Question

I’m working through ‘Dive Into Python’ on Google App Engine and came across this error while attempting to call one class’s methods from another:

ERROR __init__.py:463] create() takes exactly 1 argument (2 given)
Traceback (most recent call last):
  File "main.py", line 35, in get
    dal.create("sample-data");
  File "dataAccess/dal.py", line 27, in create
    self.data_store.create(data_dictionary);
TypeError: create() takes exactly 1 argument (2 given)

Here’s my main class:

# filename: main.py

from dataAccess.dal import DataAccess

class MySampleRequestHandler(webapp.RequestHandler):
"""Configured to be invoked for a specific GET request"""

    def get(self):
        dal = DataAccess();
        dal.create("sample-data"); # problem area

MySampleRequestHandler.get() tries to instantiate and invoke DataAccess which is defined else where:

# filename: dal.py

from dataAccess.datastore import StandardDataStore

class DataAccess:
    """Class responsible for wrapping the specific data store"""

    def __init__(self):
        self.data_store = None;

        data_store_setting = config.SETTINGS['data_store_name'];                
        if data_store_setting == DataStoreTypes.SOME_CONFIG:
            self.data_store = StandardDataStore();

        logging.info("DataAccess init completed.");

    def create(self, data_dictionary):
        # Trying to access the data_store attribute declared in __init__
        data_store.create(data_dictionary);

I thought I could call DataAccess.create() with 1 parameter for its argument, especially according to how Dive into Python notes about class method calls:

When defining your class methods, you must explicitly list self as the first
argument for each method, including __init__. When you call a method of an
ancestor class from within your class, you must include the self argument.
But when you call your class method from outside, you do not specify anything
for the self argument; you skip it entirely, and Python automatically adds the
instance reference for you.

Answer 1

In self.data_store.create(data_dictionary), the self.data_store refers to the object created by self.data_store = StandardDataStore() in the __init__ method.

It looks like the create method of a StandardDataStore object doesn’t expect an additional argument.