«

Hi all,

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

>>> class X : pass
....
>>> X.attr = 5

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.
>>> class X : pass
....
>>> attrname = "attr"
>>> eval("X.%s = val" % attrname , {"X":X, "val":5})
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 -

--
"I know what I believe. I will continue to articulate what I believe
and what I believe - I believe what I believe is right."
-- George W. Bushman

harold fellermann wrote:
> 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.
> >>> class X : pass
> ...
> >>> attrname = "attr"
> >>> eval("X.%s = val" % attrname , {"X":X, "val":5})
> 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

harold fellermann wrote:

> 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.
> >>> class X : pass
> ...
> >>> attrname = "attr"
> >>> eval("X.%s = val" % attrname , {"X":X, "val":5})
> 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'?

>>> setattr(X, 'attr', 5)

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

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

> What you are doing wrong is attempting to use eval before exhausting
> all
> the simpler techniques. Why not just call 'setattr'?
>
>>>> setattr(X, 'attr', 5)
>
> BTW, the syntax error is because eval evaluates an expression and
> an assignment statement is a statement not an expression.

--
Abandon the search for Truth -- settle for a good fantasy.
--