Duck Typing Assert in Python

People who come from strongly typed languages that offer interfaces often are confused by lack of one in Python. Python, being dynamic typing programming language, follows duck typing principal. Here we will see how programmer can assert duck typing between two Python classes. Setup environment before proceed:

$ virtualenv env
$ env/bin/pip install wheezy.core

Let play a bit with duck test `looks like`. Place the following code snippet into file `test.py`:

from wheezy.core.introspection import looks

class IFoo(object):
    def foo(self):
        pass

class Foo(object):
    def bar(self):
        pass

assert looks(Foo).like(IFoo)

and run it:

test.py:11: UserWarning: 'foo': is missing.
  assert looks(Foo).like(IFoo)
Traceback (most recent call last):
  File "test.py", line 11, in 
    assert looks(Foo).like(IFoo)
AssertionError

The error says the function `foo` is missing in `Foo` implementation. Let fix that and look at arguments checks.

from wheezy.core.introspection import looks

class IFoo(object):
    def foo(self, a, b=None):
        pass

class Foo(object):
    def foo(self):
        pass

assert looks(Foo).like(IFoo)

Here is output from run:

test.py:11: UserWarning: 'foo': argument names or defaults have no match.
  assert looks(Foo).like(IFoo)
Traceback (most recent call last):
  File "test.py", line 11, in 
    assert looks(Foo).like(IFoo)
AssertionError

So far so good. Let fix it and take a look at properties:

from wheezy.core.introspection import looks

class IFoo(object):

    def foo(self, a, b=None):
        pass
        
    @property
    def bar(self):
        pass


class Foo(object):

    def foo(self, a, b=None):
        pass

    def bar(self):
        pass

assert looks(Foo).like(IFoo)

Here is output:

test.py:21: UserWarning: 'bar': is not property.
  assert looks(Foo).like(IFoo)
Traceback (most recent call last):
  File "test.py", line 21, in 
    assert looks(Foo).like(IFoo)
AssertionError

Look at other examples available here. A simple `duck test` using `looks like` approach asserts conformance between two classes in Python.