pyxie 0.0.19

Little Python to C++ Compiler

Pyxie – A Little Python to C++ Compiler

What job does / will this do?

The aim of this project is to allow a adults and children to write code in a familiar high level language that can then be compiled to run on an arbitrary embedded system - that is devices with very low power CPUs and very little memory. (ie devices too small to host a python interpreter/runtime)

It’s pre-alpha at the moment.

It will be useful in supporting things like the The Scout Association’s “Digital Maker” badge. That’s a fair way off though.

Show me something you CAN compile

Currently it can compile very very simple types of python program that looks like this into an equivalent (simple) C++ program.

Example program:

age = 10
new_age = 10 +1
new_age_too = age + 1
new_age_three = age + new_age_too
foo = "Hello"
bar = "World"
foobar = foo + bar

print 10-1-2,7
print 1+2*3*4-5/7,25
print age, new_age, new_age_too
print foo, bar, foobar

countdown = 2147483647
print "COUNTING DOWN"
while countdown:
    countdown = countdown - 1

print "BLASTOFF"

Example results:

#include <iostream>
#include <string>

using namespace std;

int main(int argc, char *argv[])
{
    int age;
    string bar;
    int countdown;
    string foo;
    string foobar;
    int new_age;
    int new_age_three;
    int new_age_too;

    age = 10;
    new_age = (10+1);
    new_age_too = (age+1);
    new_age_three = (age+new_age_too);
    foo = "Hello";
    bar = "World";
    foobar = (foo+bar);
    cout << ((10-1)-2) << " " << 7 << endl;
    cout << ((1+((2*3)*4))-(5/7)) << " " << 25 << endl;
    cout << age << " " << new_age << " " << new_age_too << endl;
    cout << foo << " " << bar << " " << foobar << endl;
    countdown = 2147483647;
    cout << "COUNTING DOWN" << endl;
    while(countdown) {
        countdown = (countdown-1);
    };
    cout << "BLASTOFF" << endl;
    return 0;
}

What does it do?

Currently:

• Recognise simple sequential python programs with simple statements

• Can handle basic conditionals and while loops

• Custom includes, and function calls to C/C++ functions (within limits)

• Parse those to an AST

• Can represent equivalent C programs using a concrete C representation (CST)

• Can translate the AST to the CST and then generate C++ code from the CST

Python structural things it supports:

• While loop statements

• Comparisons

• If/elif/elif/else statements

• For loops - specific for X in range(Y)

• Function calls into libraries that we link with

This is close to allowing actually useful programs now.

It’s a starting point, not the end point. For that, take a look at the language spec.

Pyxie is intended to be a simple Python to C++ compiler, with a target of compiling python code such that it can run on a microcontroller - like Arduino, MSP430 or ARM mbed type devices.

The name is a play on words. Specifically, Python to C++ - can be py2cc or pycc. If you try pronouncing “pycc” it can be “pic”, “py cc” or pyc-c”. The final one leads to Pixie.

This is unlikely to ever be a completely general python to C++ compiler - if you’re after than look at Shed Skin, or things like Cython, Pyrex, and PyPy. (in terms of diminishing similarity) The difference this project has from those is that this project assumes a very small target device. Something along the lines of an Atmega 8A, Atmega 328 or more capable.

In the past I’ve written a test driven compiler suite, so I’ll be following the same approach here. It did consider actually making Pyxie use that as a frontend, but for the moment, I’d like python compatibility.

Why not micropython? Micropython is ace . If your device is large enough to support the micropython runtime, use it! The aim of this is on the really small microcontrollers- the ones too small to even support micropython - like an MSP430, or an Atmega 8A or similarly tiny MCU.

In the past I’ve written a test driven compiler suite, so I’ll be following the same approach here. It did consider actually making Pyxie use that as a frontend, but for the moment, I’d like python compatibility.

Status Overview

For the impatient: this probably does NOT do what you want, yet.

High level view of support:

• Supports variables, sequence, and assignment

• while loops controlled by expressions, possibly involving variables

• while loops can contain break/continue which allows “if” style functionality

• Also have basic conditional operators like “==”, “!=”, etc.

