Does python have an equivalent to Java Class.forName()?

Question

I have the need to take a string argument and create an object of the class named in that string in Python. In Java, I would use Class.forName().newInstance(). Is there an equivalent in Python?

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Thanks for the responses. To answer those who want to know what I'm doing: I want to use a command line argument as the class name, and instantiate it. I'm actually programming in Jython and instantiating Java classes, hence the Java-ness of the question. getattr() works great. Thanks much.

Solution 1

Reflection in python is a lot easier and far more flexible than it is in Java.

I recommend reading this tutorial

There's no direct function (that I know of) which takes a fully qualified class name and returns the class, however you have all the pieces needed to build that, and you can connect them together.

One bit of advice though: don't try to program in Java style when you're in python.

If you can explain what is it that you're trying to do, maybe we can help you find a more pythonic way of doing it.

Here's a function that does what you want:

def get_class( kls ):
    parts = kls.split('.')
    module = ".".join(parts[:-1])
    m = __import__( module )
    for comp in parts[1:]:
        m = getattr(m, comp)            
    return m

You can use the return value of this function as if it were the class itself.

Here's a usage example:

>>> D = get_class("datetime.datetime")
>>> D
<type 'datetime.datetime'>
>>> D.now()
datetime.datetime(2009, 1, 17, 2, 15, 58, 883000)
>>> a = D( 2010, 4, 22 )
>>> a
datetime.datetime(2010, 4, 22, 0, 0)
>>> 

How does that work?

We're using __import__ to import the module that holds the class, which required that we first extract the module name from the fully qualified name. Then we import the module:

m = __import__( module )

In this case, m will only refer to the top level module,

For example, if your class lives in foo.baz module, then m will be the module foo
We can easily obtain a reference to foo.baz using getattr( m, 'baz' )

To get from the top level module to the class, have to recursively use gettatr on the parts of the class name

Say for example, if you class name is foo.baz.bar.Model then we do this:

m = __import__( "foo.baz.bar" ) #m is package foo
m = getattr( m, "baz" ) #m is package baz
m = getattr( m, "bar" ) #m is module bar
m = getattr( m, "Model" ) #m is class Model

This is what's happening in this loop:

for comp in parts[1:]:
    m = getattr(m, comp)    

At the end of the loop, m will be a reference to the class. This means that m is actually the class itslef, you can do for instance:

a = m() #instantiate a new instance of the class    
b = m( arg1, arg2 ) # pass arguments to the constructor

Solution 2

Assuming the class is in your scope:

globals()['classname'](args, to, constructor)

Otherwise:

getattr(someModule, 'classname')(args, to, constructor)

Edit: Note, you can't give a name like 'foo.bar' to getattr. You'll need to split it by . and call getattr() on each piece left-to-right. This will handle that:

module, rest = 'foo.bar.baz'.split('.', 1)
fooBar = reduce(lambda a, b: getattr(a, b), rest.split('.'), globals()[module])
someVar = fooBar(args, to, constructor)

Solution 3

def import_class_from_string(path):
    from importlib import import_module
    module_path, _, class_name = path.rpartition('.')
    mod = import_module(module_path)
    klass = getattr(mod, class_name)
    return klass

Usage

In [59]: raise import_class_from_string('google.appengine.runtime.apiproxy_errors.DeadlineExceededError')()
---------------------------------------------------------------------------
DeadlineExceededError                     Traceback (most recent call last)
<ipython-input-59-b4e59d809b2f> in <module>()
----> 1 raise import_class_from_string('google.appengine.runtime.apiproxy_errors.DeadlineExceededError')()

DeadlineExceededError: 

Solution 4

Yet another implementation.

def import_class(class_string):
    """Returns class object specified by a string.

    Args:
        class_string: The string representing a class.

    Raises:
        ValueError if module part of the class is not specified.
    """
    module_name, _, class_name = class_string.rpartition('.')
    if module_name == '':
        raise ValueError('Class name must contain module part.')
    return getattr(
        __import__(module_name, globals(), locals(), [class_name], -1),
        class_name)

Solution 5

It seems you're approaching this from the middle instead of the beginning. What are you really trying to do? Finding the class associated with a given string is a means to an end.

If you clarify your problem, which might require your own mental refactoring, a better solution may present itself.

For instance: Are you trying to load a saved object based on its type name and a set of parameters? Python spells this unpickling and you should look at the pickle module. And even though the unpickling process does exactly what you describe, you don't have to worry about how it works internally:

>>> class A(object):
...   def __init__(self, v):
...     self.v = v
...   def __reduce__(self):
...     return (self.__class__, (self.v,))
>>> a = A("example")
>>> import pickle
>>> b = pickle.loads(pickle.dumps(a))
>>> a.v, b.v
('example', 'example')
>>> a is b
False

View more solutions

Comments

[Pat]

See stackoverflow.com/a/10675081/116891 for an example of using importlib.import, which was backported from python 3 to 2.7 (docs.python.org/2/library/importlib.html)

[Serafina Brocious]

Exec is extremely dangerous. It's easy to shoot yourself in the foot by overwriting names in your scope dynamically and just as easy to open a security flaw the size of Rhode Island.

[Serafina Brocious]

The reason exec is insecure here is that you can use a ';' in the class name to break out of the import. This can easily allow arbitrary code execution. The reason it can damage your code is that exec will overwrite colliding names, causing bugs and/or security flaws.

[orip]

Shouldn't it be globals()['classname']?

[Serafina Brocious]

You are indeed correct. I forgot globals() doesn't return the actual global scope, just a mapping of it. Editing as such -- thanks!

[hasen]

This is not equivalent to java's Class.forName, in Java, no assumptions are made about what's imported and what's not

[hasen]

I mean, the only input is the fully qualified class name in string format, e.g. "org.eclipse.something.utils.date.DateTimeWidget",

[cdleary]

Yep, this is what __import__ exists for. Projects like Django that do module-loading-magic use it all the time. Nice answer.

[hasen]

Yea, Django is what motivated me to learn about this magic (reflection) stuff.

[jfs]

attrs = 'foo.bar.baz'.split('.'); fooBar = reduce(getattr, attrs[1:], __import__(attrs[0]))

[jfs]

get_class = lambda name: reduce(getattr, name.split('.')[1:], __import__(name.partition('.')[0])). Though 'object.from_name' might be a better name. Examples: get_class('decimal.Decimal'), get_class(module_name).

[jfs]

Or module, _, attrs = 'foo.bar.baz'.partition('.'); fooBar = reduce(getattr, attrs, __import__(module))

[Mario Duarte]

I've tried this function but instead of a type I get a classobj. Any ideas why? and how can I actually get an instance of that class from it?

[Mario Duarte]

regarding the problem I stated in the comment above: I had to change the last line of get_class to return m().

[Pavel Vlasov]

It only took to define a seven line function. A real ease.

[Nick]

this is very interesting, but is bit different than the question

[bukzor]

The if module_name LBYL is pretty redudant, since the error you get if you simply delete those lines is: ValueError: Empty module name.

[sage88]

+1 For using importlib to do this as it prevents the need (that import causes) to recursively find the class. Simpler and cleaner than the accepted answer (though less well documented).

[sage88]

See Pat's answer below for a way to do this with importlib which is cleaner.

[Tom N Tech]

Link is dead, no longer up.