Python `eval`: "invalid syntax" error, when trying to create class attributes programmatically?

Question

I have a lot of class attributes that I want to create, so I decided to use a function to do so:

def make_index_variables(self):
    for index, label in enumerate(self.variable_labels):
        eval('self.' + label + '_index = ' + str(index))

If earlier, I defined:

self.variable_labels = ['x', 'y']

I get an error message like this:

eval('self.' + label + '_index = ' + str(index))

    self.x_index = 0
                     ^
    SyntaxError: invalid syntax

I am beginning to realize that using setattr is probably better than using eval (but I am not sure). In any case, why does eval raise this error?

Use exec for "commands" (like x = 5), and eval for expressions (like 2**5). And yes, using setattr is definitely better. — Navith
– Navith, Commented May 7, 2015 at 22:59

rafaelc · Accepted Answer · 2015-05-07 23:06:53Z

5

You want to do exec instead of eval

exec('self.' + label + '_index = ' + str(index))

eval will evaluate a expression, not run it like you want.
Think of eval like the argument of a if statement.

Also, if you want to set attributes of a class, you should definitely use setattr instead.
Actually, 99% of time there are better options for what you want rather than using exec.

answered May 7, 2015 at 23:06

rafaelc

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too honest for this site · Accepted Answer · 2015-05-07 23:48:36Z

1

Try this one:

setattr(self, name + "_index", index)

Eval evaluates an expression. Different from C, in Python an assignment is a statement, not an expression (you cannot write c = (a = b) == None, for example. The variant a = b = 3 is somewhat special syntax. It does actually not pass the value assigned to b , but the value on the right side (yes, this is a subtle, but important difference).

If it just for an index, there may be better versions which do not pullute the namespace, however.

edited May 7, 2015 at 23:48

answered May 7, 2015 at 23:43

too honest for this site

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