Using input parameters of defined function inside apply function

Question

Maybe a very naive question, but I am stuck in this: pandas.DataFrame.apply has a possibility to put inside a function.

# define function for further usage    
def get_string(df):
        string_input = []
        for id, field in enumerate(df.index):
            string_input.append('<field id="{0}" column="{1}">{2}</field>'.format(id, field, df[field]))
        return '\n'.join(string_input)

If i apply it on df i get perfectly formatted string file output as wanted

global_1 = '\n'.join(df.apply(get_string, axis=1))

output:
<field id="0" column="xxx">49998.0</field>
<field id="1" column="xxx">37492.0</field>
<field id="2" column="xxx">12029.0</field>

But why don't i have to put inside get_string() necessary input global parameter df get_string(df) like this:

global_1 = '\n'.join(df.apply(get_string(df), axis=1))

and what if i have more input global parameters? I have Googled for it a while, but I am not very clear about it. Could anyone give me some illustrative explanation how it works? Thank you for any assistance.

There's really no need to use apply here. Provide a minimal reproducible example with sample data and your expected output and you'll get a better way to do this. — ALollz
– ALollz, Commented Jul 26, 2019 at 14:05

Code Different · Accepted Answer · 2019-07-26 14:10:58Z

1

You are confusing between df the global variable and df the local variable.

The get_string function defines input variable called df and this will overshadow any variable of the same name from higher scopes. The df that get_string knows is the dataframe you called apply upon, not the global df. You can try it with different dataframes:

df = pd.DataFrame({'a': ['Lorem', 'Ipsum']})
x = pd.DataFrame({'b': ['Hello', 'World']})
y = pd.DataFrame({'c': ['Goodbye', 'World']})

global_1 = '\n'.join(df.apply(get_string, axis=1))
global_2 = '\n'.join(x.apply(get_string, axis=1))
global_3 = '\n'.join(y.apply(get_string, axis=1))

print(global_1)
print(global_2)
print(global_3)

Result:

# From the global `df`
<field id="0" column="a">Lorem</field>
<field id="0" column="a">Ipsum</field>
# From x
<field id="0" column="b">Hello</field>
<field id="0" column="b">World</field>
# From y
<field id="0" column="c">Goodbye</field>
<field id="0" column="c">World</field>

answered Jul 26, 2019 at 14:10

Code Different

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2 Comments

Jan Pansky Over a year ago

Thank you for great explanation, but problem comes, when my defined function get_string() requires more than only df like get_string(df, some_list,..), how to put this inside apply function, when it is called only on df? Thank you

Code Different Over a year ago

You pass them by keyword in the apply call: df.apply(get_string, axis=1, some_list=..., another_param=...)

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