An introduction to Python (and Jython)

Introduction: why Python

Python in a few words

Python is a scripting language which can replace most task scripts and compiled programs written in languages like C/C++/Java (no compilation is necessary).

Python is available in many platforms: Windows, Mac, Linux, etc.

It is a very high level language that provides high level data types built-in (lists, dictionnaries, tuples, sets, ...), object-oriented programming (but this is not mandatory :-)), numerous modules (I/O, system calls, threads, GUIs, ...), and dynamical prototyping (no need to declare variable type).

Python is extensible: one can write C, Fortran or Java libraries which can be called directly in a Python script.

The language is named after the BBC show “Monty Python’s Flying Circus” and has nothing to do with reptiles.


Python was created in the early 1990s by Guido van Rossum at Stichting Mathematisch Centrum (CWI) in the Netherlands as a successor of a language called ABC. Guido remains Python’s principal author, although it includes many contributions from others.

In 1995, Guido continued his work on Python at the Corporation for National Research Initiatives (CNRI) in Reston, Virginia where he released several versions of the software.

In May 2000, Guido and the Python core development team moved to to form the BeOpen PythonLabs team. In October of the same year, the PythonLabs team moved to Digital Creations (now Zope Corporation). In 2001, the Python Software Foundation (PSF) was formed, a non-profit organization created specifically to own Python-related Intellectual Property. Zope Corporation is a sponsoring member of the PSF.

All Python releases are Open Source.

Release Derived from Year Owner GPL compatible?
0.9.0 thru 1.2 n/a 1991-1995 CWI yes
1.3 thru 1.5.2 1.2 1995-1999 CNRI yes
1.6 1.5.2 2000 CNRI no
2.0 1.6 2000 no
1.6.1 1.6 2001 CNRI no
2.1 2.0+1.6.1 2001 PSF no
2.0.1 2.0+1.6.1 2001 PSF yes
2.1.1 2.1+2.0.1 2001 PSF yes
2.2 2.1.1 2001 PSF yes
2.1.2 2.1.1 2002 PSF yes
2.1.3 2.1.2 2002 PSF yes
2.2.1 2.2 2002 PSF yes
2.2.2 2.2.1 2002 PSF yes
2.2.3 2.2.2 2002-2003 PSF yes
2.3 2.2.2 2002-2003 PSF yes
2.3.1 2.3 2002-2003 PSF yes
2.3.2 2.3.1 2003 PSF yes
2.3.3 2.3.2 2003 PSF yes
2.3.4 2.3.3 2004 PSF yes
2.3.5 2.3.4 2005 PSF yes
2.4 2.3 2004 PSF yes
2.4.1 2.4 2005 PSF yes
2.4.2 2.4.1 2005 PSF yes
2.4.3 2.4.2 2006 PSF yes
2.4.4 2.4.3 2006 PSF yes
2.5 2.4 2006 PSF yes
2.5.1 2.5 2007 PSF yes
2.6 2.5 2008 PSF yes
2.6.1 2.6 2008 PSF yes
3.0 2.6 2008 PSF yes
3.1 3.0 2009 PSF yes

Python 2.x vs. Python 3.x

Python 3.0 final was released on December 3rd, 2008.

Python 3.0 (a.k.a. "Python 3000" or "Py3k") is a new version of the language that is incompatible with the 2.x line of
releases. The language is mostly the same, but many details, especially how built-in objects like dictionaries and strings 
work, have changed considerably, and a lot of deprecated features have finally been removed. Also, the standard library has 
been reorganized in a few prominent places.

The Python 3.x branch is fairly new and many third-party Python libraries are still only Python 2.x compatible. Moreover, the last version of Jython is 2.5 (see below).

The tutorial presented here will only concern the 2.x branch.

Python and Jython

Jython: Python for the Java Platform

Jython is an implementation of the Python programming language which is designed to run on the Java™ Platform. It consists of a compiler to compile Python source code down to Java bytecodes which can run directly on a JVM, a set of support libraries which are used by the compiled Java bytecodes, and extra support to make it trivial to use Java packages from within Jython.

History of Jython

The Jython project was announced on 19-oct-2000. The first alpha release occured on 26-nov-2000.

The final release of Jython-2.0 occurred on 17-jan-2001. The final release of Jython-2.1 occurred on 31-dec-2001.

