Passing additional arguments to python pandas DataFrame apply

Question

I have a DataFrame df and I tried to iterate each row to map values of two columns to new values, but I have problems passing in the dictionary containing the map to df.apply

df.apply(lambda row: (map_dict[row['colA']], map_dict[row['colB']]), axis=1, args=(map_dict,), map_dict=map_dict)

I got the error message

File "<console>", line 1
SyntaxError: Generator expression must be parenthesized if not sole argument

I tried to read the official doc , but I struggle to understand its explanation on args parameter http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.apply.html

I have also tried passing in map_dict as either keyword or positional argument, but I still get the same error

@Micheal will you please provide full stacktrace ? Its hard to find problem from single line of error. — NPatel
– NPatel, Commented Sep 7, 2015 at 11:22
Does it work if you just do df[['colA','colB']].apply(map_dict)? — EdChum
– EdChum, Commented Sep 7, 2015 at 11:22

shanmuga · Accepted Answer · 2015-09-07 12:30:31Z

6

The args argument given to apply function is passed func argument (lambda function given). You are getting this error since two arguments are given but lambda function only accepts one argument.

Hope this example helps

import pandas as pd
df = pd.DataFrame({'a':range(100,110), 'b':range(200, 210)})

def modulo(x, n=5):
    return x%n

some_dict = {0: 'a', 1:'b', 2:'c', 3:'d', 4:'e'}
print(df.apply(lambda row,n, map_dict: (map_dict[modulo(row['a'],n)], map_dict[modulo(row['b'],n)]), axis=1, args=(5, some_dict)))

answered Sep 7, 2015 at 12:30

shanmuga

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