Python tutorial | What is Python,Statement,Interpreter

This python programming tutorial covers What is Python, Statement and Interpreter.

What is Python?

An Interpreted high-level programming language similar to Perl, Ruby, Tcl, and other so-called scripting languages created by Guido Van Rossum around 1990 and named in the honor of Monty Python.

Common applications and non-uses of Python

•  Text processing/data processing
•  Application scripting
•  Systems administration/programming
•  Internet programming
•  Graphical user interfaces
•  Testing

Python is not suitable for device drivers and low-level systems,computer graphics, visualization, and games and for numerical algorithms/scientific computing.

But Python is still used in these application domains, but only as a high-level control language. Important computations are actually carried out in C, C++, Fortran, etc. For example, you would not implement matrix multiplication in Python.

Running Python

•  Python programs run inside an interpreter
•  The interpreter is a simple console-based application that normally starts from a command shell (e.g. the Unix shell)
shell % python
Python 2.5.1 (r251:54869, Apr 18 2007, 22:08:04)
[GCC 4.0.1 (Apple Computer, Inc. build 5367)] on darwin
Type "help", "copyright", "credits" or "license"
•  Expert programmers usually have no problem using the interpreter in this way , but it is not so user-friendly for beginners.
•  Python includes a simple integrated development called IDLE (which is another Monty Python reference)

The Python Interpreter

•  When you start Python, you get an "interactive" mode where you can experiment
•  If you start typing statements, they will run immediately
•  No edit/compile/run/debug cycle
•  In fact, there is no "compiler"

Interactive Mode

The interpreter runs a "read-eval" loop
>>> print "hello world"
hello world
>>> 37*42
1554
>>> for i in range(5):
... print i
...
0
1
2
3
4
>>>

Getting Help

help(name) command
Type help() with no name for interactive help
Documentation is available at https://docs.python.org

Creating python program

•  Programs are put in .py files
•  Source files are simple text files
•  Create with your favorite editor (e.g. emacs)
•  Can also edit programs with IDLE or other Python IDE

Python Statement

•  A Python program is a sequence of statements
•  Each statement is terminated by a newline
•  Statements are executed one after the other until you reach the end of the file.
•  When there are no more statements, the program stops

Comment

Comments are denoted by #
# This is a comment
height = 442 # Meters

Extend to the end of the line
There are no block comments in Python (e.g., /* ... */)

Variable

A variable is just a name for some value
Variable names follow same rules as C
You do not declare types (int, float, etc.)
height = 442 # An integer
height = 442.0 # Floating point
height = "Really tall" # A string
Differs from C++/Java where variables have a fixed type that must be declared.

Keywords

Variables can not have one of these names
These are mostly C-like and have the same meaning in most cases

Case Sensitivity

•  Python is case sensitive
•  These are all different variables:
name = "Jake"
•  Language statements are always lower-case
print "Hello World" # OK
PRINT "Hello World" # ERROR
while x < 0: # OK
WHILE x < 0: # ERROR

Loops

The while statement executes a loop
while thickness <= height:
thickness = thickness * 2
numfolds = numfolds + 1
print numfolds, thickness

Indentation

Indentation used to denote blocks of code Indentation must be consistent Colon (:) always indicates start of new block while thickness <= height: There is a preferred indentation style Always use spaces Use 4 spaces per level Avoid tabs Always use a Python-aware editor

Conditionals

•  If-else
if a < b:
print "Computer says no"
else:
print "Computer says yes"

•  If-elif-else
if a == '+':
op = PLUS
elif a == '-':
op = MINUS
elif a == '*':
op = TIMES
else:
op = UNKNOWN

Relations

Relational operators
< > <= >= == !=

Boolean expressions (and, or, not)
if b >= a and b <= c:
print "b is between a and c"

if not (b < a or b > c):
print "b is still between a and c"
Non-zero numbers, non-empty objects also evaluates as True
x = 42
if x:
# x ix non zero

The print statement

print x
print x,y,z
print "Your name is", name
print x, #Omits newline

Produces a single line of text

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