Jython 2.2 was released on August 22, 2007, with 2.2.1 released on October 13, 2007.

Jython 2.5.0 was released on June 16, 2009, with a release candidate for 2.5.1 scheduled for August 20, 2009.

Jython 2.5 implements the same language as CPython 2.5, and nearly all of the Core Python standard library modules. (CPython is the C implementation of the Python language.) Jython 2.5 uses the same regression test suite as CPython, with some minor modifications.

Differences between Python and Jython

Jython programs cannot use CPython extension modules written in C.

There are a number of other differences between the two implementations that are unlikely to go away. These range from the trivial - Jython's code objects currently do not have a co_code attribute because it is not possible to directly access Java bytecode from a class, without loading the file; to the significant - Jython uses Java's true garbage collection rather than Python's reference counting scheme.

First steps in python

The Python interpreter

To start using Python, just ask python ! (or jython :-))

 $ python
Python 2.5.2 (r252:60911, Jul 31 2008, 17:28:52)
[GCC 4.2.3 (Ubuntu 4.2.3-2ubuntu7)] on linux2
Type "help", "copyright", "credits" or "license" for more information.

You can impose the python version you want to use:

 $ python2.5
Python 2.5.2 (r252:60911, Jul 31 2008, 17:28:52)
[GCC 4.2.3 (Ubuntu 4.2.3-2ubuntu7)] on linux2
Type "help", "copyright", "credits" or "license" for more information.

To quit the python interpreter, type Ctrl-D.

Playing with the python interpreter:

 $ python
Python 2.5.2 (r252:60911, Jul 31 2008, 17:28:52)
[GCC 4.2.3 (Ubuntu 4.2.3-2ubuntu7)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> 2+3
>>> 3/5.3
>>> (2+3)*5+4**3
>>> print 'bonjour'
>>> print "un autre bonjour"
un autre bonjour
>>> print """toujours bonjour"""
toujours bonjour
>>> print "aujourd'hui"+', '+"""j'ai dit attention au "guillement"."""
aujourd'hui, j'ai dit attention au "guillement".

Python scripts

On Unix/Linux systems, Python scripts can be made directly executable, like shell scripts, by putting the line

#!/usr/bin/env python

The script can be given an executable mode, or permission, using the chmod command:

$ chmod +x

On Windows systems, there is no notion of an “executable mode”. The Python installer automatically associates .py files with python.exe so that a double-click on a Python file will run it as a script. The extension can also be .pyw, in that case, the console window that normally appears is suppressed.

Source code encoding

It is possible to use encodings different than ASCII in Python source files. The best way to do it is to put one more special comment line right after the #! line to define the source file encoding:

#!/usr/bin/env python
# -*- coding: iso8859-1 -*-

print "un pétit encodéûr"
 $ ./
un pétit encodéûr
#!/usr/bin/env python

print "un pétit encodéûr"
 $ ./
    File "./", line 3
SyntaxError: Non-ASCII character '\xe9' in file ./ on line 3, but no encoding declared; see for details

Python as a calculator

When used directly as an interpretor, python is first a calculator.


  • >>> : enter a command

  • (no prompt): the result of the command

  • ... : wait for the end of the command (continuation of the previous line or block of commands)


  • all comments in python start with a # and finish at the end of the current line
# this is the first comment
SPAM = 1                 # and this is the second comment
                         # ... and now a third!
STRING = "# This is not a comment."


 $ python
Python 2.5.2 (r252:60911, Jul 31 2008, 17:28:52)
[GCC 4.2.3 (Ubuntu 4.2.3-2ubuntu7)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> 2+2
>>> # This is a comment
... 4+4-2*3
>>> (50+6)/3
>>> # integer division returns the floor
... # if you want a real, you must perform the division using real number
... (50.+6)/3

The equal sign ('=') is used to assign a value to a variable. Afterwards, no result is displayed before the next interactive prompt:

>>> width = 20
>>> height = 5*9
>>> width * height

A value can be assigned to several variables simultaneously:

>>> x = y = z = 0  # Zero x, y and z
>>> x
>>> y
>>> z

Variables must be “defined” (assigned a value) before they can be used, or an error will occur:

>>> n
Traceback (most recent call last):
  File "`<stdin>`", line 1, in `<module>`
NameError: name 'n' is not defined

There is full support for floating point; operators with mixed type operands convert the integer operand to floating point:

>>> 3 * 3.75 / 1.5
>>> 7.0 / 2

Complex numbers are also supported; imaginary numbers are written with a suffix of j or J. Complex numbers with a nonzero real component are written as (real+imagj), or can be created with the complex(real, imag) function.

