Given a string of a Python class, e.g. my_package.my_module.MyClass
, what is the best possible way to load it?
In other words I am looking for a equivalent Class.forName()
in Java, function in Python. It needs to work on Google App Engine.
Preferably this would be a function that accepts the FQN of the class as a string, and returns a reference to the class:
my_class = load_class('my_package.my_module.MyClass')
my_instance = my_class()
This question is related to
python
reflection
python-import
If you don't want to roll your own, there is a function available in the pydoc
module that does exactly this:
from pydoc import locate
my_class = locate('my_package.my_module.MyClass')
The advantage of this approach over the others listed here is that locate
will find any python object at the provided dotted path, not just an object directly within a module. e.g. my_package.my_module.MyClass.attr
.
If you're curious what their recipe is, here's the function:
def locate(path, forceload=0):
"""Locate an object by name or dotted path, importing as necessary."""
parts = [part for part in split(path, '.') if part]
module, n = None, 0
while n < len(parts):
nextmodule = safeimport(join(parts[:n+1], '.'), forceload)
if nextmodule: module, n = nextmodule, n + 1
else: break
if module:
object = module
else:
object = __builtin__
for part in parts[n:]:
try:
object = getattr(object, part)
except AttributeError:
return None
return object
It relies on pydoc.safeimport
function. Here are the docs for that:
"""Import a module; handle errors; return None if the module isn't found.
If the module *is* found but an exception occurs, it's wrapped in an
ErrorDuringImport exception and reraised. Unlike __import__, if a
package path is specified, the module at the end of the path is returned,
not the package at the beginning. If the optional 'forceload' argument
is 1, we reload the module from disk (unless it's a dynamic extension)."""
In Google App Engine there is a webapp2
function called import_string
. For more info see here:https://webapp-improved.appspot.com/api/webapp2.html
So,
import webapp2
my_class = webapp2.import_string('my_package.my_module.MyClass')
For example this is used in the webapp2.Route
where you can either use a handler or a string.
Here is to share something I found on __import__
and importlib
while trying to solve this problem.
I am using Python 3.7.3.
When I try to get to the class d
in module a.b.c
,
mod = __import__('a.b.c')
The mod
variable refer to the top namespace a
.
So to get to the class d
, I need to
mod = getattr(mod, 'b') #mod is now module b
mod = getattr(mod, 'c') #mod is now module c
mod = getattr(mod, 'd') #mod is now class d
If we try to do
mod = __import__('a.b.c')
d = getattr(mod, 'd')
we are actually trying to look for a.d
.
When using importlib
, I suppose the library has done the recursive getattr
for us. So, when we use importlib.import_module
, we actually get a handle on the deepest module.
mod = importlib.import_module('a.b.c') #mod is module c
d = getattr(mod, 'd') #this is a.b.c.d
If you happen to already have an instance of your desired class, you can use the 'type' function to extract its class type and use this to construct a new instance:
class Something(object):
def __init__(self, name):
self.name = name
def display(self):
print(self.name)
one = Something("one")
one.display()
cls = type(one)
two = cls("two")
two.display()
If you're using Django you can use this. Yes i'm aware OP did not ask for django, but i ran across this question looking for a Django solution, didn't find one, and put it here for the next boy/gal that looks for it.
# It's available for v1.7+
# https://github.com/django/django/blob/stable/1.7.x/django/utils/module_loading.py
from django.utils.module_loading import import_string
Klass = import_string('path.to.module.Klass')
func = import_string('path.to.module.func')
var = import_string('path.to.module.var')
Keep in mind, if you want to import something that doesn't have a .
, like re
or argparse
use:
re = __import__('re')
module = __import__("my_package/my_module")
the_class = getattr(module, "MyClass")
obj = the_class()
def import_class(cl):
d = cl.rfind(".")
classname = cl[d+1:len(cl)]
m = __import__(cl[0:d], globals(), locals(), [classname])
return getattr(m, classname)
OK, for me that is the way it worked (I am using Python 2.7):
a = __import__('file_to_import', globals(), locals(), ['*'], -1)
b = a.MyClass()
Then, b is an instance of class 'MyClass'
import importlib
module = importlib.import_module('my_package.my_module')
my_class = getattr(module, 'MyClass')
my_instance = my_class()
Source: Stackoverflow.com