python equivalent of functools 'partial' for a class / constructor

I don't think there's a standard method to do it, but if you need it often, you can just put together your own small function:

import functoolsimport collectionsdef partialclass(cls, *args, **kwds):    class NewCls(cls):        __init__ = functools.partialmethod(cls.__init__, *args, **kwds)    return NewClsif __name__ == '__main__':    Config = partialclass(collections.defaultdict, list)    assert isinstance(Config(), Config)

I had a similar problem but also required instances of my partially applied class to be pickle-able. I thought I would share what I ended up with.

I adapted fjarri's answer by peeking at Python's own collections.namedtuple. The below function creates a named subclass that can be pickled.

from functools import partialmethodimport sysdef partialclass(name, cls, *args, **kwds):    new_cls = type(name, (cls,), {        '__init__': partialmethod(cls.__init__, *args, **kwds)    })    # The following is copied nearly ad verbatim from `namedtuple's` source.    """    # For pickling to work, the __module__ variable needs to be set to the frame    # where the named tuple is created.  Bypass this step in enviroments where    # sys._getframe is not defined (Jython for example) or sys._getframe is not    # defined for arguments greater than 0 (IronPython).    """    try:        new_cls.__module__ = sys._getframe(1).f_globals.get('__name__', '__main__')    except (AttributeError, ValueError):        pass    return new_cls

If you actually need working explicit type checks via isinstance, you can simply create a not too trivial subclass:

class Config(collections.defaultdict):    def __init__(self): # no arguments here        # call the defaultdict init with the list factory        super(Config, self).__init__(list)

You'll have no-argument construction with the list factory and

isinstance(Config(), Config)

will work as well.