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?

Out of curiosity: how are you generating the program?

Hugh Bothwell
– Hugh Bothwell

2014-04-07 15:24:24 +00:00
Commented Apr 7, 2014 at 15:24 — Hugh Bothwell
– Hugh Bothwell, Commented Apr 7, 2014 at 15:24
a variant of genetic programming

Chris Headleand
– Chris Headleand

2014-04-07 15:28:34 +00:00
Commented Apr 7, 2014 at 15:28 — Chris Headleand
– Chris Headleand, Commented Apr 7, 2014 at 15:28

Martijn Pieters · Accepted Answer · 2014-04-07 15:33:37Z

5

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.

edited Apr 7, 2014 at 15:33

answered Apr 7, 2014 at 15:22

Martijn Pieters

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4 Comments

wnnmaw Over a year ago

So throwing a ; in between each expression doesn't jive?

Martijn Pieters Over a year ago

@wnnmaw: no, because that's still multiple expressions. That's quite apart from including a statement like if.

Chris Headleand Over a year ago

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 Over a year ago

@ChrisHeadleand: Not automatically, no. Code meaning changes entirely based on where indentation and newlines are used.

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