Python Programming - Ultra Short Tutorial

Interactive session with an interpreter: numbers, lists and tuples ( interactive prompt ">>>" ) :

>>> 2+2, range(10), (1,2,3), 1.0, 1e-3, 2+2j
(4, [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], (1, 2, 3), 1.0, 0.001, (2+2j))

Looping over lists:

my_list = ['welcome',"to",'the','python','world']
print my_list[0], my_list[-2:], my_list.pop()   # print first and last two elements, pop last
for x in my_list:
    print x, len(x), type(x)                    # python code blocks are defined by indentation
print '\t'.join(my_list)                   # joins string list with TAB-spaces
print [ s for s in my_list if s.startswith('t') ]
print map( string.upper, my_list )              # string.upper each element of the list

Defining and using a python dictionary (a "mapping"):

d = { 'first' : 1.0 , 'second' : 2.0 }
if 'first' in d: print 'yes'
for ix in d: print d[ix]
value=d['first'] + d.get('third',0.0)           # .get yields default value 0.0
d[(77,99)]='most types/objects can be used as dictionary key'
d.clear()

Functions and conditions:

def my_function(a,b=1,c=2):                     # b=1, c=2 : default arguments
    print a,b+c
    return "thanks for calling", a+b+c          # 2 return values (tuple)
text,result = my_function( 7, c=8 )             # c=8 : keyword argument
my_list.sort( lambda a,b:-cmp(len(a),len(b)) )  # lambda (ad-hoc) function for sorting on string length
if 0<result<100:
    print "result '%s' is:%.2f" % (text,result)
elif result>500: 
    while random.random()<0.9:
        print 'looping ...'       
else:
    print "hmm..."

Defining and using a class:

class X:
    a=1
    def f(self):                                # self
        return self.a * 2
class Y(X):                                     # Y inherits from X
    b=2
    def f(self,value,*args,**kwargs):   # extra positional args and keyword kwargs
        print value,"var args:",args,kwargs
        self.value=value                        # new attribute "value" created on-the-fly 
        return self.a
y=Y(); print y.f( 1.0,2,'text', happyarg='happy' )

Modules, math and regular expressions:

import re,os,glob,thread,encodings.ascii
print re.search( r"#(\d+)", "is #123 a number?").group(1)     
from math import *
print sin( 1.8**2 % 3 ), 1|2, 1 or 2, 1^3       # sinus, to-the, modulo, bit-or, logic-or, bit-xor

We can't really show Pythons limits in this tutorial:

try: y.value + 1/0
except AttributeError,v: print "attribute missing:",v
except ZeroDivisionError: print "ZERO!"; raise
def no_delay_files_lines():                      
    """ generator function : 'yield' returns iterated values on-the-fly
        lookup this doc string by help(no_delay_files_lines) """
    for filename in glob.glob("hugedata/*.txt"):
        if os.path.isfile(filename):
            for line in open(filename):
                yield line.strip() and line[:-1] or "<empty line>"
for line in no_delay_files_lines(): print line
thread.start_new(my_function,(1,2,3))
print "methods:", [ var for var in dir(y) if isinstance(getattr(y,var),types.MethodType) ]
y.__dict__.update(d)
print "instance_dict:", y.__dict__,y.first,y.value==y.__dict__['value']
class Z(Y,list,object):
    "python classes can be types and can be derived from base types"
    def __mul__(self,other): # define a 'mul' (*) operator for Z class instances
        return [a*b for a,b in zip(self,other)]
    def ff():return "ZZZ"
    ff=staticmethod(ff)
Z.smart_slot = property( lambda self:self.a+len(self), Y.my_method )  
Y.__bases__=() 
Z.__metaclass__=...
...