How to use functions with pandas dataframe

Question

How can I use function with pandas dataframe. For example:

a       | b
london  | uk
newyork | usa
berlin  | germany

df1 = df[[\'a\', \'b\']]

def doSomething(df         


        

Answer 1

You can use:

df = pd.DataFrame({'a':['london','newyork','berlin'],
                   'b':['uk','usa','germany'],
                   'c':[7,8,9]})

print (df)
df1 = df[['a', 'b']]

def doSomething(x):
    return x.a

#function works with DataFrame 
print (doSomething(df1))
0     london
1    newyork
2     berlin
Name: a, dtype: object

#function works with Series, columns are transformed to index of Series
#return for each row value of Series with index a which is transformed to column in output df
print (df1.apply(doSomething, axis=1))
0     london
1    newyork
2     berlin
dtype: object

If need applymap it works with each element of df:

def doSomething(x):
    return x + '___'

#function works with element
print (df1.applymap(doSomething))
            a           b
0   london___       uk___
1  newyork___      usa___
2   berlin___  germany___

Answer 2

df1 = df[['a', 'b']]

def doSomething(df):
df2 = df['a']
return df2

df3 = df1.apply(doSomething, axis=1)
df3 = pd.DataFrame(df3).rename(columns={0: 'a'})