>>> 1j * 1J
>>> 1j * complex(0,1)
>>> 3+1j*3
>>> (3+1j)*3
>>> (1+2j)/(1+1j)

Complex numbers are always represented as two floating point numbers, the real and imaginary part. To extract these parts from a complex number z, use z.real and z.imag.

>>> a=1.5+0.5j
>>> a.real
>>> a.imag

In interactive mode, the last printed expression is assigned to the variable _. This means that when you are using Python as a desk calculator, it is somewhat easier to continue calculations, for example:

>>> tax = 12.5 / 100
>>> price = 100.50
>>> price * tax
>>> price + _
>>> round(_, 2)


a = 'This is a string'

b = "This is another string"

c = """And another way of defining a string"""

d = "The difference between \', \", and \"\"\" is \n the way a newline \
is handled"

e = """ using three double quotes " is better when
multiple lines are involved"""

print a
print b
print c
print d
print e

Strings can be concatenated (glued together) with the + operator, and repeated with *:

text = "This is a "+"dead parrot !"
print text
print (text+" ")*3

Strings can be subscripted (indexed); like in C, the first character of a string has subscript (index) 0. There is no separate character type; a character is simply a string of size one. Like in Icon, substrings can be specified with the slice notation: two indices separated by a colon.

text = "This is a "+"dead parrot !"
print text            # all characters
print text[0]         # character at index 0
print text[10]        # character at index 10
print text[10:14]     # characters from index 10 to index 14 (excluded)
print text[15:]       # characters from index 15 to the end of the strind
print text[:4]        # first four characters
print text[10:-9]     # from index 10 to the end of the string minus 9 characters

The built-in function len() returns the length of a string:

>>> s = 'supercalifragilisticexpialidocious'
>>> len(s)


Python knows a number of compound data types, used to group together other values. The most versatile is the list, which can be written as a list of comma-separated values (items) between square brackets. List items need not all have the same type.

>>> a = ['spam', 'eggs', 100, 1234]
>>> a
['spam', 'eggs', 100, 1234]

Like string indices, list indices start at 0, and lists can be sliced, concatenated and so on:

>>> a[0]
>>> a[3]
>>> a[-2]
>>> a[1:-1]
['eggs', 100]
>>> a[:2] + ['bacon', 2*2]
['spam', 'eggs', 'bacon', 4]
>>> 3*a[:3] + ['Boo!']
['spam', 'eggs', 100, 'spam', 'eggs', 100, 'spam', 'eggs', 100, 'Boo!']

Unlike strings, which are immutable, it is possible to change individual elements of a list:

>>> a
['spam', 'eggs', 100, 1234]
>>> a[2] = a[2] + 23
>>> a
['spam', 'eggs', 123, 1234]

Assignment to slices is also possible, and this can even change the size of the list or clear it entirely:

>>> # Replace some items:
... a[0:2] = [1, 12]
>>> a
[1, 12, 123, 1234]
>>> # Remove some:
... a[0:2] = []
>>> a
[123, 1234]
>>> # Insert some:
... a[1:1] = ['bletch', 'xyzzy']
>>> a
[123, 'bletch', 'xyzzy', 1234]
>>> # Insert (a copy of) itself at the beginning
>>> a[:0] = a
>>> a
[123, 'bletch', 'xyzzy', 1234, 123, 'bletch', 'xyzzy', 1234]
>>> # Clear the list: replace all items with an empty list
>>> a[:] = []
>>> a

The built-in function len() also applies to lists:

>>> a = ['a', 'b', 'c', 'd']
>>> len(a)

It is possible to nest lists (create lists containing other lists), for example:

>>> q = [2, 3]
>>> p = [1, q, 4]
>>> len(p)
>>> p[1]
[2, 3]
>>> p[1][0]
>>> p[1].append('xtra')     # See section 5.1
>>> p
[1, [2, 3, 'xtra'], 4]
>>> q
[2, 3, 'xtra']

Note that in the last example, p[1] and q really refer to the same object!

