Python Over View (Python for mobile app Devt)1.ppt

1. Python Overview
• Python is a high-level, interpreted, interactive and object
oriented-scripting language.
• Python was designed to be highly readable which uses English
keywords frequently where as other languages use punctuation
and it has fewer syntactical constructions than other languages.

• Python is Interpreted: This means that it is processed at
runtime by the interpreter and you do not need to compile your
program before executing it. This is similar to PERL and PHP.
• Python is Interactive: This means that you can actually sit at a
Python prompt and interact with the interpreter directly to
write your programs.
• Python is Object-Oriented: This means that Python supports
Object-Oriented style or technique of programming that
encapsulates code within objects.
• Python is Beginner's Language: Python is a great language
for the beginner programmers and supports the development
of a wide range of applications, from simple text processing to
WWW browsers to games.

Compiling and interpreting
• Many languages require you to compile (translate) your
program into a form that the machine understands.
• Python is instead directly interpreted into machine instructions.
compile execute
output
source code
Hello.java
byte code
Hello.class
interpret
output
source code
Hello.py

History of Python:
• Python was developed by Guido van Rossum in the late eighties
and early nineties at the National Research Institute for
Mathematics and Computer Science in the Netherlands.
• Python is derived from many other languages, including ABC,
Modula-3, C, C++, Algol-68, SmallTalk, and Unix shell and other
scripting languages.
• Python is copyrighted, Like Perl, Python source code is now
available under the GNU General Public License (GPL).
• Python is now maintained by a core development team at the
institute, although Guido van Rossum still holds a vital role in
directing it's progress.

Python Features
• Easy-to-learn: Python has relatively few keywords, simple
structure, and a clearly defined syntax.
• Easy-to-read: Python code is much more clearly defined and
visible to the eyes.
• Easy-to-maintain: Python's success is that its source code is
fairly easy-to-maintain.
• A broad standard library: One of Python's greatest strengths
is the bulk of the library is very portable and cross-platform
compatible on UNIX, Windows, and Macintosh.
• Interactive Mode: Support for an interactive mode in which
you can enter results from a terminal right to the language,
allowing interactive testing and debugging of snippets of code.

Python Features (cont’d)
• Portable: Python can run on a wide variety of hardware
platforms and has the same interface on all platforms.
• Extendable: You can add low-level modules to the Python
interpreter. These modules enable programmers to add to or
customize their tools to be more efficient.
• Databases: Python provides interfaces to all major commercial
databases.
• GUI Programming: Python supports GUI applications that can
be created and ported to many system calls, libraries, and
windows systems, such as Windows MFC, Macintosh, and the X
Window system of Unix.
• Scalable: Python provides a better structure and support for
large programs than shell scripting.

Python Environment
• Unix (Solaris, Linux, FreeBSD, AIX, HP/UX, SunOS, IRIX etc.)
• Win 9x/NT/2000
• Macintosh (PPC, 68K)
• OS/2
• DOS (multiple versions)
• PalmOS
• Nokia mobile phones
• Windows CE
• Acorn/RISC OS
• BeOS
• Amiga
• VMS/OpenVMS
• QNX
• VxWorks
• Psion
• Python has also been ported to the Java and .NET virtual machines.

2. Python - Basic Syntax
• Interactive Mode Programming:
>>> print "Hello, Python!";
Hello, Python!
>>> 3+4*5;
23

• Script Mode Programming :
Invoking the interpreter with a script parameter begins
execution of the script and continues until the script is finished.
When the script is finished, the interpreter is no longer active.
For example, put the following in one test.py, and run,
print "Hello, Python!";
print "I love COMP3050!";
The output will be:
Hello, Python!
I love COMP3050!

Python Identifiers:
• A Python identifier is a name used to identify a variable,
function, class, module, or other object. An identifier starts with
a letter A to Z or a to z or an underscore (_) followed by zero or
more letters, underscores, and digits (0 to 9).
• Python does not allow punctuation characters such as @, $, and
% within identifiers. Python is a case sensitive programming
language. Thus Manpower and manpower are two different
identifiers in Python.

Python Identifiers (cont’d)
• Here are following identifier naming convention for Python:
– Class names start with an uppercase letter and all other
identifiers with a lowercase letter.
– Starting an identifier with a single leading underscore
indicates by convention that the identifier is meant to be
private.
– Starting an identifier with two leading underscores indicates
a strongly private identifier.
– If the identifier also ends with two trailing underscores, the
identifier is a language-defined special name.

