CODEWITHSUNDEEP
python
Orphee
Orphee

Reputation: 53

Python: Reference to a class from a string?

How to use a string containing a class name to reference a class itself?
See this (not working) exemple...

class WrapperClass:
    def display_var(self):
        #FIXME: self.__class_name__.__name__ is a string
        print self.__class__.__name__.the_var

class SomeSubClass(WrapperClass):
    var = "abc"

class AnotherSubClass(WrapperClass):
    var = "def"

And an obvious error message:

>>> b = SomeSubClass()
>>> b.display_var()
Traceback (most recent call last):
  File "", line 1, in 
  File "", line 4, in display_var
AttributeError: 'str' object has no attribute 'the_var'
>>>

Thanks!

Upvotes: 4

Views: 11826

Answers (4)

Jim Robinson
Jim Robinson

Reputation: 309

There exists a case where one has the name of a class, but not a reference to it. A tkinter Entry widget has a validate method which returns to the callback function (%W parameter) the name of the widget, not a reference to it. If you have a window with an array of entry fields, It is inconvenient to use a different callback function for each entry. Converting the string name to the reference in the callback function is a more efficient way to associate the callback to the source of the validate event. I would have commented on Devin's answer, but don't have the reputation points to make comments yet.

Upvotes: 0

bobince
bobince

Reputation: 536725

How to use a string containing a class name to reference a class itself?

Classes aren't special, they're just values contained in variables. If you've said:

class X(object): pass

in global scope, then the variable ‘X’ will be a reference to the class object.

You can get the current script/module's global variables as a dictionary using ‘globals()’, so:

classobj= globals()[self.__class__.__name__]
print classobj.var

(locals() is also available for local variables; between them you shouldn't ever need to use the awful eval() to access variables.)

However as David notes, self.__class__ is already the classobj, so there's no need to go running about fetching it from the global variables by name; self.__class__.var is fine. Although really:

print self.var

would be the usual simple way to do it. Class members are available as members of their instances, as long as the instance doesn't overwrite the name with something else.

Upvotes: 9

Devin Jeanpierre
Devin Jeanpierre

Reputation: 95616

Depending on where you get this string, any general method may be insecure (one such method is to simply use eval(string). The best method is to define a dict mapping names to classes:

class WrapperClass:
    def display_var(self):
        #FIXME: self.__class_name__.__name__ is a string
        print d[self.__class__.__name__].the_var

class SomeSubClass(WrapperClass):
    the_var = "abc"

class AnotherSubClass(WrapperClass):
    the_var = "def"

d = {'WrapperClass': WrapperClass, 'SomeSubClass': SomeSubClass, 'AnotherSubClass': AnotherSubClass}
AnotherSubClass().display_var()
# prints 'def'

Upvotes: 3

David Berger
David Berger

Reputation: 12823

Your example would work if you called print self.__class__.var. I don't think there's any need to use the name.

Upvotes: 2

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