Python Development with Pydev and Eclipse - Tutorial

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130 Python Development with PyDev and Eclipse -

Free tutorial, donate Tutorial to support Lars Vogel

Version 1.8

01.07.2012

Revision History

Revision 0.1 12.01.2009 Lars Created by Lars Vogel Vogel

Revision 0.2 - 1.8 31.01.2009 - 01.07.2012 Lars bug fixes and enhancements Vogel

Python, Pydev and Eclipse

This article describes how to write and debug Python programs with Eclipse

This article was developed on Eclipse 3.6 (Helios), Python 2.6.2 and PyDev version 1.6.2.

Table of Contents

1. Overview 1.1. What is Python 1.2. Block concept in Python- Indentation 1.3. About this tutorial 2. Installation 2.1. Python 2.2. Eclipse Python Plugin 2.3. Configuration of Eclipse 3. Your first Python program in Eclipse 4. Debugging 5. Programming in Python 5.1. Comments 5.2. Variables 5.3. Assertions 5.4. Methods / Functions in Python 5.5. Loops and if clauses 5.6. String manipulation 5.7. Concatenate strings and numbers 5.8. Lists 5.9. Processing files in Python 5.10. Splitting strings and comparing lists. 5.11. Writing Python Scripts in Unicode 5.12. Classes in Python 6. Google App Engine 7. Thank you 8. Questions and Discussion 9. Links and Literature

1. Overview

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vogella.com Tutorials Training Services Publications Connect language. Python was develop from Guido van Rossum. The name Python is based on the TV show "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 / or functional programming style

1.2. Block concept in Python- Indentation

Python follows a different way then other programming languages to identify blocks on related code. A block is identified 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.

1.3. About this tutorial

This tutorial will first explain how to install Python and the Python plugins for Eclipse. It will then create a small Python project to show the usage of the plugin. Afterwards the general constructs of Python are explained.

2. Installation

2.1. Python

Download Python from http://www.python.org . Download version 2.6.x from Python. If you are using Windows you can use the native installer for Python.

2.2. Eclipse Python Plugin

The following assume that you have already Eclipse installed. For an installation description of Eclipse please see Eclipse IDE for Java .

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 .

2.3. Configuration of Eclipse

You also have to maintain in Eclipse the location of your Python installation. Open in the menu Window -> Preference and select Pydev-> Interpreter Python

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Press new and maintain the path to "python.exe" in your installation directory.

The result should look like the following.

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3. Your first Python program in Eclipse

Select File -> New -> Project. Select Pydev -> Pydev Project.

BACK TO TOP Create a new project with the name "de.vogella.python.first". Select Python version 2.6 and your interpreter.

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Press finish.

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".

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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!

4. Debugging

Just right-click in the source code and add a breakpoint.

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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

Command Description

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.

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5. Programming in Python

5.1. Comments

The following create a single line comment.

# This is a comment

5.2. Variables

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

5.3. Assertions

Python provides assertions. These assertions are always called.

assert(1==2)

5.4. Methods / Functions in Python

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)

5.5. Loops and if clauses

The following demonstrates a loop the usage of an if-clause.

i = 1 forforfor i in range(1, 10): ififif i <= 5 : print 'Smaller or equal then 5.\n' , elseelseelse : print 'Larger then 5.\n' ,

5.6. String manipulation

Python allows the following String operations.

Table 2.

Operations Description

len(s) Returns the length of string s BACK TO TOP s[i] Gets the element on position i in String s, position start with zero

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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

For example:

s = "abcdefg" assert (s[0:4]== "abcd" ) assert (s[4:]== "efg" ) assert ( "abcdefg" [4:0]== "" ) assert ( "abcdefg" [0:2]== "ab" )

5.7. Concatenate strings and numbers

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".

For example:

print 'this is a text plus a number ' + str(10)

5.8. Lists

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[0]) # 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 forforfor 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))

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vogella.com Tutorials Training Services Publications Connect 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 forforfor 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' ) forforfor line in f: output.write(line.rstrip() + '\n' ) f.close()

5.10. Splitting strings and comparing lists.

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= "" forforfor line in f1: s+=line f1.close()

f2 = open( 'c:\\temp\\launchconfig2.txt' , 'r' ) s2= "" forforfor 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)

5.11. Writing Python Scripts in Unicode

If you read special non ASCII sign, e.g. ö, ä. ü or ß, 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 -*-

5.12. Classes in Python

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.

Table 3.

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__str__ The method which is called if print is applied to this object

__add__ + Operator

__mul__ * Operator

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)

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Ads by LinkedIn 6. Google App Engine

Google offers free hosting of small Python based web application. Please see Google App Engine development with Python .

7. Thank you

Please help me to support this article:

8. Questions and Discussion

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.

9. Links and Literature

http://www.python.org/ Python Homepage

http://www.pydev.org/ PyDev Homepage

http://docs.python.org/tutorial/ Python Tutorial

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