Pandas groupby flatten multiindex

Any groupby operation involves one of the following operations on the original object. In many situations, we split the data into sets and we apply some functionality on each subset. Pandas object can be split into any of their objects.

With the groupby object in hand, we can iterate through the object similar to itertools. An aggregated function returns a single aggregated value for each group.

Once the group by object is created, several aggregation operations can be performed on the grouped data.

Transformation on a group or a column returns an object that is indexed the same size of that is being grouped. Thus, the transform should return a result that is the same size as that of a group chunk.

Filtration filters the data on a defined criteria and returns the subset of data. The filter function is used to filter the data. In the above filter condition, we are asking to return the teams which have participated three or more times in IPL. Python Pandas - GroupBy Advertisements. Previous Page. Next Page. Live Demo.

pandas groupby flatten multiindex

Previous Page Print Page.Pandas is a popular python library for data analysis. It provides the abstractions of DataFrames and Series, similar to those in R. In Pandas data reshaping means the transformation of the structure of a table or vector i. DataFrame or Series to make it suitable for further analysis. Some of Pandas reshaping capabilities do not readily exist in other environments e. SQL or bare bone R and can be tricky for a beginner.

The pivot function is used to create a new derived table out of a given one. Pivot takes 3 arguements with the following names: indexcolumnsand values.

As a value for each of these parameters you need to specify a column name in the original table. Then the pivot function will create a new table, whose row and column indices are the unique values of the respective parameters.

The cell values of the new table are taken from column given as the values parameter. Each client can be classified as Gold, Silver or Bronze customer and this is specified in the CType column. The following code snippet creates the depicted DataFrame. Note that we will assume these imports are present in all code snippets throughout this article.

In such a table, it is not easy to see how the USD price varies over different customer types. With Pandas, we can do so with a single line:. CType and whose rows are indexed with the unique values of d. Each cell in the newly created DataFrame will have as a value the entry of the USD column in the original table corresponding to the same Item and CType. The following diagram illustrates this.

pandas groupby flatten multiindex

Column and row indices are marked in red. In other words, the value of USD for every row in the original table has been transferred to the new table, where its row and column match the Item and CType of its original row. Cells in the new table which do not have a matching entry in the original one are set with NaN. Note that in this example the pivoted table does not contain any information about the EU column!

Thus, the pivoted table is a simplified version of the original data and only contains information about the columns we specified as parameters to the pivot method. Now what if we want to extend the previous example to have the EU cost for each item on its row as well? This is actually easy - we just have to omit the values parameter as follows:.

In this case, Pandas will create a hierarchical column index MultiIndex for the new table. You can think of a hierarchical index as a set of trees of indices. The first level of the column index defines all columns that we have not specified in the pivot invocation - in this case USD and EU. The second level of the index defines the unique value of the corresponding column. This is depicted in the following diagram:.

We can use this hierarchical column index to filter the values of a single column from the original table.In a previous postyou saw how the groupby operation arises naturally through the lens of the principle of split-apply-combine.

You checked out a dataset of Netflix user ratings and grouped the rows by the release year of the movie to generate the following figure:. This was achieved via grouping by a single column.

I mentioned, in passing, that you may want to group by several columns, in which case the resulting pandas DataFrame ends up with a multi-index or hierarchical index.

pandas groupby flatten multiindex

In this post, you'll learn what hierarchical indices and see how they arise when grouping by several features of your data. You can find out more about all of these concept and practices in our Manipulating DataFrames with pandas course.

Before introducing hierarchical indices, I want you to recall what the index of pandas DataFrame is. The index of a DataFrame is a set that consists of a label for each row. Let's look at an example. I'll first import a synthetic dataset of a hypothetical DataCamp student Ellie's activity on DataCamp.

The columns are a date, a programming language and the number of exercises that Ellie completed that day in that language. Load in the data:. You can see the Index on the left hand side of the DataFrame and that it consists of integers. This is a RangeIndex :. This index, however, is not so informative. If you're going to label the rows of your DataFrame, it would be good to label them in a meaningful manner, if at all possible. Can you do this with the dataset in question?

A good way to think about this challenge is that you want a unique and meaningful identifier for each row. Check out the columns and see if any matches these criteria. Notice that the date column contains unique dates so it makes sense to label each row by the date column. That is,you can make the date column the index of the DataFrame using the.

Also note that the. This can be slightly confusing because this says is that df. This does not mean that the columns are the index of the DataFrame.

The index of df is always given by df. Check out our pandas DataFrames tutorial for more on indices. Now it's time to meet hierarchical indices.

Each date now corresponds to several rows, one for each language.You can go pretty far with it without fully understanding all of its internal intricacies. However, sometimes that can manifest itself in unexpected behavior and errors. Ever had one of those? Then read this visual guide to Pandas groupby-apply paradigm to understand how it works, once and for all.

Source: Courtesy of my team at Sunscrapers. Solid understanding of the groupby-apply mechanism is often crucial when dealing with more advanced data transformations and pivot tables in Pandas. Here are a few things that I believe you should understand first to make working with more advanced Pandas pivot tables more straightforward:. Groupby — what does it do? Read on to get answers to these questions and some extra insights about working with pivot tables in Pandas.

The table is quite small, but well sufficient enough for our needs and will suit us nicely for demonstration purposes in this article:. You can apply groupby method to a flat table with a simple 1D index column. A very simple example can be grouping by a specific column value eg.