• Ability to pull in C++ includes on standard paths

This means we can almost start writing useful programs, but in particular can start creating simplistic benchmarks for measuring run speed. It IS getting there however, and feedback, usecases, devices very welcome.

Influences

Many moons ago, I made a generic language parser which I called SWP (semantic whitespace parser), or Gloop.

• https://github.com/sparkslabs/minisnips/tree/master/SWP

• http://www.slideshare.net/kamaelian/swp-a-generic-language-parser

It was an experiment to see if you could write a parser that had no keywords, or similar, in a completely test driven fashion. ie a bit like a parser for a Lisp like language that would look like python or ruby. It turns out that you can and there’s lots of interesting things that arise if you do. (Best seen in the slideshare link)

Which version of Python?

It’s not a complete subset of any particular python, but it’s based around the intersection points in python 2 and 3. It will be, by definition, a non-dynamic subset - at least at first.

• For detail as to what’s planned for the language, take a look at the language spec.

• For an overview as to the guiding principles, please take a look at project status

• For detail as to what’s actually implemented, take a look at language status

These are all a WIP, but becoming more solid.

Why write this?

Personally, having built something simpler in the past, I know I’d find it useful. (I use python rather than C++ often because I can write more quicker with the former). Also, I work with kids in my spare time, and it opens up options there.

I’ve written something like this for work last year, but that was much more limited and restricted in both aspiration and implementation. This rewrite is something I’ve done on my own time, with my own tools, from scratch, which allows me to share this with others.

Major changes:

• This aims to be a more rounded implementation

• This performs transforms from an AST (abstract syntax tree) to a CCR (concrete code representation), rather than munging code directly from a concrete parse tree.

That potentially allows other things, like creation of visual representations of programs from code as well.

Is this part of any larger project?

No. It could be used by others, but it’s got a definite goal - to allow the use of a “little” python to program devices which are too small to host a python runtime.

If anything, it’s a continuation of the personal itch around SWP (mentioned above) from about 10 years ago. Unlike that though, it’s much, much better structured.

One thing that may happen though is the ability to take python classes and derive iotoy device implementations/interfaces directly. (since iotoy was inspired heavily by python introspection) That’s quite some time off.

Release History

Release History:

• 0.0.19 - 2016-01-31 - Continued work on arduino profile and initial Python3 support

• 0.0.18 - 2016-01-10 - Grammar changes to support object attributes and methods, start of servo support in arduino profile.

• 0.0.17 - 2015-08-12 - Add pass statement, enable “for” on arduino, update documentation, refactor pyxie harness

• 0.0.16 - 2015-08-02 - Adds initial Arduino LEONARDO support, improved function call, release build scripts

• 0.0.15 - 2015-07-18 - clib converted to py clib for adding to build directory

• 0.0.14 - 2015-07-18 - For loops implemented. Added clib code, C++ generator implementation, FOR loop style test harness, parsing and basic analysis of of FOR loops using a range interator

• 0.0.13 - 2015-06-21 - if/elif/else,conditionals/boolean/parenthesised expressions.

• 0.0.12 - 2015-06-16 - While loops, break/continue, Website, comparison operators, simple benchmark test

• 0.0.11 - 2015-06-06 - Function calls; inclusion of custom C++ headers; empty statements; language spec updates

• 0.0.10 - 2015-06-03 - Analysis phase to make type inference work better. Lots of related changes. Implementation of expression statements.

• 0.0.9 - 2015-05-23 - Grammar changed to be left, not right recursive. (Fixes precedence in un-bracketed expressions) Added standalone compilation mode - outputs binaries from python code.

• 0.0.8 - 2015-05-13 - Internally switch over to using node objects for structure - resulting in better parsing of expressions with variables and better type inference.

