Pandas apply() — A Helpful Illustrated Guide

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The Pandas apply( ) function is used to apply the functions on the Pandas objects. We have so many built-in aggregation functions in pandas on Series and DataFrame objects. But, to apply some application-specific functions, we can leverage the apply( ) function. Pandas apply( ) is both the Series method and DataFrame method.

Pandas apply function to one column – apply( ) as Series method

Let’s construct a DataFrame in which we have the information of 4 persons.

>>> import pandas as pd
>>> df = pd.DataFrame(
...         {
...             'Name': ['Edward', 'Natalie', 'Chris M', 'Priyatham'],
...             'Sex' : ['M', 'F', 'M', 'M'],
...             'Age': [45, 35, 29, 26],
...             'weight(kgs)': [68.4, 58.2, 64.3, 53.1]
...         }
...     )

>>> print(df)
        Name Sex  Age  weight(kgs)
0     Edward   M   45         68.4
1    Natalie   F   35         58.2
2    Chris M   M   29         64.3
3  Priyatham   M   26         53.1

pandas.Series.apply takes any of the below two different kinds of functions as an argument.  They are:

  • Python functions
  • Numpy’s universal functions (ufuncs)

1. Python functions

In Python, there are 3 different kinds of functions in general;

  • Built-in functions
  • User-defined functions
  • Lambda functions

a) Applying Python built-in functions on Series

If we would like to know the length of the names of each person, we can do so using the len( ) function in python.

For example, if we want to know the length of the “Python” string, we can get by the following code;

>>> len("Python")
6

A single column in the DataFrame is a Series object. Now, we would like to apply the same len( ) function on the whole “Name” column of the DataFrame.  This can be achieved using the apply( ) function in the below code;

>>> df['Name'].apply(len)
0    6
1    7
2    7
3    9
Name: Name, dtype: int64

If you observe the above code snippet, the len inside the apply( ) function is not taking any argument. In general, any function takes some data to operate on them. In the len(“Python”) code snippet, it’s taking the “Python” string as input data to calculate its length. Here, the input data is directly taken from the Series object that called the function using apply( ).

When applying the Python functions, each value in the Series is applied one by one and returns the Series object.

The above process can be visualised as:

In the above visualisation, you can observe that each element of Series is applied to the function one by one.

b) Applying user-defined functions on Series

Let’s assume that the data we have is a year old. So, we would like to update the age of each person by adding 1. We can do so by applying a user-defined function on the Series object using the apply( ) method.

The code for it is,

>>> def add_age(age):
...     return age + 1

>>> df['Age'].apply(add_age)
0    46
1    36
2    30
3    27
Name: Age, dtype: int64

>>> df['Age'] = df['Age'].apply(add_age)

>>> df
        Name Sex  Age  weight(kgs)
0     Edward   M   46         68.4
1    Natalie   F   36         58.2
2    Chris M   M   30         64.3
3  Priyatham   M   27         53.1

From the above result, the major point to be noted is,

  • The index of the resultant Series is equal to the index of the caller Series object. This makes the process of adding the resultant Series as a column to the DataFrame easier.

It operates in the same way as applying built-in functions. Each element in the Series is passed one by one to the function.

  •  User-defined functions are used majorly when we would like to apply some application-specific complex functions.

c) Applying Lambda functions on Series

Lambda functions are used a lot along with the apply( ) method. We used a user-defined function for an easy addition operation in the above section. Let’s achieve the same result using a Lambda function.

The code for it is,

>>> df['Age'].apply(lambda x: x+1)
0    46
1    36
2    30
3    27
Name: Age, dtype: int64

>>> # Comparing the results of applying both the user-defined function and Lambda function
>>> df['Age'].apply(lambda x: x+1) == df['Age'].apply(add_age)
0    True
1    True
2    True
3    True
Name: Age, dtype: bool

From the above result, you can observe the results of applying the user-defined function and Lambda function are the same.

  • Lambda functions are used majorly when we would like to apply some application-specific small functions.

2. Numpy’s universal functions (ufuncs)

Numpy has so many built-in universal functions (ufuncs). We can provide any of the ufuncs as an argument to the apply( ) method on Series. A series object can be thought of as a NumPy array.

The difference between applying Python functions and ufuncs is;

  • When applying the Python Functions, each element in the Series is operated one by one.
  • When applying the ufuncs, the entire Series is operated at once.

Let’s choose to use a ufunc to floor the floating-point values of the weight column. We have numpy.floor( ) ufunc to achieve this.

The code for it is,

>>> import numpy as np

>>> df['weight(kgs)']
0    68.4
1    58.2
2    64.3
3    53.1
Name: weight(kgs), dtype: float64 
>>> df['weight(kgs)'].apply(np.floor)
0    68.0
1    58.0
2    64.0
3    53.0
Name: weight(kgs), dtype: float64

In the above result, you can observe the floored to the nearest lower decimal point value and maintain its float64 data type.

We can visualise the above process as:

In the above visualisation, you can observe that all elements of Series are applied to the function at once.

