Friday, June 14, 2013

More Reasons to Love Python - A Lesson on KISS

Recently I've been doing some work in the area of programming language design. At one point I wanted to define a Python subset which allows only the simplest Python statements without loops, conditionals, functions, classes and a bunch of other high-level constructs. So I looked into the grammar specification of the Python language and I was astonished by its simplicity and succinctness. Click here to take a look for yourself. It's no longer than 125 lines of text, and the whole thing can be printed on one side of an A4 sheet. This is definitely one of those instances where the best design is also the simplest design. No wonder everybody loves Python.
However that's not the whole point. Having selected a suitable Python subset, I was looking into ways for implementing a simple parser for those grammar rules. I've done some work with JavaCC in the past, so I straightaway jumped into implementing a Java-based parser for the selected Python subset using JavaCC. After a few hours of coding I managed to get it working too. The next step of my project required me to do some analysis on the abstract syntax tree (AST) produced by the parser. I was looking around for some existing work that fits my requirements, and I came across Python's native ast module. I immediately realized that all those hours I spent on implementing the JavaCC-based parser is a complete waste. The ast module provides excellent support for parsing Python code and constructing ASTs. This is all you have to do parse some Python code using the ast module and obtain an AST representation of the code.
import ast

# The variable 'source' contains the Python statement to be parsed
source = 'x = y + z'
tree = ast.parse(source)
The ast module supports several modes. The default mode is exec which supports parsing a sequence of Python statements. The module also supports a special eval mode which can be used to parse simple one-liner Python statements. It turned out the eval mode supports more or less the same exact Python subset I wanted to use. So I threw away my JavaCC-based parser and wrote the following snippet of Python code to get my job done.
import ast

# The variable 'source' contains the Python statement to be parsed
source = 'x = y + z'
tree = ast.parse(source, mode='eval')
Now when it came to analyzing the AST produced by the parser, the ast module again turned out to be useful. The module provides two helper classes, namely NodeVisitor and NodeTransformer which can be used to either traverse or transform a given Python AST. To use these helper classes, we just need to extend them and implement the appropriate visit methods. There's a unique top level visit method and one visit_ method per AST node type (e.g. visit_Str, visit_Num, visit_BoolOp etc.). Here's an example NodeVisitor implementation, that flattens a given Python AST into a list.
class NodeEnumerator(ast.NodeVisitor):
  def get_node_list(self, tree):
    self.nodes = []
    self.visit(tree)
    return self.nodes

  def visit(self, node):
    self.generic_visit(node)
    self.nodes.append(node)
These helper classes can be used to do virtually anything with a given AST. If you want you can even implement a Python interpreter in Python using this approach. In my case I'm running some search and isomorphism detection algorithms on the Python AST's.
So once again I've been pleasantly surprised and deeply impressed by the simplicity and richness of Python. It looks like the designers of Python have thought of everything. Kudos to Python aside, this whole experience taught me to always looks for existing, simple solutions before doing it in my own complicated way. It actually reminds me of the good old KISS principle - "Keep It Simple, Stupid". 

1 comment:

Haridas Gowra said...

Nice to share this.............