Using eval on string throws error.

Question

I have a program, which generates a python program as a string which I then need to execute. However, when I try running the string it throws a syntax error.

For example:

program = "self.move() self.turnRight() if x > 0: self.turnLeft() else: self.turnRight()"
eval(program)

when this runs a syntax error is thrown at whatever the second command is. I'm assuming this is because the string lacks tabs or newlines. Is there a way to automatically add these when the string goes through the eval command?

• Hugh Bothwell: Out of curiosity: how are you generating the program?
• Chris Headleand: a variant of genetic programming

Answer 1

eval can handle only a single Python expression, and no statement (simple or compound).

Your string contains multiple expressions and statements. You'd have to use exec instead:

program = '''\
self.move()
self.turnRight()
if x > 0:
 self.turnLeft()
else:
 self.turnRight()
'''
exec program

If you were to use a conditional expression you can make it 3 separate expressions:

program = ['self.move()', 'self.turnRight()',
 'self.turnLeft() if x > 0 else self.turnRight']
for line in program:
 eval(program)

Note that it is always a better idea to implement a more specific language rather than re-use Python and eval or exec; you'll create more problems than you'll solve, especially when it comes to security.

• wnnmaw: So throwing a ; in between each expression doesn't jive?
• Martijn Pieters: @wnnmaw: no, because that's still multiple expressions. That's quite apart from including a statement like if.
• Chris Headleand: Thanks for the answer, is there anyway to add the newlines and tab's in awell as exec still throws a syntax error./
• Martijn Pieters: @ChrisHeadleand: Not automatically, no. Code meaning changes entirely based on where indentation and newlines are used.