  • Whenever we have a ufunc to achieve our functionality, we can use it instead of defining a Python function.

Pandas apply( ) as a DataFrame method

We will take a look at the official documentation of the apply( ) method on DataFrame:

pandas.DataFrame.apply has two important arguments;

  • func – Function to be applied along the mentioned axis
  • axis – Axis along which function is applied

Again the axis also has 2 possible values;

  1. axis=0 – Apply function to multiple columns
  2. axis=1 – Apply function to every row

1. Pandas apply function to multiple columns

Let’s say the people in our dataset provided their height (in cms) information. It can be added using the following code,

>>> df['height(cms)'] = [178, 160, 173, 168]
>>> df
        Name Sex  Age  weight(kgs)  height(cms)
0     Edward   M   45         68.4          178
1    Natalie   F   35         58.2          160
2    Chris M   M   29         64.3          173
3  Priyatham   M   26         53.1          168

We’ll make the “Name” column the index of the DataFrame. Also, we’ll get the subset of the DataFrame with “Age”, “weight(kgs)”, and “height(cms)” columns.

>>> data = df.set_index('Name')
>>> data
          Sex  Age  weight(kgs)  height(cms)
Name                                       
Edward      M   45         68.4          178
Natalie     F   35         58.2          160
Chris M     M   29         64.3          173
Priyatham   M   26         53.1          168

>>> data_subset = data[['Age', 'weight(kgs)', 'height(cms)']]
>>> data_subset
           Age  weight(kgs)  height(cms)
Name                                   
Edward      45         68.4          178
Natalie     35         58.2          160
Chris M     29         64.3          173
Priyatham   26         53.1          168

If we would like to get the average age, weight, and height of all the people, we can use the numpy ufunc numpy.mean( ).

The code for it is,

>>> import numpy as np
>>> data_subset.apply(np.mean, axis=0)
Age             33.75
weight(kgs)     61.00
height(cms)    169.75
dtype: float64

We directly have a Pandas DataFrame aggregation function called mean( ) which does the same as above;

>>> data_subset.mean()
Age             33.75
weight(kgs)     61.00
height(cms)    169.75
dtype: float64

If you observe the results above, the results of Pandas DataFrame aggregation function and applying ufunc are equal. So, we don’t use the apply( ) method in such simple scenarios where we have aggregation functions available.

  • Whenever you have to apply some complex functions on DataFrames, then use the apply( ) method.

2. Pandas apply function to every row

Based upon the height and weight, we can know whether they’re fit or thin, or obese. The fitness criteria are different for men and women as setup by international standards. Let’s grab the fitness criteria data for the heights and weights of the people in our data.

This can be represented using a dictionary;

>>> male_fitness = {
...     #height  : (weight_lower_cap, weight_upper_cap)
...     178      : (     67.5       ,      83         ),
...     173      : (     63         ,      70.6       ),
...     168      : (     58         ,      70.7       )
... }
>>> female_fitness = {
...     #height  : (weight_lower_cap, weight_upper_cap)
...     160      : (     47.2       ,      57.6       )
... }

In the above dictionary, the keys are the heights and the values are tuples of the lower and upper limit of ideal weight respectively.

If someone is below the ideal weight for their respective height, they are “Thin”. If someone is above the ideal weight for their respective height, they are “Obese”. If someone is in the range of ideal weight for their respective height, they are “Fit”.

Let’s build a function that can be used in the apply( ) method that takes all the rows one by one.

>>> def fitness_check(seq):
...     if seq.loc['Sex'] == 'M':
...         if (seq.loc['weight(kgs)'] > male_fitness[seq.loc['height(cms)']][0]) & (seq.loc['weight(kgs)'] < male_fitness[seq.loc['height(cms)']][1]):
...             return "Fit"
...         elif (seq.loc['weight(kgs)'] < male_fitness[seq.loc['height(cms)']][0]):
...             return "Thin"
...         else:
...             return "Obese"
...     else:
...         if (seq.loc['weight(kgs)'] > female_fitness[seq.loc['height(cms)']][0]) & (seq.loc['weight(kgs)'] < female_fitness[seq.loc['height(cms)']][1]):
...             return "Fit"
...         elif (seq.loc['weight(kgs)'] < female_fitness[seq.loc['height(cms)']][0]):
...             return "Thin"
...         else:
...             return "Obese"

The function returns whether a given person is “Fit” or “Thin” or “Obese”. It uses the different fitness criteria dictionaries for male and female created above.

Finally, let’s apply the above function to every row using the apply( ) method;

>>> data.apply(fitness_check, axis=1)
Name
Edward         Fit
Natalie      Obese
Chris M        Fit
Priyatham     Thin
dtype: object

From the above result, we got to know who is Fit or Thin or Obese.

Conclusion and Next Steps

Using the apply( ) method when you want to achieve some complex functionality is preferred and recommended. Mostly built-in aggregation functions in Pandas come in handy. If you liked this tutorial on the apply( ) function and like quiz-based learning, please consider giving it a try to read our Coffee Break Pandas book.