Python Basics

Debugging

http://docs.python.org/library/pdb.html

python -m pdb my.py

# launch pdb after a crash.
import pdb; pdb.set_trace()   

# compile without executing it
python -m py_compile test.py 

Lib Path

export PYTHONPATH=/home/python
$ python
Python 2.4.3 (#1, May 24 2008, 13:47:28) 
[GCC 4.1.2 20070626 (Red Hat 4.1.2-14)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.path
['', '/home/python', '/usr/lib/python24.zip', '/usr/lib/python2.4', '/usr/lib/python2.4/plat-linux2', '/usr/lib/python2.4/lib-tk', '/usr/lib/python2.4/lib-dynload', '/usr/lib/python2.4/site-packages', '/usr/lib/python2.4/site-packages/Numeric', '/usr/lib/python2.4/site-packages/gtk-2.0']

Useful

help(int)
dir(int)
locals()
globals()

Variables

everything in python is an object. 

All python variables hold value of references to objects.

parameter passing is by value.

String, numbers, tuples are immutable. 

Integers, floats, complex numbers, booleans

x = 5
y = 6
print `x`;
print "x is %i y is %i" % (x, y)

Lists

a=[1,4,5,6]
b=[1,"two",3,["a","b"],4]

Tuples

Same as lists but not changeable

a=(1,2)
b=(1,"two",3,["a","b"],4)

Dictionaries

a={1:"one", 2:"two"}
a["first"]="one";
list(a.keys())
a.get(1, "not availabe)

Sets

unorder collection of objects.

>>> x=set([1,2,3,2,4,1])
>>> x
set([1, 2, 3, 4])
>>> 1 in x
True
>>> 6 in x
False

Conditionals

if x < 10:
   y = 1
elif x > 12:
   y = 2
else:
   y = 3

Loops

For

for x in [1,2,3,4,5]:
  print x;

While

x=5
while x < 10:
  print x
  x=x+1

Functions

def func(x, y, z):
   print (x, y, z)
   print x, y, z
func(1,2,3)
mult=lambda x,y: x * y   # return is implicit
map( lambda x: x*x, [1,2,3,4])
filter(lambda x: x > 3, [ 1,2,3,4,5,6])

Class

class X:
  "Sample class"
  x=5
  def setVal(self,val):
    self.x=val

External Commands

os.system

import os
res=os.system("ls myfile");
if res>>8 != 0:
  print "err"

commands

import commands
>>> res, output = commands.getstatusoutput('ls /bin/ls')
>>> res
0
>>> output
'/bin/ls'
>>> commands.getoutput('ls /bin/ls')
'/bin/ls'
>>> commands.getstatus('/bin/ls')
'-rwxr-xr-x. 1 root root 105112 Feb  8  2011 /bin/ls'