Reserved Words:
and exec not
assert finally or
break for pass
class from print
continue global raise
def if return
del import try
elif in while
else is with
except lambda yield
Keywords contain lowercase letters only.

Lines and Indentation:
• One of the first caveats programmers encounter when learning
Python is the fact that there are no braces to indicate blocks of
code for class and function definitions or flow control. Blocks of
code are denoted by line indentation, which is rigidly enforced.
• The number of spaces in the indentation is variable, but all
statements within the block must be indented the same
amount. Both blocks in this example are fine:
if True:
print "Answer“;
print "True" ;
else:
print "Answer“;
print "False"

Multi-Line Statements:
• Statements in Python typically end with a new line. Python
does, however, allow the use of the line continuation character
() to denote that the line should continue. For example:
total = item_one +
item_two +
item_three
• Statements contained within the [], {}, or () brackets do not
need to use the line continuation character. For example:
days = ['Monday', 'Tuesday', 'Wednesday',
'Thursday', 'Friday']

Quotation in Python:
• Python accepts single ('), double (") and triple (''' or """) quotes
to denote string literals, as long as the same type of quote
starts and ends the string.
• The triple quotes can be used to span the string across multiple
lines. For example, all the following are legal:
word = 'word'
sentence = "This is a sentence."
paragraph = """This is a paragraph. It is made up
of multiple lines and sentences."""

Comments in Python:
• A hash sign (#) that is not inside a string literal begins a
comment. All characters after the # and up to the physical line
end are part of the comment, and the Python interpreter
ignores them.

Using Blank Lines:
• A line containing only whitespace, possibly with a comment, is
known as a blank line, and Python totally ignores it.
• In an interactive interpreter session, you must enter an empty
physical line to terminate a multiline statement.

Multiple Statements on a Single Line:
• The semicolon ( ; ) allows multiple statements on the single line
given that neither statement starts a new code block. Here is a
sample snip using the semicolon:
import sys; x = 'foo'; sys.stdout.write(x + 'n')

Multiple Statement Groups as Suites:
• Groups of individual statements making up a single code block
are called suites in Python.
Compound or complex statements, such as if, while, def, and
class, are those which require a header line and a suite.
Header lines begin the statement (with the keyword) and
terminate with a colon ( : ) and are followed by one or more
lines which make up the suite.
if expression :
suite
elif expression :
suite
else :
suite

3. Python - Variable Types
• Variables are nothing but reserved memory locations to store
values. This means that when you create a variable you reserve
some space in memory.
• Based on the data type of a variable, the interpreter allocates
memory and decides what can be stored in the reserved
memory. Therefore, by assigning different data types to
variables, you can store integers, decimals, or characters in
these variables.

Assigning Values to Variables:
• Python variables do not have to be explicitly declared to reserve
memory space. The declaration happens automatically when
you assign a value to a variable. The equal sign (=) is used to
assign values to variables.
counter = 100 # An integer assignment
miles = 1000.0 # A floating point
name = "John" # A string
print counter
print miles
print name

Multiple Assignment:
• You can also assign a single value to several variables
simultaneously. For example:
a = b = c = 1
a, b, c = 1, 2, "john"

Standard Data Types:
Python has five standard data types:
• Numbers
• String
• List
• Tuple
• Dictionary

Python Numbers:
• Number data types store numeric values. They are immutable data
types, which means that changing the value of a number data type
results in a newly allocated object.
• Number objects are created when you assign a value to them. For
example:
var1 = 1
var2 = 10
Python supports four different numerical types:
• int (signed integers)
• long (long integers [can also be represented in octal and
hexadecimal])
• float (floating point real values)
• complex (complex numbers)

Number Examples:
int long float complex
10 51924361L 0 3.14j
100 -0x19323L 15.2 45.j
-786 0122L -21.9 9.322e-36j
80 0xDEFABCECBDAECBFBAEl 32.3+e18 .876j
-490 535633629843L -90 -.6545+0J
-0x260 -052318172735L -3.25E+101 3e+26J
0x69 -4721885298529L 70.2-E12 4.53e-7j

Python Strings:
• Strings in Python are identified as a contiguous set of
characters in between quotation marks.
• Python allows for either pairs of single or double quotes.
Subsets of strings can be taken using the slice operator ( [ ] and
[ : ] ) with indexes starting at 0 in the beginning of the string
and working their way from -1 at the end.
• The plus ( + ) sign is the string concatenation operator, and the
asterisk ( * ) is the repetition operator.

