rspec/rules/S6742/python/rule.adoc

This rule raises an issue when 7 or more commands are applied on a data frame.

== Why is this an issue?

The pandas library provides many ways to filter, select, reshape and modify a data frame.
Pandas supports as well method chaining, which means that many ``++DataFrame++`` methods return a modified ``++DataFrame++``. 
This allows the user to chain multiple operations together, making it effortless perform several of them in one line of code:

[source,python]
----
import pandas as pd

schema = {'name':str, 'domain': str, 'revenue': 'Int64'}
joe = pd.read_csv("data.csv", dtype=schema).set_index('name').filter(like='joe', axis=0).groupby('domain').mean().round().sample()
----

While this code is correct and concise, 
it can be challenging to follow its logic and flow, making it harder to debug or modify in the future.

To improve code readability, debugging, and maintainability, it is recommended to break down long chains of pandas instructions into smaller, more modular steps. 
This can be done with the help of the pandas ``++pipe++`` method, which takes a function as a parameter. 
This function takes the data frame as a parameter, operates on it and returns it for further processing.
Grouping complex transformations of a data frame inside a function with a meaningful name can further enhance the readability and maintainability of the code.

== How to fix it

To fix this issue refactor chains of instruction into a function that can be consumed by the ``++pandas.pipe++`` method.

=== Code examples

==== Noncompliant code example

[source,python,diff-id=1,diff-type=noncompliant]
----
import pandas as pd

def foo(df: pd.DataFrame):
  return df.set_index('name').filter(like='joe', axis=0).groupby('team').mean().round().sort_values('salary').take([0]) # Noncompliant: too many operations happen on this data frame.
----

==== Compliant solution

[source,python,diff-id=1,diff-type=compliant]
----
import pandas as pd

def select_joes(df):
  return df.set_index('name').filter(like='joe', axis=0)

def compute_mean_salary_per_team(df):
  return df.groupby('team').mean().round()

def foo(df: pd.DataFrame):
  return df.pipe(select_joes).pipe(compute_mean_salary_per_team).sort_values('salary').take([0]) # Compliant
----


== Resources

=== Documentation

* Pandas Documentation - https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.pipe.html#pandas-dataframe-pipe[pandas.DataFrame.pipe]