While loop Invalid syntax

Question

I'm trying to use this code to create a while loop from a string, but it's not working. This is the string that I call with the eval() function:

    'while True:\n\vprint("What role do you want?")\n\vAnswer = Reformat(raw_input())\n\vif Answer in RoleArray:\n\v\vPlayArray[0].append(Answer)\n\vif Answer == "Mafia":\n\v\v\vMafia.append(User)\n\v\velif Answer == "Sheriff":\n\v\v\vSheriff = User\n\v\velif Answer == "Doctor":\n\v\v\vDoctor = User\n\v\vdel RoleArray[RoleArray.index(Answer)]\n\velse:\n\v\vprint("That is not a role. Please check you spelling. [Mafia/Sheriff/Doctor/Townsperson]")'

And this is the error that shows up:

    Traceback (most recent call last):
File "<pyshell#2>", line 1, in <module>
eval('while True:\n\vprint("What role do you want?")\n\vAnswer = Reformat(raw_input())\n\vif Answer in RoleArray:\n\v\vPlayArray[0].append(Answer)\n\vif Answer == "Mafia":\n\v\v\vMafia.append(User)\n\v\velif Answer == "Sheriff":\n\v\v\vSheriff = User\n\v\velif Answer == "Doctor":\n\v\v\vDoctor = User\n\v\vdel RoleArray[RoleArray.index(Answer)]\n\velse:\n\v\vprint("That is not a role. Please check you s')
File "<string>", line 1
while True:
    ^
SyntaxError: invalid syntax

There is an alternate way to execute this code properly, but I would like to know why it isn't working. Thanks.

Also, for whatever you're doing, I strongly recommend against using `eval()\exec()` — Christian Dean, Dec 06 '16 at 00:49
@Blorgbeard Your correct. He needs `exec()`. `eval()` is for Python expressions, not compound statements. — Christian Dean, Dec 06 '16 at 00:49
@Blorgbeard He needs `eval()`. He attempts to execute Python code withing Python interpreter. — DYZ, Dec 06 '16 at 00:56
As another side note, this sounds a bit like an X/Y problem. Don't ask how to fix your solution. Ask about how to solve your _actual_ problem. — Christian Dean, Dec 06 '16 at 00:56
@DYZ No, he needs `exec()`. Either way, both should be avoid. — Christian Dean, Dec 06 '16 at 00:57

score 2 · Answer 1 · edited May 23 '17 at 12:13

The most likely culprit is both your use of \v instead of \t for tabs and your use of eval. You need to fix both:

Fixing both removes the syntax error:

>>> exec("while True:\n\tprint('yup')\n\tbreak")
yup

Using the vertical tab \v causes a syntax error:

>>> exec("while True:\n\vprint('yup')\n\vbreak")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 2
    ♂print('yup')
    ^
SyntaxError: invalid syntax

And so does using eval, which is intended only for expressions:

>>> eval("while True:\n\tprint('yup')\n\tbreak")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1
    while True:
        ^
SyntaxError: invalid syntax

See this answer for an explanation of the differences between eval and exec.

It's possible you have other syntax errors as well, but I'm not going to try to debug that very long line.

Additionally, you should strongly reconsider your use of the dynamic code consuming functions like eval and exec. Even if your code's intended use doesn't present any security risk, they make things much more difficult to debug and maintain. The cases where they can't be avoided are exceedingly rare.

The use of \v is working fine, but I will keep \t in mind. Thx. — GreenHawk1220, Dec 06 '16 at 01:19
@GreenHawk1220 Hmm... what OS and python version are you using? That's odd that it would cause a syntax error for me and not you. — skrrgwasme, Dec 06 '16 at 01:20

score 0 · Answer 2 · answered Dec 06 '16 at 01:17

0

After veiwing comments, I realize that I was being really dumb as to not use exec() instead of eval(), since one gives me a variable, and the other gives me a variable. Thank you, Blorgbeard!

answered Dec 06 '16 at 01:17

GreenHawk1220

133
1
10

Are you _sure_ it's working? Whenever I run it using `exec()` I get a `SyntaxError`. – Christian Dean Dec 06 '16 at 01:21
@leaf You may be seeing a similar issue that I'm getting. Are you using `\v` or `\t` for the tabs? – skrrgwasme Dec 06 '16 at 01:23
@skrrgwasme I am using `\v` and it does not work. In fact, when I print the escape character in the idle, it prints a strange empty box. – Christian Dean Dec 06 '16 at 02:20
Yeah, it doesn't. I was misreading it. Exec() with \t solved the problem. Thx guys. – GreenHawk1220 Dec 06 '16 at 03:02

While loop Invalid syntax

2 Answers2