CODEWITHSUNDEEP
pythonunit-testinglambda
user2833457
user2833457

Reputation: 3

Mocking inherited methods

Please forgive my noob status, but I have come across a construct I don't really understand and hope someone can explain it for me.

class Base(object):
    def mogrify(self, column):
        return self.mogrifiers.get(column.lower().strip()) or (lambda x: x)

...

class MyClass(some.package.Base):
    def mogrifiers(self):
        return {
            'column1': (lambda x: datetime.datetime.fromtimestamp(int(x)))
        }

...

class MyOtherClass(object):
    def convert_columns:
        ...
        new_row[colkey] = self.myclass.mogrify(colkey)(value)

This all works, but I'm trying to write a unit test and mock out MyClass.

As far as I can tell, mogrifiers returns a dictionary of all the columns and any transformations that are required.

The code I am testing calls mogrify (inherited from the Base class) with a specific column name in a string.

This tries to extract the column from the dictionary and returns the lambda function ? or if it doesn't exist in the dictionary, it returns a lambda that just gives the string back ?

So that just leaves me with the (value) bit in the code I'm trying to test. It's no clear what it does.

If I don't want to test the underlying conversion/transformation my mock could just return the simple lambda.

So I've done that, but it throws an exception on the call to mogrify saying:

E TypeError: 'str' object is not callable

Can anyone provide some clues what I'm missing here?

Upvotes: 0

Views: 569

Answers (1)

Duncan
Duncan

Reputation: 95722

As far as I can tell, mogrifiers returns a dictionary of all the columns and any transformations that are required.

That is correct, though as you've shown it it will create a fresh dictionary each time which seems unnecessary.

The code I am testing calls mogrify (inherited from the Base class) with a specific column name in a string.

This tries to extract the column from the dictionary and returns the lambda function ? or if it doesn't exist in the dictionary, it returns a lambada that just gives the string back ?

Yes, that is also correct (except that a lambada is a dance, but I think you meant lambda again).

So that just leaves me with the (value) bit in the code I'm trying to test. It's no clear what it does.

The call self.myclass.mogrify(colkey) returns a callable, the (value) simply calls it. It may be clearer if I rewrite like this:

fn = self.myclass.mogrify(colkey)
new_row[colkey] = fn(value)

splitting it into two lines will also make it clearer whether the problem is with the call self.myclass.mogrify(colkey) or fn(value). If as seems likely it is the fn(value) call it means your mocked mogrify is returning a str instead of returning a callable; it could however be that you got the mock wrong and the mocked mogrify method is actually a string.

I would suggest you rewrite as shown and also insert a print between the two lines and see what is actually being returned.

Upvotes: 1

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