Evaluate condition of if in python

Question

I have the AST of a python program and want to manually evaluate the condition of an if statement.

cond = node.test
b = eval(compile(cond,"<string>","eval"))
print b

Where node is the If-Node, gives me TypeError: expected Expression node, got Compare, even if Compare is an expression according to the grammar in the python doc of ast.

Any ideas?

Answer 1

You have a ast.expr subclass, not a ast.Expression top-level node.

compile() can only take a mod object, so one of Module, Interactive or Expression, depending on the third argument to compile(). For 'eval', use ast.Expression().

You can create one containing the ast.Compare node:

expr = ast.Expression(cond)

because the abstract grammar defines it as:

Expression(expr body)

and this you can compile:

compile(expr, '<file>', 'eval')

Demo:

>>> import ast
>>> code = "if foo == 'bar': pass"
>>> tree = ast.parse(code, '<file>', 'exec')
>>> cond = tree.body[0].test
>>> expr = ast.Expression(cond)
>>> compile(expr, '<file>', 'eval')
<code object <module> at 0x1067f6230, file "<file>", line 1>
>>> foo = 'baz'
>>> eval(compile(expr, '<file>', 'eval'))
False
>>> foo = 'bar'
>>> eval(compile(expr, '<file>', 'eval'))
True