Programming in Python

A first example

Python can be used for more complicated tasks than adding two and two together. For instance, we can write an initial sub-sequence of the Fibonacci series as follows:

>>> # Fibonacci series:
... # the sum of two elements defines the next
... a, b = 0, 1
>>> while b < 10:
...     print b
...     a, b = b, a+b

This example introduces several new features.

multiple variable assignment

The first line contains a multiple assignment: the variables a and b simultaneously get the new values 0 and 1. On the last line this is used again, demonstrating that the expressions on the right-hand side are all evaluated first before any of the assignments take place. The right-hand side expressions are evaluated from the left to the right.

conditional loop

The while loop executes as long as the condition (here: b < 10) remains true. In Python, like in C/Java, any non-zero integer value is true; zero is false. The condition may also be a string or list value, in fact any sequence; anything with a non-zero length is true, empty sequences are false. The test used in the example is a simple comparison. The standard comparison operators are written the same as in C: < (less than), > (greater than), == (equal to), <= (less than or equal to), >= (greater than or equal to) and != (not equal to).

blocks are made by indenting the code

The body of the loop is indented: indentation is Python’s way of grouping statements. Each line within a basic block must be indented by the same amount.

The print statement writes the value of the expression(s) it is given. It differs from just writing the expression you want to write (as we did earlier in the calculator examples) in the way it handles multiple expressions and strings. Strings are printed without quotes, and a space is inserted between items, so you can format things nicely, like this:

>>> i = 256*256
>>> print 'The value of i is', i
The value of i is 65536

A trailing comma avoids the newline after the output:

>>> a, b = 0, 1
>>> while b < 1000:
...     print b,

...     a, b = b, a+b
1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987

if statements

x = int(raw_input("Please enter an integer: "))
if x < 0:
     x = 0
     print 'Negative changed to zero'
elif x == 0:
     print 'Zero'
elif x == 1:
     print 'Single'
     print 'More'

There can be zero or more elif parts, and the else part is optional. The keyword elif is short for else if, and is useful to avoid excessive indentation. An if ... elif ... elif ... sequence is a substitute for the switch or case statements found in other languages.

for statement

Python’s for statement iterates over the items of any sequence (a list or a string), in the order that they appear in the sequence.

>>> # Measure some strings:
... a = ['cat', 'window', 'defenestrate']
>>> for x in a:
...     print x, len(x)
cat 3
window 6
defenestrate 12

the range() function

If you do need to iterate over a sequence of numbers, the built-in function range() comes in handy. It generates lists containing arithmetic progressions:

>>> range(10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

The given end point is never part of the generated list; range(10) generates a list of 10 values, the legal indices for items of a sequence of length 10. It is possible to let the range start at another number, or to specify a different increment (even negative; sometimes this is called the ‘step’):

>>> range(5, 10)
[5, 6, 7, 8, 9]
>>> range(0, 10, 3)
[0, 3, 6, 9]
>>> range(-10, -100, -30)
[-10, -40, -70]

To iterate over the indices of a sequence, you can combine range() and len() as follows:

>>> a = ['Mary', 'had', 'a', 'little', 'lamb']
>>> for i in range(len(a)):
...     print i, a[i]
0 Mary
1 had
2 a
3 little
4 lamb

break, continue, and else (again)

The break statement breaks out of the smallest enclosing for or while loop.

The continue statement continues with the next iteration of the loop.

Loop statements may have an else clause; it is executed when the loop terminates through exhaustion of the list (with for) or when the condition becomes false (with while), but not when the loop is terminated by a break statement. This is exemplified by the following loop, which searches for prime numbers:

>>> for n in range(2, 10):
...     for x in range(2, n):
...         if n % x == 0:
...             print n, 'equals', x, '*', n/x
...             break
...     else:
...         # loop fell through without finding a factor
...         print n, 'is a prime number'
2 is a prime number
3 is a prime number
4 equals 2 * 2
5 is a prime number
6 equals 2 * 3
7 is a prime number
8 equals 2 * 4
9 equals 3 * 3


The pass statement does nothing. It can be used when a statement is required syntactically but the program requires no action. For example:

>>> while True:
...     pass  # Busy-wait for keyboard interrupt (Ctrl+C)

This is commonly used for creating minimal classes:

>>> class MyEmptyClass:
...     pass

Another place pass can be used is as a place-holder for a function or conditional body when you are working on new code, allowing you to keep thinking at a more abstract level. The pass is silently ignored:

>>> def initlog(*args):
...     pass   # Remember to implement this!

defining functions

a first example

>>> def fib(n):    # write Fibonacci series up to n
...     """Print a Fibonacci series up to n."""
...     a, b = 0, 1
...     while b < n:
...         print b,
...         a, b = b, a+b
>>> # Now call the function we just defined:
... fib(2000)
1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597

The keyword def introduces a function definition. It must be followed by the function name and the parenthesized list of formal parameters. The statements that form the body of the function start at the next line, and must be indented.

