Python won't eval() code

Question

I have the following eval() thing:

c = Customer()
eval("c.name = row.value('customer', '{c}')".format(c=column_name), { 'c': c, 'row': row})

When I try to run that, I get this:

Traceback (most recent call last):
  File "./import.py", line 19, in <module>
    c = Customer.save_from_row(row)
  File "/home/jason/projects/mcifdjango/mcif/models/customer.py", line 43, in save_from_row
    eval("c.name = row.value('customer', '{c}')".format(c=column_name), { 'c': c, 'row': row})
  File "<string>", line 1
    c.name = row.value('customer', 'name')
           ^
SyntaxError: invalid syntax

What am I doing wrong?

Edit: Because it looks like I didn't explain the context of my problem well enough, here's what I ended up doing, if anyone's curious:

@classmethod
def save_from_row(cls, row):
    c = cls()
    map(lambda column_name: setattr(c, column_name, row.value('customer', column_name)), c.distinguishing_column_names())
    return c.upsert()

Before I found out about setattr() I was separately setting several different attributes on c.

So... what's wrong with `c.name = row.value('customer', column_name)`? Why get `eval()` involved? — Mike DeSimone, Feb 01 '11 at 19:39
Because I have a whole list of different column names and properties of `Customer` that I need to assign. — Jason Swett, Feb 01 '11 at 19:42
Because I didn't know about them. I tried `setattr` and it did exactly what I wanted. If you post `setattr` as an answer I'll accept it. — Jason Swett, Feb 01 '11 at 19:53

score 11 · Accepted Answer · edited May 23 '17 at 09:58

11

eval evaluates expressions. Assignment is a statement, not an expression.

And don't even get me started on how easily misused and - in 99.99% of all cases - utterly unnecesary eval is. Just refer to the numerous other eval questions, I bet each has at least one such rant in an answer or comment - so I'll save my breath and link to one I like. (That being said, exec works like eval for statements.)

edited May 23 '17 at 09:58

Community

1
1

answered Feb 01 '11 at 19:36

Any recommendation for what I might do instead of eval? – Jason Swett Feb 01 '11 at 19:41
1

@Jason: Depends on what you need. In general, if you are emulating [PHP's variable variables](http://www.php.net/manual/en/language.variables.variable.php) - i.e. associate values with strings - a dictionary will do work as well while preserving sanity (and be orders of magnitude faster while we're at it). – Feb 01 '11 at 19:47

score 2 · Answer 2 · answered Feb 01 '11 at 19:50

2

Wouldn't this do what you need?:

c = Customer()
name_cols = (('name', 'custname'), ('addr', 'cust_addr'))
for name, col in name_cols:
    setattr(c, name, row.value('customer', col))

answered Feb 01 '11 at 19:50

Ned Batchelder

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Python won't eval() code

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