Copyright © 2009, 2010, 2011, 2012, 2013 Lars Vogel
|Revision 0.2 - 1.9||31.01.2009 - 01.05.2013||Lars
|bug fixes and enhancements|
Table of Contents
Python is an interpreted programming language and claims to be a very effective programming language. Python was develop by Guido van Rossum.
The name Python is based on the TV show called Monty Python's Flying Circus. During execution the Python source code is translated into bytecode which is then interpreted by the Python interpreter. Python source code can also run on the Java Virtual Machine, in this case you are using Jython.
Key features of Python are:
high-level data types, as for example extensible lists
statement grouping is done by indentation instead of brackets
variable or argument declaration is not necessary
supports for object-orientated, procedural and functional programming style
Python identify blocks of code by indentation. If you have an if statement and the next line is indented then it means that this indented block belongs to the if. The Python interpreter supports either spaces or tabs, e.g. you can not mix both. The most "pythonic" way is to use 4 spaces per indentation level.
Download Python from http://www.python.org. Download the version 3.3.1 or higher of Python. If you are using Windows you can use the native installer for Python.
For Python development under Eclipse you can use the PyDev Plugin which is an open source project. Install PyDev via the Eclipse update manager via the following update site: http://pydev.org/updates
Select File -> New -> Project. Select Pydev -> Pydev Project.
Create a new project with the name "de.vogella.python.first". Select Python version 2.6 and your interpreter.
Select Window->Open Perspective ->Other. Select the PyDev perspective.
Select the "src" folder of your project, right-click it and select New -> PyDev Modul. Create a module "FirstModule".
Create the following source code.
''' Created on 18.06.2009 @author: Lars Vogel ''' def add(a,b): return a+b def addFixedValue(a): y = 5 return y +a print add(1,2) print addFixedValue(1)
Right-click your model and select Run As -> Python run.
Congratulations! You created your first (little) Python modul and ran it!
Just right-click in the source code and add a breakpoint.
Then select Debug as -> Python Run
You can now inspect and modify the variables in the variables view.
Via the debug buttons (or shortcuts F5, F6, F7, F8) you can move in your program.
You can use F5 / F6, F7 and F8 to step through your coding.
Table 1. Debugging Key bindings
|F5||Goes to the next step in your program. If the next step is a method / function this command will jump into the associated code.|
|F6||F6 will step over the call, e.g. it will call a method / function without entering the associated code.|
|F7||F7 will go to the caller of the method/ function. So this will leave the current code and go to the calling code.|
|F8||Use F8 to go to the next breakpoint. If no further breakpoint is encountered then the program will normally run.|
You can of course use the ui to debug. The following displays the keybindings for the debug buttons.
Python provides dynamic typing of its variables, e.g. you do not have to define a type of the variable Python will take care of this for you.
# This is a text s= "Lars" # This is an integer x = 1 y=4 z=x+y
Python allows to define methods via the keyword def. As the language is interpreted the methods need to be defined before using it.
def add(a,b): return a+b print add(1,2)
The following demonstrates a loop the usage of an if-clause.
i = 1 for i in range(1, 10): if i <= 5 : print 'Smaller or equal then 5.\n', else: print 'Larger then 5.\n',
Python allows the following String operations.
|len(s)||Returns the length of string s|
|s[i]||Gets the element on position i in String s, position start with zero|
|s[-i]||Get the i-tes Sign of the string from behind the string, e.g. -1 returns the last element in the string|
|"abcdefg"[0:4]||Gets the first 4 elements (abcd)|
|"abcdefg"[4:]||Gets the elements after the first 4 elements (abcd)|
|`a`+`b` + `c`||Concatenates the int varibles a, b,c, e.g. if a=1, b=2, c=3 then the result is 123.|
|s.lower()||Result will be s in lower cases|
|s.upper()||Result will be s in upper cases|
|s.startswith(t)||True, if s startsWith t|
|s.rstrip()||Removes the end of line sign from the string|
s = "abcdefg" assert (s[0:4]=="abcd") assert (s[4:]=="efg") assert ("abcdefg"[4:0]=="") assert ("abcdefg"[0:2]=="ab")
Python does not allow to concatenate a string directly with a number. It requires you to turn the number first into a string with the str() function.
If you do not use str() you will get "TypeError: cannot concatenate 'str' and 'int' objects".
print 'this is a text plus a number ' + str(10)
Python has good support for lists. See the following example how to create a list, how to access individual elements or sublists and how to add elements to a list.
''' Created on 14.09.2010 @author: Lars Vogel ''' mylist = ["Linux", "Mac OS" , "Windows"] # Print the first list element print(mylist) # Print the last element # Negativ values starts the list from the end print(mylist[-1]) # Sublist - first and second element print(mylist[0:2]) # Add elements to the list mylist.append("Android") # Print the content of the list for element in mylist: print(element)
If you want to remove the duplicates from a list you can use:
mylist = ["Linux", "Linux" , "Windows"] # remove duplicates from the list mylist = list(set(mylist))
The following example is contained in the project "de.vogella.python.files".
The following reads a file, strips out the end of line sign and prints each line to the console.
''' Created on 07.10.2009 @author: Lars Vogel ''' f = open('c:\\temp\\wave1_new.csv', 'r') print f for line in f: print line.rstrip() f.close()
The following reads the same file but write the output to another file.
''' @author: Lars Vogel ''' f = open('c:\\temp\\wave1_new.csv', 'r') output = open('c:\\temp\\sql_script.text', 'w') for line in f: output.write(line.rstrip() + '\n') f.close()
The following example is contained in the project "de.vogella.python.files". It reads to files which contain one long comma separated string. This string is splitted into lists and the lists are compared.
f1 = open('c:\\temp\\launchconfig1.txt', 'r') s= "" for line in f1: s+=line f1.close() f2 = open('c:\\temp\\launchconfig2.txt', 'r') s2= "" for line in f2: s2+=line f2.close() list1 = s.split(",") list2 = s2.split(","); print(len(list1)) print(len(list2)) difference = list(set(list1).difference(set(list2))) print (difference)
If you read special non ASCII sign, e.g. ??, ??. ?? or ??, you have to tell Python which character set to use. Include the following in the first or second line of your script.
# -*- coding: UTF-8 -*-
The following is a defined class in Python. Python uses the naming convension __name__ for internal functions.
Python allows operator overloading, e.g. you can define what the operator + will to for a specific class.
|__init__||Constructor of the class|
|__str__||The method which is called if print is applied to this object|
The empty object (null) is called None in Python.
class Point: def __init__(self, x=0, y=0): self.x = x self.y = y def __str__(self): return "x-value" + str(self.x) + " y-value" + str(self.y) def __add__(self,other): p = Point() p.x = self.x+other.x p.y = self.y+other.y return p p1 = Point(3,4) p2 = Point(2,3) print p1 print p1.y print (p1+p2)
Google offers free hosting of small Python based web application. Please see Google App Engine development with Python .
Before posting questions, please see the vogella FAQ. If you have questions or find an error in this article please use the www.vogella.com Google Group. I have created a short list how to create good questions which might also help you.