pombredanne / libpypa

libpypa is a Python parser implemented in pure C++

libpypa - A Python Parser Library in C++

• Introduction
• Motivation
• Goal
• Example
• Error Reporting
• Requirements
• Structure
• Lexer
• Parser
• AST
• License

## Introduction **libpypa** is a Python parser implemented in pure *C++*. It neither uses any tools like [flex][1], [yacc][2], [bison][3] etc, nor is it using any parser framework like [Boost.Spirit][4]. It's implementation is pure C++ code. ### Motivation I started getting involved into the [pyston project][5] where it had an entry in their getting involved list for implementing a parser for Python. Never having properly tackled the problem of creating a parser library for any language, I decided it might be worth a try, since most of the libraries I found, where basically just using the builtin Python parser or where implemented in Python itself. ### Goal The first goal of the library is to support python 2.7 syntax, later on 3.x syntax might be added. ## Example

An example file:

$cat hello_world.py
#! /usr/bin/env python
# -*- coding: utf-8 -*-
#

"""
    A "Hello World" example for the pypa parser
"""
import sys

print >> sys.stdout, "Hello", "World!"

And here the output of the test parser:

$ ./parser-test hello_world.py
Parsing successfull

[Module]
  - body:
    [Suite]
      - items: [
            [DocString]
              - doc:
    A "Hello World" example for the pypa parser


            [Import]
              - names:
                [Alias]
                  - as_name: <NULL>
                  - name:
                    [Name]
                      - context: Load
                      - dotted: False
                      - id: sys

            [Print]
              - destination:
                [Attribute]
                  - attribute:
                    [Name]
                      - context: Load
                      - dotted: False
                      - id: stdout
                  - context: Load
                  - value:
                    [Name]
                      - context: Load
                      - dotted: False
                      - id: sys
              - newline: True
              - values: [
                    [Str]
                      - value: Hello

                    [Str]
                      - value: World!
                    ]
            ]
  - kind: Module

And here the parse tree of python: (astdump.py can be found in tools)

[Module]
    - body: [

        [Expr]
            - value:
            [Str]
                - s:
    A "Hello World" example for the pypa parser


        [Import]
            - names: [

                [alias]
                    - asname: None
                    - name: sys
            ]

        [Print]
            - dest:
            [Attribute]
                - attr: stdout
                - ctx: Load
                - value:
                [Name]
                    - ctx: Load
                    - id: sys
            - nl: True
            - values: [

                [Str]
                    - s: Hello

                [Str]
                    - s: World!
            ]
    ]

## Error Reporting The parser supports also SyntaxError and IndentionError reporting:

Let's take a look at this file syntax_error.py which clearly has a syntax error:

#! /usr/bin/env python
# -*- coding: utf-8 -*-
"""
    Syntax error example
"""

print x y z

This is the output of the test parser:

$./parser-test syntax_error.py
  File "syntax_error.py", line 7
    print x y z
            ^
SyntaxError: Expected new line after statement
-> Reported @pypa/parser/parser.cc:944 in bool pypa::simple_stmt(pypa::{anonymous}::State&, pypa::AstStmt&)

Parsing failed

And this of cpython 2.7:

$ python syntax_error.py
  File "syntax_error.py", line 7
    print x y z
            ^
SyntaxError: invalid syntax

libpypa uses different error messages than python, however in the hopes that that would increase the clarity.

## Requirements To be able using **libpypa**, you have to have a *C++11* compiler available. **libpypa** was developed on top of *g++ 4.8.2* and it heavily uses *C++11* features where seen fit.

libpypa currently does not depend on any other libraries than the C++11 standard library with the exception of the class FileBuf which currently uses system libraries, but might be changed to just use fopen/fread/ fclose.

## Structure **libpypa** currently consists of 3 major parts:

1. Lexer
2. Parser
3. AST

### Lexer The `Lexer` portion of the library tokenizes the input for the `Parser` and distinguishes the different types of tokens for the `Parser`. ### Parser The `Parser` utilizes the `Lexer` to parse the input and generates a preliminary `AST` from the input. ### AST The AST contains the definition of all syntax elements in the code. The main parts of the definition are in `pypa/ast/ast.hh` which makes heavily use of preprocessor macros to define typedefs, mappings for compile time type lookups by AstType (enum class), and an implementation for a switch based visitor.

The AST types do not implement any methods, they are just structures with data. The only thing which is in there for some of the bases is the constructor, to set the type id value and initialize the line and column values.

## License Copyright 2014 Vinzenz Feenstra

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

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