SyntaxError: invalid syntax, clause on '=' in eval() function

Question

I'm having the following trouble with my program:

File "C:/Users/6ixpage/Documents/SoftwareDevelopment/Python/Programs/CLI/
PrintandCopy/P&C Job System.py", line 75, in set
     eval('self.details['+a+'].'+b+'"'+c+'"');
File "<string>", line 1
     self.details[0].file = "image"
                          ^
SyntaxError: invalid syntax

Here's the code snippet for reference:

IN = ('details','0','file','image')
if IN[0] == 'details':
    a = IN[1]
    b = IN[2]
    c = IN[3]
    print('self.details['+a+'].'+b+' = "'+c+'"');
    eval('self.details['+a+'].'+b+' = "'+c+'"');

IN[0] - used to identify that the command specifies the details of the folder. *
IN[1] - the task within the folder. *
IN[2] - the attribute of the task (any of type, file, quantity, tally). *
IN[3] - the value to set the attribute to.

The entire code is here, unfortunately undocumented as I originally wanted to only have a simple CUI setup. But do quiz me if you have any questions about particular snippets. I have paper documentation too, that shows how the program is supposed to work.

If you can suggest some changes to how the CUI works please do, especially if they will cause less headaches than the one I currently have. (And I'm certain that I don't have to use eval() for every single function.)

`eval` evaluates an expression. Assignment is a statement, not an expression so you can't do it with `eval`. Use the answer from @Jérôme — Duncan, Feb 12 '17 at 16:05

score 7 · Accepted Answer · edited May 23 '17 at 12:08

From Duncan's comment:

eval evaluates an expression. Assignment is a statement, not an expression so you can't do it with eval.

You could use exec (see this answer for the explanation about exec and eval), but seriously, there is no need to obfuscate your code with these.

Here's what you could do:

inputs = ('details','0','file','image')
if inputs[0] == 'details':
    a = inputs[1]
    b = inputs[2]
    c = inputs[3]
    setattr(self.details[int(a)], b, c)

(Note there is no test. All inputs are considered valid.)

Explanation:

int(a) turns string '0' into integer 0.
setattr sets attribute 'file' of self.details[0] to 'image'

You should consider eval and exec last resort things. Most likely you don't need those anywhere in your code.

For instance

eval('print(self.'+str(arg)+')')

should be written

print(getattr(self, arg))

Thank you very much, this change to the program works perfectly and makes it look a whole lot neater ^O^ — Connor Turlan, Feb 14 '17 at 06:05

score 1 · Answer 2 · answered Feb 12 '17 at 16:15

1

Assignments are not allowed in the eval function; try using exec instead.

answered Feb 12 '17 at 16:15

FluxIX

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exec might work here, but please don't use it when you can write simple code. – Jérôme Feb 12 '17 at 16:19
1

In general, `eval` and `exec` are not to be used; their usefulness is cases where dynamic execution of code from an external source is required. – FluxIX Feb 12 '17 at 16:55
1

As such though, there are all of the risks associated with the arbitrary execution of unvetted code. Also, debugging such code execution (especially errors in the external code) can be a challenge as normal debugging methods may not be available. (I apologize for the late edit -- accidental submission of original comment.) – FluxIX Feb 12 '17 at 17:05

SyntaxError: invalid syntax, clause on '=' in eval() function

2 Answers2