Compile AST to bytecode?

[Rob De Almeida]

Hi,

I would like to compile an AST to bytecode, so I can eval it later. I
tried using parse.compileast, but it fails:

>>import compiler, parser
ast = compiler.parse("42")
parser.compileast(ast)

Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: compilest() argument 1 must be parser.st, not instance

Any hints?

TIA,
--Rob

[Duncan Booth]

"Rob De Almeida" <ra******@gmail.comwrote:

I would like to compile an AST to bytecode, so I can eval it later. I
tried using parse.compileast, but it fails:

>>>import compiler, parser
ast = compiler.parse("42")
parser.compileast(ast)

Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: compilest() argument 1 must be parser.st, not instance

Any hints?

There are two distinct varieties of AST in the Python standard library.
compiler and parser ast objects are not compatible.

I'm not sure there are any properly documented functions for converting an
AST to a code object, so your best bet may be to examine what a
pycodegen class like Expression or Module actually does.

>>import compiler
def ast_to_evalcode(ast):

return compiler.pycodegen.ExpressionCodeGenerator(ast).ge tCode()

>>def parse_to_ast(src):

return compiler.pycodegen.Expression(src, "<string>")._get_tree()

>>ast = parse_to_ast("40+2")
ast

Expression(Add((Const(40), Const(2))))

>>ast_to_evalcode(ast)

<code object <expressionat 0122D848, file "<string>", line -1>

>>eval(_)

42

[Rob De Almeida]

Duncan Booth wrote:

I would like to compile an AST to bytecode, so I can eval it later.

I'm not sure there are any properly documented functions for converting an
AST to a code object, so your best bet may be to examine what a
pycodegen class like Expression or Module actually does.

Thanks, Duncan. It worked perfectly. :-)

For arbitrary nodes I just had to wrap them inside an Expression node:

>>ast = compiler.ast.Expression(node)
ast.filename = 'dummy'
c = compiler.pycodegen.ExpressionCodeGenerator(ast)
obj = eval(c.getCode(), scope)

--Rob

[fumanchu]

Rob De Almeida wrote:

Duncan Booth wrote:

I would like to compile an AST to bytecode, so I can eval it later.

I'm not sure there are any properly documented functions for converting an
AST to a code object, so your best bet may be to examine what a
pycodegen class like Expression or Module actually does.

Thanks, Duncan. It worked perfectly. :-)

For arbitrary nodes I just had to wrap them inside an Expression node:

>ast = compiler.ast.Expression(node)
ast.filename = 'dummy'
c = compiler.pycodegen.ExpressionCodeGenerator(ast)
obj = eval(c.getCode(), scope)

If you're only worried about expressions, then you can have CPython do
the compilation for you much faster by wrapping the expression in a
lambda:

>>f = lambda: 42
f.func_code.co_code

'd\x01\x00S'

>>map(ord, _)

[100, 1, 0, 83]

>>g = lambda x, y: (x * 3) + len(y)
map(ord, g.func_code.co_code)

[124, 0, 0, 100, 1, 0, 20, 116, 1, 0, 124, 1, 0, 131, 1, 0, 23, 83]

For a complete round-trip Expression library, see
http://projects.amor.org/dejavu/browser/trunk/logic.py
Robert Brewer
System Architect
Amor Ministries
fu******@amor.org