Why isn’t my Pandas ‘apply’ function referencing multiple columns working? [closed]

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I have some problems with the Pandas apply function, when using multiple columns with the following dataframe

df = DataFrame ({'a' : np.random.randn(6),
                 'b' : ['foo', 'bar'] * 3,
                 'c' : np.random.randn(6)})

and the following function

def my_test(a, b):
    return a % b

When I try to apply this function with :

df['Value'] = df.apply(lambda row: my_test(row[a], row[c]), axis=1)

I get the error message:

NameError: ("global name 'a' is not defined", u'occurred at index 0')

I do not understand this message, I defined the name properly.

I would highly appreciate any help on this issue

Update

Thanks for your help. I made indeed some syntax mistakes with the code, the index should be put ”. However I still get the same issue using a more complex function such as:

def my_test(a):
    cum_diff = 0
    for ix in df.index():
        cum_diff = cum_diff + (a - df['a'][ix])
    return cum_diff

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