Adding methods to Class doesnot work in python3.7.0 same works with python 2.7

Question

Below is my code and it works with python 2 and not with python 3, minor changes have been added to make syntax compaitable with python3. Please help with this. ANd i get the error as

TypeError: get() missing 1 required positional argument: 'self'

import collections
from types import MethodType

_ApiMethod = collections.namedtuple('_ApiMethod', ['name', 'path', 'http_method', 'query_params'])

API = [
    _ApiMethod('print_hello', 'api/hello', "GET", ['limit']),

]

class HelloClient(object):

    def __repr__(self):
        return "HelloClient(%s)" % ', '.join('%s=%s' % (a, repr(getattr(self, a))) for a in ['url', 'headers'])

    def get(self, path,query_params=None, headers=None, **kwargs):

        return "I am Hello Get Method"


def _add_methods():

    def _build_method(path, http_method, expected_query_params):

        if http_method == "GET":

            def _method(self, **kwargs):
                query_params = kwargs.setdefault('query_params', {})
                query_params.update({qp: kwargs[qp] for qp in expected_query_params if qp in kwargs})
                return self.get(path=path, **kwargs)

        return _method

    for api_method in API:        
        setattr(HelloClient, api_method.name, MethodType(_build_method(api_method.path, api_method.http_method, api_method.query_params or []), HelloClient))

_add_methods()

and i would call this method as

client = HelloClient()
response = client.print_hello()

You are adding the method to the class object. I could not find a way to add an instance method to the class, so I used metaclasses, as you can see from gist.github.com/fcracker79/bec718d0214db7c1b7bb67b31e1c605a — fcracker79
– fcracker79, Commented Jan 3, 2019 at 8:36

Matan Shahar · Accepted Answer · 2019-01-03 09:18:57Z

4

The main issue here is that you are mixing the old unbound methods from python 2 which were dropped in python 3. Since python 3 classes no longer expose their function as unbound methods, running the following snippet:

class A:
  def f(self):
    pass

type(A.f)

Gives function in python 3 and <type 'instancemethod'> in python2. With this simplification in python 3 classes no longer have the correct mechanism to deal with MethodType as you'd expect.

The solution would be to just add a simple function (_build_method result) to the class.

The solution in this specific case would be changing:

setattr(HelloClient, api_method.name, MethodType(_build_method(api_method.path, api_method.http_method, api_method.query_params or []), HelloClient)

into:

setattr(HelloClient, api_method.name, _build_method(api_method.path, api_method.http_method, api_method.query_params or [])

edited Jan 3, 2019 at 9:18

answered Jan 3, 2019 at 8:51

Matan Shahar

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1 Comment

santosh Over a year ago

Thanks for the info. Bit confused with this, please help with an example and will be really helpful

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