The first statement of the function body can optionally be a string literal; this string literal is the function’s documentation string, or docstring.

Instead of printing the results, one can write a function that returns a list of the numbers of the Fibonacci series.

>>> def fib2(n): # return Fibonacci series up to n
...     """Return a list containing the Fibonacci series up to n."""
...     result = []
...     a, b = 0, 1
...     while b < n:
...         result.append(b)    # see below
...         a, b = b, a+b
...     return result
>>> f100 = fib2(100)    # call it
>>> f100                # write the result
[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]

The return statement returns with a value from a function. return without an expression argument returns None. Falling off the end of a function also returns None.

A function can return more than one argument using a tuple:

def euclidean(n,m):
  """return the integer division and the modulo of n/m"""
  return n/m, n%m

a, b = euclidean(18,7)
print "Euclidean division"
print "%d = %d * %d + %d" % (18, a, 7, b)

default argument values

Python allows the use of default argument values. This creates a function that can be called with fewer arguments than it is defined to allow.

def f(x = 2, y = 3):
  return x+y
print f(1, 2), f(1), f(x=2), f(y=3), f(y=5,x=-2)

Documentation strings

Python allows the code documentation (use it ! :-))

>>> def my_function():
...     """Do nothing, but document it.
...     No, really, it doesn't do anything.
...     """
...     pass
>>> print my_function.__doc__
Do nothing, but document it.

    No, really, it doesn't do anything.

More on lists

The list data type has some more methods. Here are all of the methods of list objects:

list.append(x): Add an item to the end of the list; equivalent to a[len(a):] = [x].

list.extend(L): Extend the list by appending all the items in the given list; equivalent to a[len(a):] = L.

list.insert(i, x): Insert an item at a given position. The first argument is the index of the element before which to insert, so a.insert(0, x) inserts at the front of the list, and a.insert(len(a), x) is equivalent to a.append(x).

list.remove(x): Remove the first item from the list whose value is x. It is an error if there is no such item.

list.pop([i]): Remove the item at the given position in the list, and return it. If no index is specified, a.pop() removes and returns the last item in the list. (The square brackets around the i in the method signature denote that the parameter is optional, not that you should type square brackets at that position. You will see this notation frequently in the Python Library Reference.)

list.index(x): Return the index in the list of the first item whose value is x. It is an error if there is no such item.

list.count(x): Return the number of times x appears in the list.

list.sort(): Sort the items of the list, in place.

list.reverse(): Reverse the elements of the list, in place.

Functional programming tools

There are three built-in functions that are very useful when used with lists: filter(), map(), and reduce().

filter(function, sequence) returns a sequence consisting of those items from the sequence for which function(item) is true. For example, to compute some primes:

>>> def f(x): return x % 2 != 0 and x % 3 != 0
>>> filter(f, range(2, 25))
[5, 7, 11, 13, 17, 19, 23]

map(function, sequence) calls function(item) for each of the sequence’s items and returns a list of the return values. For example, to compute some cubes:

>>> def cube(x): return x*x*x
>>> map(cube, range(1, 11))
[1, 8, 27, 64, 125, 216, 343, 512, 729, 1000]

reduce(function, sequence) returns a single value constructed by calling the binary function function on the first two items of the sequence, then on the result and the next item, and so on. For example, to compute the sum of the numbers 1 through 10:

>>> def add(x,y): return x+y
>>> reduce(add, range(1, 11))

List Comprehension

List comprehensions provide a concise way to create lists without resorting to use of map(), or filter(). The resulting list definition tends often to be clearer than lists built using those constructs.

