Mocking new instance of class and verifying the arguments in python with flexmock

mock-with-arg-verification.py

# Mock new instances of a class (verify that it's initialized with specific arguments)
# First argument to __new__ is the class, so pass `object` because we don't care about that part
flexmock(MyClass).should_receive('__new__').once.with_args(object, 'arg1', 'arg2').and_return(fake_object)

MyClass('wrong', 'arguments') # => MethodSignatureError
MyClass('arg1', 'arg2')       # => <fake_object>

stub-without-arg-verification.py

# Stub new instances of a class (simply replace any new instance with another object)
flexmock(MyClass).new_instances(fake_object)

MyClass('wrong', 'arguments') # => <fake_object>
MyClass('arg1', 'arg2')       # => <fake_object>

Just ran into this little buzz saw...

Remember to use should_call if you just want to spy on the constructor, but still pass the call onto the actual implementation.

The following just returns None:

flexmock(MyClass).should_receive('__new__').once.with_args(object, 'arg1', 'arg2')