dynamic variable declaration

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April 17th, 2004, 09:37 AM

escaria

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dynamic variable declaration

hi ,

how can i declare variables dynamically?
for example , in php i can do

$var_num = 1;
${'var'.$var_num} = 5;

so i get

$var1 = 5

how can i do that in python?

thanks

April 17th, 2004, 10:52 AM

percivall

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If I understood what you wanted, this can be done with:

Code:

setattr(object, name, value)

    Set a named attribute on an object; setattr(x, 'y', v) is equivalent to
    ``x.y = v''.

April 17th, 2004, 11:25 AM

escaria

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yes, that's what i need , thanks man

April 17th, 2004, 12:57 PM

DevCoach

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The solution that percivall gave will set an attribute on an object, as he said. To create a local or global variable (as in your PHP example) you can use the builtin functions locals() and globals(), which will return the dictionaries used for those namespaces. If you are in the top level namespace (i.e. outside of a function or class) then the two are equivalent.

The Python code corresponding to your example is:

Code:

>>> var_num = 1
>>> globals()['var%s' % var_num] = 5
>>> var1
5

However the docs warn that modifying the locals() dictionary may not always work and should be avoided. It worked OK when I tested it, but may fail in some circumstances. An alternative that will always work is to use exec:

Code:

>>> exec 'var%s = 23' % var_num
>>> var1
23

Dave - The Developers' Coach

April 17th, 2004, 01:12 PM

DevCoach

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I said:

Quote:

Originally Posted by DevCoach

However the docs warn that modifying the locals() dictionary may not always work and should be avoided. It worked OK when I tested it, but may fail in some circumstances.

After a little more experimenting and research, I have found that modifying locals() works ok at the top level namespace since it then locals() and globals() are the same. Inside a function locals() returns a copy of the namespace, so any changes to it do not affect the local variables:

Code:

>>> var1 = 0
>>> def fn():
... 	var_num = 1
... 	locals()['var%s' % var_num] = 6
... 	print var1
... 
>>> fn()
0

so use either globals() or exec, depending on your requirements.
Dave - The Developers' Coach

April 17th, 2004, 04:58 PM

Strike

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A better solution is, of course, to not do this at all but instead to use a list of variables.

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April 23rd, 2004, 01:38 AM

netytan

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I agree with Strike here although the topic itself is very intersting - i saw a similar question in the perl forum ages ago and the recomendation there was also to use an array (list) - the best way to go IMO.

Mark.

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dynamic variable declaration

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