>>> vec = [2, 4, 6]
>>> [3*x for x in vec]
[6, 12, 18]
>>> [3*x for x in vec if x > 3]
[12, 18]
>>> [3*x for x in vec if x < 2]
>>> vec1 = [2, 4, 6]
>>> vec2 = [4, 3, -9]
>>> [x*y for x in vec1 for y in vec2]
[8, 6, -18, 16, 12, -36, 24, 18, -54]
>>> [x+y for x in vec1 for y in vec2]
[6, 5, -7, 8, 7, -5, 10, 9, -3]
>>> [vec1[i]*vec2[i] for i in range(len(vec1))]
[8, 12, -54]


A tuple consists of a number of values separated by commas, for instance:

>>> t = 12345, 54321, 'hello!'
>>> t[0]
>>> t
(12345, 54321, 'hello!')
>>> # Tuples may be nested:
... u = t, (1, 2, 3, 4, 5)
>>> u
((12345, 54321, 'hello!'), (1, 2, 3, 4, 5))


It is best to think of a dictionary as an unordered set of key: value pairs, with the requirement that the keys are unique (within one dictionary). A pair of braces creates an empty dictionary: {}. Placing a comma-separated list of key:value pairs within the braces adds initial key:value pairs to the dictionary.

>>> tel = {'jack': 4098, 'sape': 4139}
>>> tel['guido'] = 4127
>>> tel
{'sape': 4139, 'guido': 4127, 'jack': 4098}
>>> tel['jack']
>>> del tel['sape']
>>> tel['irv'] = 4127
>>> tel
{'guido': 4127, 'irv': 4127, 'jack': 4098}
>>> tel.keys()
['guido', 'irv', 'jack']
>>> 'guido' in tel

Looping techniques:

>>> knights = {'gallahad': 'the pure', 'robin': 'the brave'}
>>> for (key, value) in knights.iteritems():
...     print key, value
gallahad the pure
robin the brave


A module is a file containing Python definitions and statements. The file name is the module name with the suffix .py appended.

# Fibonacci numbers module

def fib(n):    # write Fibonacci series up to n
    a, b = 0, 1
    while b < n:
        print b,
        a, b = b, a+b

def fib2(n): # return Fibonacci series up to n
    result = []
    a, b = 0, 1
    while b < n:
        a, b = b, a+b
    return result
>>> import fibo
>>> fibo.fib(1000)
1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987
>>> fibo.fib2(100)
[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]

Python comes with a library of standard modules, described in a separate document, the Python Library Reference.


old string formatting

The % operator can also be used for string formatting.

>>> import math
>>> print 'The value of PI is approximately %5.3f.' % math.pi
The value of PI is approximately 3.142.

reading and writing files

open() returns a file object, and is most commonly used with two arguments: open(filename, mode).

To read a file’s contents, call, which reads some quantity of data and returns it as a string. size is an optional numeric argument. When size is omitted or negative, the entire contents of the file will be read and returned.

>>> f = open('/tmp/workfile', 'w')
>>> print f
`<open file '/tmp/workfile', mode 'w' at 80a0960>`
'This is the entire file.\n'
>>> f.close()

f.readline() reads a single line from the file.

>>> f.readline()
'This is the first line of the file.\n'
>>> f.readline()
'Second line of the file\n'
>>> f.readline()

An alternative approach to reading lines is to loop over the file object. This is memory efficient, fast, and leads to simpler code:

>>> for line in f:
        print line,

This is the first line of the file.
Second line of the file

f.write(string) writes the contents of string to the file, returning None.

>>> f.write('This is a test\n')

To write something other than a string, it needs to be converted to a string first:

>>> value = ('the answer', 42)
>>> s = str(value)
>>> f.write(s)


Even if a statement or expression is syntactically correct, it may cause an error when an attempt is made to execute it. Errors detected during execution are called exceptions.

>>> 10 * (1/0)
Traceback (most recent call last):
  File "`<stdin>`", line 1, in ?
ZeroDivisionError: integer division or modulo by zero
>>> 4 + spam*3
Traceback (most recent call last):
  File "`<stdin>`", line 1, in ?
NameError: name 'spam' is not defined
>>> '2' + 2
Traceback (most recent call last):
  File "`<stdin>`", line 1, in ?
TypeError: cannot concatenate 'str' and 'int' objects

The last line of the error message indicates what happened. Exceptions come in different types, and the type is printed as part of the message: the types in the example are ZeroDivisionError, NameError and TypeError. The string printed as the exception type is the name of the built-in exception that occurred. This is true for all built-in exceptions, but need not be true for user-defined exceptions.

