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Re: Getting a set of lambda functions

On Sun, 25 May 2008 18:46:27 -0700
John Nagle <na***@animats. comwrote:
>>func_string s=['x', 'x+1', 'x+2', 'x']
funclist = [eval('lambda x:' + func) for func in func_strings]

What are you actually trying to do, anyway? What's the
application?

You probably don't want to use "eval" that way. If you want
function objects out, use "compile".
I am writing on an application that I call pyspread
(http://pyspread.sourceforge.net).
It provides a grid (mapped to a numpy.array) into which users can type
in strings that contain python expressions.
Each expression is transformed into a function object in a second
numpy.array of the same size, so that each function can be called by
accessing the respective cell of the grid.

eval seems to work quite fine and provides nice access to globals,
system functions, modules, etc. Therefore, one can do general
programming tasks within the grid without worrying about cell
precedence.

compile returns code objects instead of function objects. I can of
course evaluate these code objects. However, when I store the code
objects, it is an extra operation each time I want to call the
function. So what is the benefit?

More specific: Is there any benefit of calling:

eval(compile(my string, '', 'eval'))

instead of

eval(mystring)

?

What are the drawbacks of my approach that I may be missing?
Any suggestions?

Best Regards

Martin
Jun 27 '08 #1
1 1643
On May 27, 1:11*am, Martin Manns <mma...@gmx.net wrote:
On Sun, 25 May 2008 18:46:27 -0700

John Nagle <na...@animats. comwrote:
>>>func_strings =['x', 'x+1', 'x+2', 'x']
>>>funclist = [eval('lambda x:' + func) for func in func_strings]
* *What are you actually trying to do, anyway? *What's the
application?
* *You probably don't want to use "eval" that way. *If you want
function objects out, use "compile".

I am writing on an application that I call pyspread
(http://pyspread.sourceforge.net).
It provides a grid (mapped to a numpy.array) into which users can type
in strings that contain python expressions.
Each expression is transformed into a function object in a second
numpy.array of the same size, so that each function can be called by
accessing the respective cell of the grid.

eval seems to work quite fine and provides nice access to globals,
system functions, modules, etc. Therefore, one can do general
programming tasks within the grid without worrying about cell
precedence.

compile returns code objects instead of function objects. I can of
course evaluate these code objects. However, when I store the code
objects, it is an extra operation each time I want to call the
function. So what is the benefit?

More specific: Is there any benefit of calling:

eval(compile(my string, '', 'eval'))

instead of

eval(mystring)

?

What are the drawbacks of my approach that I may be missing?
Any suggestions?

Best Regards

Martin
Yes, there is.
Your original example can be rewritten
>>func_string s=['x', 'x+1', 'x+2', 'x']
funclist = [compile('lambda x: %s' % func, '<string>', 'eval') for func in func_strings]
len(funclis t)
4
>>len(set(funcl ist))
3
>>eval(funcli st[0])(1)
1

Ivan
Jun 27 '08 #2

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