• 0.0.7 - 2015-04-29 - Structural, testing improvements, infix operators expressions (+ - * / ) for integers, precdence fixes

• 0.0.6 - 2015-04-26 - Character Literals, “plus” expressions, build/test improvements

• 0.0.5 - 2015-04-23 - Core lexical analysis now matches language spec, including blocks

• 0.0.4 - 2015-04-22 - Mixed literals in print statements

• 0.0.3 - 2015-04-21 - Ability to print & work with a small number of variables

• 0.0.2 - 2015-03-30 - supports basic assignment

• 0.0.1 - Unreleased - rolled into 0.0.2 - Initial structure

Language Status

program : statements
statements : statement
           | statement statements

statement_block : INDENT statements DEDENT

statement : assignment_statement
          | print_statement
          | general_expression
          | EOL
          | while_statement
          | break_statement
          | continue_statement
          | pass_statement
          | if_statement
          | for_statement

assignment_statement -> IDENTIFIER ASSIGN general_expression # ASSIGN is currently limited to "="

while_statement : WHILE general_expression COLON EOL statement_block

break_statement : BREAK

pass_statement : PASS

continue_statement : CONTINUE

if_statement : IF general_expression COLON EOL statement_block
             | IF general_expression COLON EOL statement_block extended_if_clauses

extended_if_clauses : else_clause
                    | elif_clause

else_clause : ELSE COLON EOL statement_block

elif_clause : ELIF general_expression COLON EOL statement_block
            | ELIF general_expression COLON EOL statement_block extended_if_clauses

print_statement : 'print' expr_list # Temporary - to be replaced by python 3 style function

for_statement | FOR IDENTIFIER IN general_expression COLON EOL statement_block

expr_list : general_expression
          | general_expression COMMA expr_list

general_expression : boolean_expression

boolean_expression : boolean_and_expression
                   | boolean_expression OR boolean_and_expression

boolean_and_expression : boolean_not_expression
                       | boolean_and_expression AND boolean_not_expression

boolean_not_expression : relational_expression
                       | NOT boolean_not_expression

relational_expression : expression
                      | relational_expression COMPARISON_OPERATOR expression

expression : arith_expression
           | expression '+' arith_expression
           | expression '-' arith_expression
           | expression '**' arith_expression

arith_expression : negatable_expression_atom
                 | arith_expression '*' negatable_expression_atom
                 | arith_expression '/' negatable_expression_atom


negatable_expression_atom : "-" negatable_expression_atom
                          | expression_atom

expression_atom : value_literal
                | IDENTIFIER '(' ')' # Function call, with no arguments
                | IDENTIFIER '(' expr_list ')' # Function call
                | '(' general_expression ')'

value_literal : number
              | STRING
              | CHARACTER
              | BOOLEAN
              | IDENTIFIER

number : NUMBER
       | FLOAT
       | HEX
       | OCTAL
       | BINARY
       | LONG         (suffice is L)
       | UNSIGNEDLONG (suffice is l)
       | '-' number

Current Lexing rules used by the grammar:

NUMBER : \d+
FLOAT : \d+.\d+ # different from normal python, which allows .1 and 1.
HEX : 0x([abcdef]|\d)+
OCTAL : 0o\d+
BINARY : 0b\d+
STRING - "([^"]|\.)*" or '([^']|\.)*' # single/double quote strings, with escaped values
CHARACTER : c'.' /  c"." # Simplification - can be an escaped character
BOOLEAN : True|False
IDENTIFIER : [a-zA-Z_][a-zA-Z0-9_]*

Limitations

Most expressions currently rely on the C++ counterparts. As a result not all combinations which are valid are directly supported yet. Notable ones:

• Combinations of strings with other strings (outlawing /*, etc)

• Combinations of strings with numbers

Why a python 2 print statement?

Python 2 has print statement with special notation; python 3’s version is a function call. The reason why this grammar currently has a python-2 style print statement with special notation is to specifically avoid implementing general function calls yet. Once those are implemented, special cases - like implementing print - can be implemented, and this python 2 style print statement WILL be removed. I expect this will occur around version 0.0.15, based on current rate of progress.

Keeping it for now also simplifies “yield” later

Michael Sparks, January 2016