Handling exceptions

>>> while True:
...     try:
...         x = int(raw_input("Please enter a number: "))
...         break
...     except ValueError:
...         print "Oops!  That was no valid number.  Try again..."

The try statement works as follows.

  • First, the try clause (the statement(s) between the try and except keywords) is executed.
  • If no exception occurs, the except clause is skipped and execution of the try statement is finished.
  • If an exception occurs during execution of the try clause, the rest of the clause is skipped. Then if its type matches the exception named after the except keyword, the except clause is executed, and then execution continues after the try statement.
  • If an exception occurs which does not match the exception named in the except clause, it is passed on to outer try statements; if no handler is found, it is an unhandled exception and execution stops with a message as shown above.

A try statement may have more than one except clause, to specify handlers for different exceptions. At most one handler will be executed. Handlers only handle exceptions that occur in the corresponding try clause, not in other handlers of the same try statement. An except clause may name multiple exceptions as a parenthesized tuple, for example:

... except (RuntimeError, TypeError, NameError):
...     pass

Raising Exceptions

The raise statement allows the programmer to force a specified exception to occur. For example:

>>> raise NameError('HiThere')
Traceback (most recent call last):
  File "`<stdin>`", line 1, in ?
NameError: HiThere

Programs may name their own exceptions by creating a new exception class. Exceptions should typically be derived from the Exception class, either directly or indirectly.


Class Definition

The simplest form of class definition looks like this:

class MyFirstClass(object):
   # list of statements 

The object statement is not mandatory but highly recommended since it allows the use of "modern" python classes.

Class Instantiation

The creation (instantiation) of python objects is performed by calling the class:

x = MyFirstClass()

Object Attributes

Attribute references use the standard syntax used for all attribute references in Python: Valid attribute names are all the names that were in the class’s namespace when the class object was created.

Data attributes need not be declared; like local variables, they spring into existence when they are first assigned to.

class MyFirstClass(object):

x = MyFirstClass()
x.a = 1
x.b = 'toto'
print x.a
print x.b

class ClassWithAttributes(object):
  i = 12345

x = ClassWithAttributes()
print x.i

Object Methods

A method is a function that “belongs to” an object.

Usually, a method is called right after it is bound:


When an object method is called by python, a reference to the object is given as the first parameter of the function. Therefore, a method defined in a class body has always a mandatory first parameter, conventionally named self.

class MyClass(object):
  def addition(self, a, b):  # <-- three parameters !
    """return the addition of two parameters"""
    return a+b

a = MyClass()
print a.addition(2,3) # <-- Two parameters ! (the parameter 'self' is automatically include by python)


Class constructor is a special function as defined in the class: __init__(). When a class defines an __init__() method, class instantiation automatically invokes __init__() for the newly-created class instance.

class MyClass(object):
  def __init__(self):
    print "MyClass instance is generated"

x = MyClass()

Of course, the __init__() method may have arguments for greater flexibility. In that case, arguments given to the class instantiation operator are passed on to __init__().

class MyClass(object):
  def __init__(self, value):
    print "MyClass instance is generated with value:"+repr(value)

x = MyClass('toto')

Object Attributes (bis)

Using the self reference, it is possible to distinguish in a method definition between local variables and object attributes:

>>> class MyClass(object):
...   a = 1          # object attribute
...   def f1(self):
...     a = 2        # local variable
...   def f2(self):
...     self.a = -3  # self attribute
>>> x = MyClass()
>>> x.a
>>> x.f1()
>>> x.a
>>> x.f2()
>>> x.a

Static Methods

A static method is a method that you can call on a class, or on any instance of the class, without the special behavior and constraints of ordinary methods (which are bound on the first argument).

In python, a static method can be built using the staticmethod built-in type:

class Complexe(object):
  def __init__(self, r, i):
    self.real = r
    self.img = i

  def conjugate(self):
    """return the conjugate of the complex"""
    return Complexe(self.real, -self.img)

  def addition(a, b):    # <-- no 'self' parameter
    """return the addition of two complexes"""
    return Complexe(a.real+b.real, a.img+b.img)
  addition = staticmethod(addition)

  def __repr__(self):
    """return a representation of the complex"""
    return "(%f, %f)" % (self.real, self.img)

z1 = Complexe(1.4, 2.3)
z2 = z1.conjugate()
zsomme = Complexe.addition(z1, z2)
print repr(z1)
print repr(z2)
print repr(zsomme)


A property is an instance attribute with special functionality. You reference the attribute with the normal syntax (obj.x). However, rather than following the usual semantics for attribute reference, specific access call methods are used:

class C(object):
    def __init__(self):
        self._x = None

    def getx(self):
        return self._x
    def setx(self, value):
        self._x = value
    def delx(self):
        del self._x
    x = property(getx, setx, delx, "I'm the 'x' property.")

