deedee

Dependency inversion for python, the easy way

Nutshell

Deedee provides an API which enables the developer to use late binding for default values in function definition.

Example:

import deedee

class Database:
    def select(self, query):
        print("select", query)


@deedee.resolve
def handler(param_a, param_b, database=deedee.context.database):
    print(param_a, param_b)  # param_a and param_b are unchanged
    print(database) # this will print a database instance
    database.select("select * from table order by id")  # this call will be successful


def main():
    # register the default value (which will be used when calling handler)
    deedee.context.register("database", Database())

    # call the handler, the 3rd parameter will be the previously registered value
    handler("param_1", "param_2")


if __name__ == '__main__':
    main()

In python the binding of the default value happens at the function defintion, so it is not possible to set the default parameter later on, the expression after the '=' sign gets evaluated at the function definition.

With deedee this changes, so you can specify a reference instead, and later on, specify the value for that reference.

When the function is called later on, the decorator deedee.resolve will resolve the reference of the default value.

This can be used to implement dependency injection and dependency inversion patterns without using any boilerplate classes or functions.

Features

• Lazy evaluation of the default parameters
• Default value can be overriden by specifying it during the call (same behaviour as 'normal' default values)
• It can be traced which function references to a given default parameter (soon)
• It can be checked that all references can be resolved in the future, so it can be ensured that there will be no error with resolving during function calls
• Context objects can be implemented by the user (it is not required to use deedee.context)
• Annotations can be use freely, the library doesn't use or change them
• Nearly straightforward implementation, good test coverage

Name

According to urban dictionary, deedee is:

A person who is always there when you need them. A strong survivor who can make it through anything.

In deedee the values you are registering to a context behave like this – they are always there when you need them.