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Python for Java Programmers

Quick Start

% python   # interactive python interpreter, quit via ctrl-D
>>> help() # enter interactive help mode

>>> help("str") # help for string class
>>> dir(str) # list contents of any Python entity

Syntax

• Indentation determines blocks, not braces. In a way, the colon can be seen as an opening brace.
  
• One statement per line
• If you want one statement to span multiple lines, there must be an opening parenthesis somewhere (or you could escape the return with a backslash)
• If you want several statements per line, you can append 1 single statement after a colon (handy for if-statemements)
• Additionally, there is a core syntax that is not usually used, but available, that allows one to
• use braces for blocks
• semicolons for command separation
• Use for hard cases only

• if-statement does not need parentheses
• null is None in Python, true is True, false is False
• Komments are per-line only: // in Java is # in Python
• There is no decrement operator, use: a-=1
• There are no char literals. Therefore: you can use either single or double quotes for strings
• Strings can be destructively changed
  

>>> a = "bla"
>>> b = "b" + "la"
>>> a == b      # Java: .equals
True
>>> a is b      # Java: ==
False
>>> (3 != 5) or not (7 == 8) and True   # Java: || && etc.
True
>>> a=5
>>> a-=1     # there is no decrement operator --
>>> a
4

Loops

>>> for i in ["a", "b", "c"]:
...     print i
... 
a
b
c
>>> for i in range(1,5):
...     print i
... 
1
2
3
4

Built-In Functions

>>> str(334)
'334'
>>> repr('hallo')
"'hallo'"
>>> str('hallo')
'hallo'
>>> repr(334)
'334'
>>> len([3,4,5])
3

Classes

>>> class MyClass:
...	classVar = 555
...     def __init__(self, arg):    # constructor
...             self.instanceVar = arg
... 
>>> a = MyClass("hi")   # Python does not have the 'new' keyword
>>> a.instanceVar
'hi'
>>> a.newInstanceVar = "externally created"
>>> a.newInstanceVar
'externally created'.

Importing

• Qualified: import javax.swing. Now you have to always qualify your class names: javax.swing.JButton("hi")
• Unqualified single: from javax.swing import JButton. Leads to unqualified class names: JButton("hi")
• Unqualified all: from javax.swing import *. Every class in that package can be used unqualified: JLabel("hello")

Neat Things That Java Can't Do

>>> class A:
...     def myMethod(self, arg):
...             print "Look:", arg
... 
>>> a = A()
>>> b = A.myMethod     # a "method pointer"
>>> b(a, "world")
Look: world
>>> A.myMethod(a, "lala")
Look: lala

>>> myName = "Mr. Boombastic"
>>> "My name is: %s" % myName
'My name is: Mr. Boombastic'
>>> 'My name is: %(myKey)s' % { "myKey": "Roger Rabit" }
'My name is: Roger Rabit'
>>> 'Still my name: %(myName)s' % locals()     # lists all currently active local vars
'Still my name: Mr. Boombastic'

>>> r'\n'      # raw strings
'\\n'
>>> """spans   # multi-line string literals
... several
... lines
... """
'spans\nseveral\nlines\n'

And Much More...

• dictionary literals
• higher-order functions (functions as actual parameters for functions)
  
• iterators (similar to Java)
• several quick ways to transform and generate lists (some of them borrowed from function programming languages)

• Python Home Page
• Jython: The Python JVM implementation
  
• Red Robin: Jython plugin for Eclipse

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