Python (programming language)
Python is a dynamic object-oriented, general purpose interpreted programming language which runs on many different computer platforms and mobile devices. Python is open source software. Python aims to be a language that is efficient, coherent, readable, and fun to use. Because Python is an interpreted language, Python programs run immediately without the need for lengthy compile and link steps.
Python was first published by the Dutch computer scientist (and applied mathematician) Guido van Rossum in pre-release (early-adopter) form in 1991, and to this day he remains the project leader and final arbiter of Python Enhancement Proposals (PEPs). The language takes its name from the English comedy group, Monty Python. 
Python 2.7.1 is the current production release, and is very stable.
Python's major (standard) releases were:
- Python 2.5.1 (April 2007)
- Python 2.4.4 (October 2006)
- Python 2.3.6 (November 2006)
- Python 2.2.3 (May 2003)
- Python 2.1.3 (April 2002)
- Python 2.0.1 (June 2001)
- Python 1.6.1 (September 2000)
- Python 1.5.2 (April 1999)
- Python 1.4 (October 1996)
- Python 1.3 (October 1995)
- Python 1.2 (April 1995)
- Python 1.1 (1995)
- Python 1.0 (1994)
The code for the "hello world" program can hardly be more simple:
print 'Hello World'
This can be put in a file "hello.py", for example, and executed with
from your operating system's command line. Alternatively, the code can be typed directly into an interactive Python environment (the Python command line interpreter or IDLE, both of which are included in the standard Python distribution).
The Python interpreter can be invoked from the command line and used as a scientific calculator. At the prompt (denoted here by
>>> ), type
the interpreter will print 8. Division of integers returns an integer result, so
More interesting functions can be found in the math module. It must be imported before it can be used.
>>> from math import *
>>> print sin(pi/2)
High quality graphs may be obtained with the matplotlib library
A useful Python construction is related to working with files. Line-by-line Perl-like file processing can be realized with a standard
for loop as in the following simple word count script. Below, the comments start with the hash '#' sign.
(char_count,word_count,line_count)=0,0,0 # multiple assignment is available file = open ('myfile.txt') # standard opening of the file for line in file: # this is "idiomatic" use of of the "for" loop wordlist = line.split() # splitting line into the list of words word_count += len(wordlist) # counting words line_count += 1 # counting lines char_count += len(line) - 1 # counting characters print line_count, word_count, char_count # we're done
The indentation indicates what code is executed within the loop. This is part of Python's syntax. When the end of file is reached, the loop terminates and the results are printed by the last line of the code. Note also that the variable
line contains the terminating newline character, so that 1 is subtracted from its length for the character counting. Another interesting observation can be made: the method (i.e. function) of splitting a string is 'provided' by this very string. Indeed, Python is an objective language; the string variables are objects and related methods are its attributes.
The following script counts the images on the Citizendium Main Page.
1 import urllib2 2 cnt=0 3 for line in urllib2.urlopen('http://en.citizendium.org/wiki/Main_Page'): 4 cnt += line.count('<img src') 5 print cnt(note that the line numbers were added for clarity - they are not needed in an actual Python script)
- In line 1, we import a module from the standard library. Python's standard library is said to be rich and often regarded as Python's strong point. In fact, Python official pages declare a "batteries included" philosophy.
- In line 2, the variable named 'cnt' is set to zero initially (a best practice in programming)
- In line 3, the URL is fetched using the urllib2 function. This merits some discussion. In Python, the source HTML files of web pages can be treated much like the local files, e.g. processed line-by-line with a for loop. In the above example the variable line is a string containing a piece of HTML code of Citizendium's Main Page. Embedded images are inserted in this code with a text that begins with '<img src'. So it's enough to count instances of this last string.
- In line 4 (inside the for loop), the appropriate method count() is used to increment the cnt variable by one every time the HTML tag beginning with "<img src" is encountered.
- In line 5, after the for loop exits (presumably because all instances of the 'img' tag have been counted), the variable 'cnt' is printed.
A ready-to-run copy of this script is available here.
Remarkable global features of the Python syntax include high readability of the code, which is aided by the use of indentation to separate blocks of code and a general "one statement per line" principle.
Generally, a high-level interpreted scripting language is far not as fast as a compiled program. However, a specializing just-in-time compiler "Psyco" runs existing Python programs much faster, with no change in the source text. In addition, the Python library contains a number of modules performing useful operations quite effectively. For example, "anydbm" can effectively maintain a large indexed file.
Python can be supported as the programming language in an integrated development environment (IDE).
Popular Python IDEs include the following:
- IDLE, the default Integrated Development Environment for Python
- Eclipse pydev, the pydev plug-in to the Eclipse IDE for Python
- ActiveState Komodo, the multiple scripting languages IDE
- Wingware Wing, the multiplatform Python-language-specific IDE
Notes and references
- Why is it called Python?. General Python FAQ. Python Software Foundation. (Mar 02, 2011).
- This is installed separately, see its introduction and a couple of examples.
- Backslash "\" at the end of line allows to break e.g. a long assignment over multiple lines. There is also a formal possibility to put more than one statement in a line by separating them with a semicolon. Still, the general principle shapes the code.
- Psyco: High-level languages need not be slower than low-level ones.
- anydbm -- Generic access to DBM-style databases