Changing an AST

[beza1e1]

Is it possible compiler.parse a statement, then change and then
execute/resolve it?

Background:
I'm probably to lazy to write my own parser.

I have such a statement as string: "distance = x**2 + y**2"
x and y are undefined, so it is no executable Python code, but it is
parseable. Now i'd like traverse through the AST and change Name('x')
for the value i have elsewhere. And finally let Python resolve the
computation.

More Background:
I want to write a simulation game with many interdepended values. I
don't want to create a class with dozens of functions, but let Python
create/evaluate them.

I hope this can be understood

 

[Fredrik Lundh]

I have such a statement as string: "distance = x**2 + y**2"
x and y are undefined, so it is no executable Python code, but it is
parseable. Now i'd like traverse through the AST and change Name('x')
for the value i have elsewhere. And finally let Python resolve the
computation.

you can use the parser class for this purpose:

http://www.effbot.org/librarybook/parser.htm

instead of manipulating the parse tree, you can use the AST to identify
the variables, create a dictionary with the current values, and use exec
or eval to evaluate it.

or you can use a regular expression to dig out all variable names.

or you can compile the expression and analyze the code object to find
the variable names, and use the dictionary/exec approach:

>>> expr = compile("distance = x**2 + y**2", "", "exec")
>>> expr.co_varnames ('distance',)
>>> list(set(expr.co_names) - set(expr.co_varnames)) ['y', 'x']
>>> context = {"x": 10, "y": 20}
>>> exec expr in context
>>> context["distance"]

500

(the regular expression approach is probably the only one that's
guaranteed to work on all python implementations)

</F>

 

[Leif K-Brooks]

Is it possible compiler.parse a statement, then change and then
execute/resolve it?

This should work:

42

Also, if you need to turn an AST back into Python code, I have some ugly
code for doing that: <http://ecritters.biz/asttosource.py>.

 

[beza1e1]

Thank you! this "compile/exec in context" is the thing i wanted.

It is not that performant i think. But it should ease the prototyping.

 

[Fredrik Lundh]

Thank you! this "compile/exec in context" is the thing i wanted.

It is not that performant i think.

it's about as fast as it can get, as long as you only call compile when
the expressions change. (the example didn't show it, but the "expr"
code object can of course be reused with different contexts)

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