cybernetics/pyjava

PyJava is a small Java library that aims to bring some Python fun to Java

PyJava

PyJava is a small Java library that aims to bring some Python to Java land. If you are familiar with Python, then with PyJava you can code in Java similar to Python. PyJava tries to bring some fun to Java users.

Let's start with some examples to see what it looks like. First:

// Write a function that receives a whole number (integer) and
// returns its reverse as a whole number.

int reverse(int n) {
    return Py.to_int(Py.str.reverse(Py.to_str(n)));
}

Py.print(2016);             // 2016
Py.print(reverse(2016));    // 6102

Second:

// Using a for loop, print the lower case letters of the English alphabet:
//
// a b c d e f g h i j k l m n o p q r s t u v w x y z

int start = Py.ord('a');
int end = Py.ord('z');

for (int i = start; i <= end; ++i) {
    Py.print(Py.chr(i), " ");    // " " means: end=" "
}
Py.print();

Third:

// removing duplicates
//
// Consider the following list: [5, 2, 3, 5, 1, 4, -200, 5, 1, 3, 2, 2, 5] . 
// Remove the duplicates, i.e. an element should be present in the list maximum once. 
// Let the result be a sorted list.

List<Integer> li = Py.as_list(5, 2, 3, 5, 1, 4, -200, 5, 1, 3, 2, 2, 5);
Py.print(Py.sorted(Py.to_list(Py.to_set(li))));

See the .../pyjava/examples package for more examples.

But why?

I know about Jython and Groovy, but I wanted to use plain Java. I also wanted something lighweight, nothing complicated, that is familiar to Python programmers. This project was made for fun, so I don't think anyone should program in Java like this :) But sometimes I need to use Java, and I always miss the Python goodies. That's why this library was born.

How to use it?

Simply import the class Py:

import com.github.jabbalaci.pyjava.Py;

All its functions are static, so there is no need to instantiate it. Py contains the built-in functions of Python (some of them). String-related functions (that are in Python's str class) are in Py.str. As an alternative, you can also use the class PyStr. List-related functions (that are in Python's list class) are in Py.list. As an alternative, you can also use the class PyList.

Python to PyJava

Built-in functions

Python                             PyJava                     remark
======                             ======                     ======

print(...)                         Py.print(...)              newline added
print(..., end=" ")                Py.print(..., " ")         specify the end character
print()                            Py.print()                 print just a newline
print("...".format(...))           Py.printf(...)             shortcut for System.out.printf(...)
input()                            Py.input()                 read from stdin
input(prompt)                      Py.input(prompt)           add a prompt
str(obj)                           Py.to_str(obj)             try to convert anything to String
int("5")                           Py.to_int("5")             convert to int
float("3.14")                      Py.to_float("3.14")        returns a float
                                   Py.to_double("3.14")       returns a double (use this)
range(end)                         Py.range2(end)             Python 2's range, i.e. returns a list
range(start, end)                  Py.range2(start, end)
range(start, end, step)            Py.range2(start, end, step)
li = []                            List<Integer> li = Py.new_list(0)    0 is the integer type
                                   List<String> li = Py.new_list("")    "" is the String type
min(my_list)                       Py.min(my_list)
max(my_list)                       Py.max(my_list)
sum(my_list)                       Py.sum(my_list)             works with int and double lists
--                                 Py.prod(my_list)            product of the elements
                                                               works with int and double lists
chr(65)                            Py.chr(65)                  ASCII value to char
ord('a')                           Py.ord('a')                 char to ASCII value
                                   Py.ord("a")
bin(n)                             Py.bin(n)                   0b prefix is present
--                                 Py.bin(n, false)            0b prefix is removed
sorted(my_list)                    Py.sorted(my_list)          returns a sorted shallow copy
sorted(my_list, reverse=True)      Py.sorted(my_list, true)    descending order
reversed(my_list)                  Py.reversed(my_list)        returns a reversed shallow copy
li = [6, 4, 3]                     List<Integer> li = Py.as_list(6, 4, 3)
li = ["aa", "bb", "cc"]            List<String> li = Py.as_list("aa", "bb", "cc")
list("pyjava")                     Py.to_list("pyjava")        explode to characters (1-long strings, actually)
list(set(1, 1, 2))                 Py.to_list(Set set)
set([1, 3, 6])                     Set<Integer> bag = Py.to_set(Py.as_list(1, 3, 6))
print("-" * 20)                    Py.sep("-", 20)             print a separator line

PyStr class (available as Py.str too)

Python                             PyJava                               remark
======                             ======                               ======

str.capitalize("john")             Py.str.capitalize("john")
"pyjava"[::-1]                     Py.str.reverse("pyjava")
"-" * 20                           Py.str.repeat("-", 20)
str.center("py", 10)               Py.str.center("py", 10)
str.split("aa:bb", ":")            Py.str.split("aa:bb", ":")           returns a List<String>
str.split("  a  b ")               Py.str.split("  a  b ")              split by whitespaces
str.join(":", ["a", "b"])          Py.str.join(":", List<String> li)

PyList class (available as Py.list too)

Python                             PyJava                        remark
======                             ======                        ======

my_list.sort()                     Py.list.sort(my_list)         sort in place
my_list.sort(reverse=True)         Py.list.sort(my_list, true)   descending order
my_list.reverse()                  Py.list.reverse(my_list)      reverse in place

TODO

Unit tests are added but the documentation is not yet perfect. Generating the javadoc (with maven javadoc:javadoc for instance) fails. Contributions are welcome here :)

Contributors

Peter Jeszenszky