How to mock functions in Python in order to change default keyword arguments

Question

I'm using the mock library and unittest2 in order to test different aspects of my software project.

At the moment I have the following question: is it possible to mock a function so that the default keyword argument is different, but the functionality remains?

Say I have the following code

class C(): def fun(self, bool_arg = True): if bool_arg: return True else return False

What if I want to mock C.fun:

C.fun = mock.Mock(???)

so that every instance of C will replace keyword 'bool_arg' with False, instead of True and the result of:

c = C() c.fun()

returns:

False

Francesco Montesano · Accepted Answer · 2013-01-31 08:43:33Z

you can also try to wrap your function. Something on the line of

def wrapper(func, bool_arg): def inner(*args, **kwargs): kwargs['bool_arg']=bool_arg return func(*args, **kwargs) return inner

and

class C(): def fun(...): ... c = C() c.fun = wrapper(fun, False)

should work

Edit

If you want to change the default for the class and not for a particular instance you can create a derived class and redefine fun wrapping the method of C. Something on the line (I don't have now the time to test it):

class D(C): def fun(self, *args, **kwargs): f = wrapper(C.f, False) return f(*args, **kwargs)

Then about the suggestion of @Ber, you can define def wrapper(func, **wrapkwargs) and then instead of kwargs['bool_arg']=bool_arg do

for i in wrapkwargs.iteritems(): #wrapkwargs is a dictionary kwargs[i[0]] = i[1]

Nice. Thid could be extended to wrap any function, supplying any list of keyword args.
Thanks for your answer! I was actually looking for replacing the Class default keyword argument to be replaced for every instance of it. But I adapted your solution a little so that instead of: c.fun = wrapper(...) I did: class C.fun = wrapper(...). And it worked for all instances!
Ber, how would you generalize the above to accept any keyword argument that one wants to wrap ?
@mpaf you mean that you have changed the implementation of class C substituting self.fun with a wrapped version?
@mpaf: if you want to get a better understanding of wrapper (and consequently decorators) I suggest you to read stackoverflow.com/a/1594484/1860757 and simeonfranklin.com/blog/2012/jul/1/…

jvallver · Accepted Answer · 2013-01-30 14:04:43Z

2

You can try to use this code:

>>> import mock >>> >>> class C(): ... def fun(self, bool_arg = True): ... if bool_arg: ... print "True" ... else: ... print "False" ... >>> c = C() >>> funCopy = c.fun >>> c.fun = mock.Mock(side_effect=lambda bool_arg=False: funCopy(bool_arg=bool_arg)) >>> c.fun() False

Hope this helps

edited Jan 30, 2013 at 14:04

answered Jan 30, 2013 at 13:53

jvallver

2,3602 gold badges12 silver badges20 bronze badges

5 Comments

jdotjdot Over a year ago

Did not know that you could do default variable assignment inside a lambda.

jvallver Over a year ago

@jdotjdot You are right, but I think if mpaf make this question is because he can call the function changing the default variable value.

Francesco Montesano Over a year ago

@jdotjdot in english please

jvallver Over a year ago

@jdotjdot OK! no problem, and sorry for my english ;)

mpaf Over a year ago

Thanks for this solution, I would have accepted it as it was the first one, but I couldn't make it work for the case that I was looking for (maybe I wasn't too explicit in my question) which was that every instance of class C gets that keyword argument replaced.

Collectives™ on Stack Overflow

How to mock functions in Python in order to change default keyword arguments

2 Answers 2

8 Comments

5 Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

8 Comments

5 Comments

Linked

Related