Python Primer If You Know Java

This document is a very quick primer that compares the basic constructs and data structures between Java and Python.

Python is a great language for this course because it requires much fewer boiler plate code, has very lightweight servers as opposed to J2EE, excellent packages, a REPL and an incredible wealth of documentation and tutorials for the language.

To get started see:

• Python tutorial
• Learn Python The Hard Way

Main program structure

Java main classes have a special method called main that denotes an executable class. They are also typically defined in a file with the same name as the class name. For example, the file HelloWorld.java may contain class HelloWorld:

    class HelloWorld {
      public static void main(String[] args) {
        System.out.println("Hello World");
      }
    }

In contrast, every python file is a script that can be run by typing python <filename> in the shell. The script is then executed from top to bottom. For example, helloworld.py may look like:

    print "Hello World"

Functions

Java requires that functions are defined as part of Class definitions:

    class HelloWorld {
      public void f() {
        System.out.println("Hello World");
      }
    }

In python, functions can be directly defined independent of class definitions:

    def f():
      print "Hello World"

This means you can then directly call f:

    f()

Indentation

In Java, statement grouping is defined by using curly braces { }. However Python groups statements based on indentation level. This is why in the above function definition, the print "Hello World" statement is indented – so it is grouped as part of the def f() statement. You will see this for the other control structures below.

Control structures

If statements

Java:

    int n = 0;
    if (1 > 1) {
      n = 1;
    } else if (1 < 1) {
      n = 2;
    } else {
      n = 3;
    }

Python:

    n = 0
    if 1 > 1:
      n = 1
    elif 1 < 1:
      n = 2
    else:
      n = 3

Switch statements

Java:

    switch(i) {
      case 0: 
        i = 0;
        break;
      case 1: 
        i = 1;
      default: 
        i = 2;
    }

Python doesn’t have a switch statement, so you need to use if/else (why does the elif clause contain i = 2?)

    if i == 0: 
      i = 0;
    elif i == 1: 
      i = 1
      i = 2
    else: 
      i = 2

for loops

Java has a for loop:

    for (int i = 0; i < 100; i++) {
    }

The above can be mechanically translated to:

    i = 0
    while i < 100:
      # note: python doesn't have the ++ shorthand, 
      # but has the += shorthand
      i += 1    

Java has a more convenient for loop for iterables:

    List<Integer> a_list = ...;
    for (int i : a_list) {
      System.out.println(i);
    }

Python’s for loop is similar

    l = [0,1,2]
    for i in l:
      print i

But has a powerful for-comprehension capabilitiy:

    l = [0, 1, 2]
    r = range(3)
    # l and r contain the same values

    for i in l:
      print i

    # prints: [0, 2, 4]
    l2 = [i*2 for i in l]
    print l2

    # prints: [0, 2, 4]
    print [i*2 for i in range(3)]

The for-comprehension also accepts boolean conditions in the if clause:

    # prints: [0, 4]
    print [i*2 for i in range(3) if i != 1]

Maps and Dictionaries

Java has Maps that map keys to values:

    Map<String, String> map = new HashMap<String, String>();
    map.put("hello", "world");

Python has a similar data structure called a dictionary.
There are multiple ways to instantiate it (notice that you can directly create a data structure with initial data without resorting to .put() calls:

    # Method 1
    d1 = dict()
    d1["hello"] = "world"


    # Method 2, passing key values in constructor
    d2 = dict(hellor="world")

    # Method 3, using {}
    d3 = {}
    d3["hello"] = "world"

    # Method 4, initializing {}.  Note that the key has string quotes around it
    d4 = {"hello": "world"}

It can be accesses easily:

    # "world"
    print d4["hello"]

The collections~ package that comes with Python has a useful defaultdict` data structure. Instead of throwing an error when trying to access a key that doesn’t exist, it calls a user defined method:

    from collections iport defaultdict
    d = defaultdict(lambda: list())
    d[1] = 99
    d[2].append(3)

    # { 1: 99, 2: [3] }
    print d

Lists

Python uses the following syntax for lists

    l1 = [0, 1, 2]

Python also has a useful range() function:

    # [0, 1, 2]
    print range(3)

    # [0, 2, 4]
    print range(0, 5, 2)

    # [0, 1, 2]
    print [0, 1, 2]