Example:
str = 'Hello World!'
print str # Prints complete string
print str[0] # Prints first character of the string
print str[2:5] # Prints characters starting from 3rd to 6th
print str[2:] # Prints string starting from 3rd character
print str * 2 # Prints string two times
print str + "TEST" # Prints concatenated string
Output:
Hello World!
H
llo
llo World!
Hello World!Hello World!
Hello World!TEST

Python Lists:
• Lists are the most versatile of Python's compound data types. A
list contains items separated by commas and enclosed within
square brackets ([]).
• To some extent, lists are similar to arrays in C. One difference
between them is that all the items belonging to a list can be of
different data type.
• The values stored in a list can be accessed using the slice
operator ( [ ] and [ : ] ) with indexes starting at 0 in the
beginning of the list and working their way to end-1.
• The plus ( + ) sign is the list concatenation operator, and the
asterisk ( * ) is the repetition operator.

list = [ 'abcd', 786 , 2.23, 'john', 70.2 ]
tinylist = [123, 'john']
print list # Prints complete list
print list[0] # Prints first element of the list
print list[1:3] # Prints elements starting from 2nd till 3rd
print list[2:] # Prints elements starting from 3rd element
print tinylist * 2 # Prints list two times
print list + tinylist # Prints concatenated lists
Output:
['abcd', 786, 2.23, 'john', 70.2]
abcd
[786, 2.23]
[2.23, 'john', 70.2]
[123, 'john', 123, 'john']
['abcd', 786, 2.23, 'john', 70.2, 123, 'john']

Python Tuples:
• A tuple is another sequence data type that is similar to the list.
A tuple consists of a number of values separated by commas.
Unlike lists, however, tuples are enclosed within parentheses.
• The main differences between lists and tuples are: Lists are
enclosed in brackets ( [ ] ), and their elements and size can be
changed, while tuples are enclosed in parentheses ( ( ) ) and
cannot be updated. Tuples can be thought of as read-only lists.

tuple = ( 'abcd', 786 , 2.23, 'john', 70.2 )
tinytuple = (123, 'john')
print tuple # Prints complete list
print tuple[0] # Prints first element of the list
print tuple[1:3] # Prints elements starting from 2nd till 3rd
print tuple[2:] # Prints elements starting from 3rd element
print tinytuple * 2 # Prints list two times
print tuple + tinytuple # Prints concatenated lists
OUTPUT:
('abcd', 786, 2.23, 'john', 70.2)
abcd
(786, 2.23)
(2.23, 'john', 70.2)
(123, 'john', 123, 'john')
('abcd', 786, 2.23, 'john', 70.2, 123, 'john')

Python Dictionary:
• Python 's dictionaries are hash table type. They work like
associative arrays or hashes found in Perl and consist of key-
value pairs.
• Keys can be almost any Python type, but are usually numbers
or strings. Values, on the other hand, can be any arbitrary
Python object.
• Dictionaries are enclosed by curly braces ( { } ) and values can
be assigned and accessed using square braces ( [] ).

dict = {}
dict['one'] = "This is one"
dict[2] = "This is two“
tinydict = {'name': 'john','code':6734, 'dept': 'sales'}
print dict['one'] # Prints value for 'one' key
print dict[2] # Prints value for 2 key
print tinydict # Prints complete dictionary
print tinydict.keys() # Prints all the keys
print tinydict.values() # Prints all the values
OUTPUT:
This is one
This is two
{'dept': 'sales', 'code': 6734, 'name': 'john'}
['dept', 'code', 'name']
['sales', 6734, 'john']

Data Type Conversion:
Function Description
int(x [,base]) Converts x to an integer. base specifies the base if x is a string.
long(x [,base] ) Converts x to a long integer. base specifies the base if x is a
string.
float(x) Converts x to a floating-point number.
complex(real
[,imag])
Creates a complex number.
str(x) Converts object x to a string representation.
repr(x) Converts object x to an expression string.
eval(str) Evaluates a string and returns an object.
tuple(s) Converts s to a tuple.
list(s) Converts s to a list.
set(s) Converts s to a set.
dict(d) Creates a dictionary. d must be a sequence of (key,value) tuples.
frozenset(s) Converts s to a frozen set.
chr(x) Converts an integer to a character.
unichr(x) Converts an integer to a Unicode character.
ord(x) Converts a single character to its integer value.
hex(x) Converts an integer to a hexadecimal string.
oct(x) Converts an integer to an octal string.

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