python: creating instance from variable name

Question

***EDIT2: Sorry about the broken code. Here is working code which illustrates the same issue:

class bead():
    def printmsg(a):
        print('test message')

chain1=bead()
x='chain1'
eval(x + '.printmsg()')

***EDIT: Thank you gnibbler for answering the original question. Here's a better worded version of my question:

class bead():
    def msg():
        print('test message')

x='chain1'
y='bead1'

eval(x + '.' + y + '=bead()')

chain1.bead1.msg()

output: 'test message'

What's a better way to do that?

Original question:

script:

class testClass():
    test1='test1 text'

x='testClass'
y='test1'

eval(x + '.' + y)

output: 'test1 text'

Is there a better way of doing this?

***EDIT: getattr() works for pulling the information from the class. Thank you gnibbler. Let me change the question a little bit though:

what could I use instead of:

x='chain1' y='mol1'

I don't understand - your title says "creating instance from variable name", but it looks like you are trying to access testClass.test1. This is not an instance of the testClass class, and you aren't creating it. — Karl Knechtel
– Karl Knechtel, Commented Jun 16, 2011 at 5:06
I don't understand the edited question. eval(x + '.' + y + '=bead()') doesn't work. You can't do an assignment in there. Are you asking for a way to dynamically create variables or are you asking something about the class? — John La Rooy
– John La Rooy, Commented Jun 16, 2011 at 5:42
yes. I'm trying to dynamically create an instance of a class from a variable of a string. — jonH
– jonH, Commented Jun 16, 2011 at 5:46
@jHoro, you'd better cut-and-paste rather than type your questions in. your examples never work as typed. — jcomeau_ictx
– jcomeau_ictx, Commented Jun 16, 2011 at 5:52

John La Rooy · Accepted Answer · 2011-06-16 04:44:35Z

12

Instead of the eval(), you could say

getattr(locals()[x], y)

or

getattr(locals().get(x), y)

Is that what you mean?

answered Jun 16, 2011 at 4:44

John La Rooy

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jonH Over a year ago

thank you for the answer. However I didn't phrase the question well. Please notice the edit.

jcomeau_ictx · Accepted Answer · 2011-06-16 07:45:14Z

1

I think I understand. What you probably want is:

>>> class testClass():
...     test1='test1 text'
... 
>>> getattr(testClass, 'test1')
'test1 text'

Still that isn't creating an instance. Riffing on gnibbler's answer, here you make an instance:

>>> locals().get(x)()
<__main__.testClass instance at 0xb73b056c>
>>> getattr(locals().get(x)(), y)
'test1 text'

For your latest edit:

>>> class bead():
...     def printmsg(a):
...         print('test message')
... 
>>> chain1=bead()
>>> x='chain1'
>>> eval(x + '.printmsg()')
test message
>>> getattr(locals()[x], 'printmsg')()
test message

edited Jun 16, 2011 at 7:45

answered Jun 16, 2011 at 4:43

jcomeau_ictx

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1 Comment

wong2 Over a year ago

But that is : "testClass" not testClass

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