An example:

class PositiveInteger(object):
  def __init__(self):
    self.__n = 0        # the two underscores mean private attribute
  def setN(self, n):
    if (n < 0):
      raise TypeError, "%d is not a positive integer" % n
    self.__n = n         
  def getN(self):
    return self.__n
  n = property(getN, setN)

a = PositiveInteger()
a.n = 2
print a.n
a.n = -2

Set, delete and doc attributes are not mandatory. This enables the use of properties when an attribute is only allowed to be read and not set.

class Rectangle(object):
  def __init__(self, width, height):
    self.width = width
    self.height = height

  def getArea(self):
    return self.width*self.height
  area = property(getArea)

r = Rectangle(10, 22)
print r.area


The syntax for a derived class definition looks like this:

class DerivedClassName(BaseClassName):

Python has two built-in functions that work with inheritance:

  • Use isinstance() to check an object’s type: isinstance(obj, int) will be True only if obj.__class__ is int or some class derived from int.
  • Use issubclass() to check class inheritance: issubclass(subclass, class) will be True if subclass is a subclass of class
>> a = 3
>>> isinstance(a, int)       # a is an instance of the 'int' class
>>> isinstance(a, float)     # but not of the 'float' class
>>> issubclass(bool, int)    # the 'bool' class is derived from 'int'
>>> issubclass(bool, float)  # but not from 'float'

class Rectangle(object):
  def __init__(self, width, height):
    print "Rectangle __init__"
    self.width = width
    self.height = height
  def getArea(self):
    print "Rectangle getArea"
    return self.width*self.height
  area = property(getArea)

class Trapeze(Rectangle):
  def __init__(self, base, height):
    self.width = base
    self.height = height

class Square(Rectangle):
  def __init__(self, size):
    print "Square __init__"
    Rectangle.__init__(self, size, size)      # call the super constructor

print "issubclass(Square, Rectangle) ", issubclass(Square, Rectangle)
print "issubclass(Rectangle, Square) ", issubclass(Rectangle, Square)
print "issubclass(Rectangle, object) ", issubclass(Rectangle, object)
print "issubclass(Square, object) ",    issubclass(Square, object)
print "issubclass(Square, Trapeze) ",   issubclass(Square, Trapeze)

t = Trapeze(10, 20)
print "isinstance(t, Trapeze) ", isinstance(t, Trapeze)
print "isinstance(t, Rectangle) ", isinstance(t, Rectangle)
print "isinstance(t, Square) ", isinstance(t, Square)
print t.area

a = Square(20)
print "isinstance(a, Trapeze) ", isinstance(a, Trapeze)
print "isinstance(a, Rectangle) ", isinstance(a, Rectangle)
print "isinstance(a, Square) ", isinstance(a, Square)
print a.area

General-Purpose Special Methods

Some special methods relate to general-purpose operations. A class that defines or inherits these methods allows its instances to control such operations.

class Complexe(object):
  def __init__(self, r, i):
    self.real = r
    self.img = i
  def __add__(self, other):
    return Complexe(self.real + other.real, self.img + other.img)
  def __sub__(self, other):
    return Complexe(self.real - other.real, self.img - other.img)
  def __mul__(self, other):
    return Complexe(self.real*other.real-self.img*other.img, self.real*other.img + self.img*other.real)
  def __div__(self, other):
    denominator = other.real**2 + other.img**2
    return Complexe( (self.real*other.real + self.img*other.img)/denominator,
                     (self.img*other.real - self.real*other.img)/denominator)
  def __repr__(self):
    return "%f + %fi" % (self.real, self.img)

z1 = Complexe(1, 2.4)
z2 = Complexe(-3.2, 5.1)
print z1+z2, z1-z2, z1*z2, z1/z2

Brief Tour of the Standard Library

See the Python documentation.