In python, I have to instantiate certain class, knowing its name in a string, but this class 'lives' in a dynamically imported module. An example follows:
loader-class script:
import sys
class loader:
def __init__(self, module_name, class_name): # both args are strings
try:
__import__(module_name)
modul = sys.modules[module_name]
instance = modul.class_name() # obviously this doesn't works, here is my main problem!
except ImportError:
# manage import error
some-dynamically-loaded-module script:
class myName:
# etc...
I use this arrangement to make any dynamically-loaded-module to be used by the loader-class following certain predefined behaviours in the dyn-loaded-modules...
This question is related to
python
Use getattr
to get an attribute from a name in a string. In other words, get the instance as
instance = getattr(modul, class_name)()
Import the root module with importlib.import_module
and load the class by its name using getattr
function:
# Standard import
import importlib
# Load "module.submodule.MyClass"
MyClass = getattr(importlib.import_module("module.submodule"), "MyClass")
# Instantiate the class (pass arguments to the constructor, if needed)
instance = MyClass()
You probably don't want to use __import__
to dynamically import a module by name, as it does not allow you to import submodules:
>>> mod = __import__("os.path")
>>> mod.join
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'join'
Here is what the python doc says about __import__
:
Note: This is an advanced function that is not needed in everyday Python programming, unlike importlib.import_module().
Instead, use the standard importlib
module to dynamically import a module by name. With getattr
you can then instantiate a class by its name:
import importlib
my_module = importlib.import_module("module.submodule")
MyClass = getattr(my_module, "MyClass")
instance = MyClass()
You could also write:
import importlib
module_name, class_name = "module.submodule.MyClass".rsplit(".", 1)
MyClass = getattr(importlib.import_module(module_name), class_name)
instance = MyClass()
This code is valid in python = 2.7 (including python 3).
Copy-paste snippet:
import importlib
def str_to_class(module_name, class_name):
"""Return a class instance from a string reference"""
try:
module_ = importlib.import_module(module_name)
try:
class_ = getattr(module_, class_name)()
except AttributeError:
logging.error('Class does not exist')
except ImportError:
logging.error('Module does not exist')
return class_ or None
I couldn't quite get there in my use case from the examples above, but Ahmad got me the closest (thank you). For those reading this in the future, here is the code that worked for me.
def get_class(fully_qualified_path, module_name, class_name, *instantiation):
"""
Returns an instantiated class for the given string descriptors
:param fully_qualified_path: The path to the module eg("Utilities.Printer")
:param module_name: The module name eg("Printer")
:param class_name: The class name eg("ScreenPrinter")
:param instantiation: Any fields required to instantiate the class
:return: An instance of the class
"""
p = __import__(fully_qualified_path)
m = getattr(p, module_name)
c = getattr(m, class_name)
instance = c(*instantiation)
return instance
If you want this sentence from foo.bar import foo2
to be loaded dynamically, you should do this
foo = __import__("foo")
bar = getattr(foo,"bar")
foo2 = getattr(bar,"foo2")
instance = foo2()
One can simply use the pydoc.locate
function.
from pydoc import locate
my_class = locate("module.submodule.myclass")
instance = my_class()
Source: Stackoverflow.com