The strings discussed until now were normal strings. Other than these there are two other types of strings namely, raw strings and unicode strings.

Raw strings are strings which are unformatted, that is the backslashes(\) are not parsed and are left as it is in the string. Raw strings are represented with an 'r' at the start of a string. Let us look at an example

    >>> x = r"Let's go \nto Pycon"
    >>> print x
    Let's go \nto Pycon

Note: The '\n' is not being parsed into a new line and is left as it is.

Try this yourself:

    >>> x = r"Let's go to Pycon\"

Unicode strings are strings where the characters are Unicode characters as opposed to ASCII characters. Unicode strings are represented with a 'u' at the start of the string. Let us look at an example:

    >>> x = u"Let's go to Pycon!"
    >>> print x
    Let's go to Pycon!

### 4.4. Boolean

Python also provides special Boolean datatype. A boolean variable can assume a value of either True or False (Note the capitalizations).

Let us look at examples:

    >>> t = True
    >>> f = not t
    >>> print f
    False
    >>> f or t
    True
    >>> f and t
    False

## 5. The while loop

The Python while loop is similar to the C/C++ while loop. The syntax is as follows:

    statement 0
    while condition:
      statement 1 #while block
      statement 2 #while block
    statement 3 #outside the while block.

Let us look at an example:

    >>> x = 1
    >>> while x <= 5:
    ...   print x
    ...   x += 1
    ...
    1
    2
    3
    4
    5

## 6. The if conditional

The Python if block provides the conditional execution of statements. If the condition evaluates as true the block of statements defined under the if block are executed.

If the first block is not executed on account of the condition not being satisfied, the set of statements in the else block are executed.

The elif block provides the functionality of evaluation of multiple conditions as shown in the example.

The syntax is as follows:

    if condition :
        statement_1
        statement_2

    elif condition:
        statement_3
        statement_4
    else:
        statement_5
        statement_6

Let us look at an example:

    >>> n = raw_input("Input a number:")
    >>> if n < 0:
          print n," is negative"
          elif n > 0:
          print n," is positive"
          else:
          print n, " is 0"

## 7. raw_input()

In the previous example we saw the call to the raw_input() subroutine. The raw_input() method is used to take user inputs through the console. Unlike input() which assumes the data entered by the user as a standard python expression, raw_input() treats all the input data as raw data and converts everything into a string. To illustrate this let us look at an example.

    >>> input("Enter a number thats a palindrome:")
    Enter a number thats a palindrome:121
    121

    >>> input("Enter your name:")
    Enter your name:PythonFreak
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "<string>", line 1, in <module>
    NameError: name 'PythonFreak' is not defined

As shown above the input() assumes that the data entered is a valid Python expression. In the first call it prompts for an integer input and when entered it accepts the integer as an integer, whereas in the second call, when the string is entered without the quotes, input() assumes that the entered data is a valid Python expression and hence it raises and exception saying PythonFreak is not defined.

    >>> input("Enter your name:")
    Enter your name:'PythonFreak'
    'PythonFreak'
    >>>

Here the name is accepted because its entered as a string (within quotes). But its unreasonable to go on using quotes each time a string is entered. Hence the alternative is to use raw_input().

Let us now look at how raw_input() operates with an example.

    >>> raw_input("Enter your name:")
    Enter your name:PythonFreak
    'PythonFreak'

Observe that the raw_input() is converting it into a string all by itself.

    >>> pal = raw_input("Enter a number thats a palindrome:")
    Enter a number thats a palindrome:121
    '121'

Observe that raw_input() is converting the integer 121 also to a string as '121'. Let us look at another example:

    >>> pal = raw_input("Enter a number thats a palindrome:")
    Enter a number thats a palindrome:121
    >>> pal + 2
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: cannot concatenate 'str' and 'int' objects
    >>> pal
    '121'

Observe here that the variable pal is a string and hence integer operations cannot be performed on it. Hence the exception is raised.

## 8. int() method

Generally for computing purposes, the data used is not strings or raw data but on integers, floats and similar mathematical data structures. The data obtained from raw_input() is raw data in the form of strings. In order to obtain integers from strings we use the method int().

Let us look at an example.

    >>> intpal = int(pal)
    >>> intpal
    121

In the previous example it was observed that pal was a string variable. Here using the int() method the string pal was converted to an integer variable.

Try This Yourself:

    >>> stringvar = raw_input("Enter a name:")
    Enter a name:Guido Van Rossum
    >>> stringvar
    'Guido Van Rossum'
    >>> numvar = int(stringvar)

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