Convert string to Python class object

Question

Given a string as user input to a Python function  I d like to get a class object out of it if there s a class with that name in the currently defined namespace  Essentially  I want the implementation for a function which will produce this kind of result   class Foo      pass  str to class  Foo      gt   lt class   main   Foo at 0x69ba0 gt    Is this  at all  possible

User · Accepted Answer

Warning  eval   can be used to execute arbitrary Python code  You should never use eval   with untrusted strings   See Security of Python  39 s eval   on untrusted strings     This seems simplest    gt  gt  gt  class Foo object           pass       gt  gt  gt  eval  Foo    lt class    main   Foo  gt

User · Answer

import sys import types  def str to class field       try          identifier   getattr sys modules   name     field      except AttributeError          raise NameError   s doesn t exist     field      if isinstance identifier   types ClassType  types TypeType            return identifier     raise TypeError   s is not a class     field    This accurately handles both old-style and new-style classes

User · Answer

Yes  you can do this  Assuming your classes exist in the global namespace  something like this will do it   import types  class Foo      pass  def str to class s       if s in globals   and isinstance globals   s   types ClassType               return globals   s      return None  str to class  Foo       gt   lt class   main   Foo at 0x340808cc gt

User · Answer

I ve looked at how django handles this  django utils module loading has this  def import string dotted path               Import a dotted module path and return the attribute class designated by the     last name in the path  Raise ImportError if the import failed              try          module path  class name   dotted path rsplit      1      except ValueError          msg     s doesn t look like a module path    dotted path         six reraise ImportError  ImportError msg   sys exc info   2        module   import module module path       try          return getattr module  class name      except AttributeError          msg    Module   s  does not define a   s  attribute class                  module path  class name          six reraise ImportError  ImportError msg   sys exc info   2     You can use it like import string  module path to all the way to your class

User · Answer

This could work   import sys  def str to class classname       return getattr sys modules   name     classname

User · Answer

You could do something like   globals   class name

User · Answer

You want the class Baz  which lives in module foo bar  With Python 2 7  you want to use importlib import module    as this will make transitioning to Python 3 easier    import importlib  def class for name module name  class name         load the module  will raise ImportError if module cannot be loaded     m   importlib import module module name        get the class  will raise AttributeError if class cannot be found     c   getattr m  class name      return c   With Python  lt  2 7   def class for name module name  class name         load the module  will raise ImportError if module cannot be loaded     m     import   module name  globals    locals    class name        get the class  will raise AttributeError if class cannot be found     c   getattr m  class name      return c   Use   loaded class   class for name  foo bar    Baz

User · Answer

If you really want to retrieve classes you make with a string  you should store  or properly worded  reference  them in a dictionary  After all  that ll also allow to name your classes in a higher level and avoid exposing unwanted classes  Example  from a game where actor classes are defined in Python and you want to avoid other general classes to be reached by user input  Another approach  like in the example below  would to make an entire new class  that holds the dict above  This would   Allow multiple class holders to be made for easier organization  like  one for actor classes and another for types of sound   Make modifications to both the holder and the classes being held easier  And you can use class methods to add classes to the dict   Although the abstraction below isn t really necessary  it is merely for     quot illustration quot     Example  class ClassHolder      def   init   self           self classes           def add class self  c           self classes c   name      c      def   getitem   self  n           return self classes n   class Foo      def   init   self           self a   0      def bar self           return self a   1  class Spam Foo       def   init   self           self a   2      def bar self           return self a   4  class SomethingDifferent      def   init   self           self a    quot Hello quot       def add world self           self a     quot  World quot       def add word self  w           self a     quot   quot    w      def finish self           self a     quot   quot          return self a  aclasses   ClassHolder   dclasses   ClassHolder   aclasses add class Foo  aclasses add class Spam  dclasses add class SomethingDifferent   print aclasses print dclasses  print  quot         quot  print  quot o quot  print aclasses  quot Foo quot   print aclasses  quot Spam quot   print  quot o quot  print dclasses  quot SomethingDifferent quot    print  quot         quot  g   dclasses  quot SomethingDifferent quot     g add world   print g finish    print  quot         quot  s      s append aclasses  quot Foo quot      s append aclasses  quot Spam quot       for a in s      print a a     print a bar       print  quot -- quot   print  quot Done experiment  quot   This returns me   lt   main   ClassHolder object at 0x02D9EEF0 gt   lt   main   ClassHolder object at 0x02D9EF30 gt          o  lt class    main   Foo  gt   lt class    main   Spam  gt  o  lt class    main   SomethingDifferent  gt          Hello World          0 1 -- 2 6 -- Done experiment   Another fun experiment to do with those is to add a method that pickles the ClassHolder so you never lose all the classes you did     UPDATE  It is also possible to use a decorator as a shorthand  class ClassHolder      def   init   self           self classes           def add class self  c           self classes c   name      c        -- the decorator     def held self  c           self add class c             Decorators have to return the function class passed  or a modified variant thereof   however I d rather do this separately than retroactively change add class  so             quot held quot  is more succint  anyway          return c       def   getitem   self  n           return self classes n   food types   ClassHolder     food types held class bacon      taste    quot salty quot    food types held class chocolate      taste    quot sweet quot    food types held class tee      taste    quot bitter quot    coffee  ftw      food types held class lemon      taste    quot sour quot   print food types  bacon   taste    No manual add class needed   D

User · Answer

In terms of arbitrary code execution  or undesired user passed names  you could have a list of acceptable function class names  and if the input matches one in the list  it is eval d   PS  I know    kinda late    but it s for anyone else who stumbles across this in the future

User · Answer

I don t see why this wouldn t work  It s not as concrete as everyone else s answers but if you know the name of your class then it s enough  def str to class name       if name     quot Foo quot           return Foo     elif name     quot Bar quot           return Bar

User · Answer

Using importlib worked the best for me   import importlib  importlib import module  accounting views      This uses string dot notation for the python module that you want to import

[python] Convert string to Python class object?

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