syntax error in eval()

[harold fellermann]

Hi all,

I am trying to dynamically add class attributes at runtime using the
function eval(),
i.e. I want to do something like

but without knowing either the attributes name nor its value.
However, I encounter a syntax error I cannot understand:

Python 2.4 (#1, Dec 30 2004, 08:00:10)
[GCC 3.3 20030304 (Apple Computer, Inc. build 1495)] on darwin
Type "help", "copyright", "credits" or "license" for more information.Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<string>", line 1
X.attr = val
^
SyntaxError: invalid syntax


Does anyone have a clue what might be wrong? Thanks in advance.

- harold -

 

[Steven Bethard]

Python 2.4 (#1, Dec 30 2004, 08:00:10)
[GCC 3.3 20030304 (Apple Computer, Inc. build 1495)] on darwin
Type "help", "copyright", "credits" or "license" for more information.Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<string>", line 1
X.attr = val
^
SyntaxError: invalid syntax

You may want to use exec instead of eval:

py> class X(object):
.... pass
....
py> attrname = "attr"
py> exec "X.%s = val" % attrname in dict(X=X, val=5)
py> X.attr
5

But personally, I'd use setattr, since that's what it's for:

py> class X(object):
.... pass
....
py> attrname = "attr"
py> setattr(X, attrname, 5)
py> X.attr
5

Steve

 

[Duncan Booth]

Python 2.4 (#1, Dec 30 2004, 08:00:10)
[GCC 3.3 20030304 (Apple Computer, Inc. build 1495)] on darwin
Type "help", "copyright", "credits" or "license" for more information.Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<string>", line 1
X.attr = val
^
SyntaxError: invalid syntax


Does anyone have a clue what might be wrong? Thanks in advance.

What you are doing wrong is attempting to use eval before exhausting all
the simpler techniques. Why not just call 'setattr'?

BTW, the syntax error is because eval evaluates an expression and
an assignment statement is a statement not an expression.

 

[harold fellermann]

Thank you, Duncan and Steven.
I completely forgot about setattr.
Of course that's the way ... as its name might suggest *g*