Alternatively, we can specify which columns are to be summed up. A common mistake made by some is calculating the sum first and then sticking a column selector at the end like this:. That means the summation is carried out first on every applicable column numeric or string and then a specified column is selected for output.

Note that since only a single column will be summed, the resulting output is a pd. Series object:. Instead, provide the column name as a list to the column selection essentially, use double brackets like that:.

Useful tip: When working with MultiIndex tables, you can use.

Python | Pandas MultiIndex.droplevel()

The resulting output is usually also a dataframe object. Instead of using one of the stock functions provided by Pandas to operate on the groups we can define our own custom function and run it on the table via the apply method. To write a custom function well, you need to understand how the two methods work with each other in the so-called Groupby-Split-Apply-Combine chain mechanism more on this here. As I already mentioned, the first stage is creating a Pandas groupby object DataFrameGroupBy which provides an interface for the apply method to group rows together according to specified column s values.

We split the groups transiently and loop them over via an optimized Pandas inner code. We then pass each group to a specified function as either a Series or a DataFrame object.

The output of a function is stored temporarily until all groups have been processed. In the last stage all the results from each function invocation are finally combined into a single output.DataFrames data can be summarized using the groupby method. This tutorial assumes you have some basic experience with Python pandas, including data frames, series and so on. If you are new to Pandas, I recommend taking the course below.

The idea of groupby is pretty simple: create groups of categories and apply a function to them. Groupby has a process of splitting, applying and combining data. You can then summarize the data using the groupby method. In our example there are two columns: Name and City. The function. Then define the column s on which you want to do the aggregation.

The groupby operation can be applied to any pandas data frame. Lets do some quick examples. The data frame below defines a list of animals and their speed measurements. The Iris flower data set contains data on several flower species and their measurements.

You can load it the whole data set from a csv file like this:. You can read any csv file with the. You can apply groupby while finding the average sepal width. You can now apply the function to any data frame, regardless of wheter its a toy dataset or a real world dataset. If you are interested in learning more about Pandas, check out this course: Data Analysis with Python and Pandas: Go from zero to hero. Pandas groupby example Start by importing pandas, numpy and creating a data frame.

This then returns the average sepal width for each species. Posted in Pandas. Leave a Reply Cancel reply Login disabled.If you find this content useful, please consider supporting the work by buying the book! Up to this point we've been focused primarily on one-dimensional and two-dimensional data, stored in Pandas Series and DataFrame objects, respectively. Often it is useful to go beyond this and store higher-dimensional data—that is, data indexed by more than one or two keys.

While Pandas does provide Panel and Panel4D objects that natively handle three-dimensional and four-dimensional data see Aside: Panel Dataa far more common pattern in practice is to make use of hierarchical indexing also known as multi-indexing to incorporate multiple index levels within a single index.

In this way, higher-dimensional data can be compactly represented within the familiar one-dimensional Series and two-dimensional DataFrame objects.

In this section, we'll explore the direct creation of MultiIndex objects, considerations when indexing, slicing, and computing statistics across multiply indexed data, and useful routines for converting between simple and hierarchically indexed representations of your data. Let's start by considering how we might represent two-dimensional data within a one-dimensional Series.

Python Pandas - GroupBy

For concreteness, we will consider a series of data where each point has a character and numerical key. Suppose you would like to track data about states from two different years.

Using the Pandas tools we've already covered, you might be tempted to simply use Python tuples as keys:. With this indexing scheme, you can straightforwardly index or slice the series based on this multiple index:. But the convenience ends there. For example, if you need to select all values fromyou'll need to do some messy and potentially slow munging to make it happen:.

This produces the desired result, but is not as clean or as efficient for large datasets as the slicing syntax we've grown to love in Pandas. Fortunately, Pandas provides a better way.

Our tuple-based indexing is essentially a rudimentary multi-index, and the Pandas MultiIndex type gives us the type of operations we wish to have. We can create a multi-index from the tuples as follows:. Notice that the MultiIndex contains multiple levels of indexing—in this case, the state names and the years, as well as multiple labels for each data point which encode these levels. If we re-index our series with this MultiIndexwe see the hierarchical representation of the data:.

Here the first two columns of the Series representation show the multiple index values, while the third column shows the data. Notice that some entries are missing in the first column: in this multi-index representation, any blank entry indicates the same value as the line above it. Now to access all data for which the second index iswe can simply use the Pandas slicing notation:.

The result is a singly indexed array with just the keys we're interested in. This syntax is much more convenient and the operation is much more efficient!

We'll now further discuss this sort of indexing operation on hieararchically indexed data. You might notice something else here: we could easily have stored the same data using a simple DataFrame with index and column labels.

How do I apply a function to a pandas Series or DataFrame?

In fact, Pandas is built with this equivalence in mind. The unstack method will quickly convert a multiply indexed Series into a conventionally indexed DataFrame :. Seeing this, you might wonder why would we would bother with hierarchical indexing at all. The reason is simple: just as we were able to use multi-indexing to represent two-dimensional data within a one-dimensional Serieswe can also use it to represent data of three or more dimensions in a Series or DataFrame.

Each extra level in a multi-index represents an extra dimension of data; taking advantage of this property gives us much more flexibility in the types of data we can represent. Concretely, we might want to add another column of demographic data for each state at each year say, population under 18 ; with a MultiIndex this is as easy as adding another column to the DataFrame :. In addition, all the ufuncs and other functionality discussed in Operating on Data in Pandas work with hierarchical indices as well.

Here we compute the fraction of people under 18 by year, given the above data:. The most straightforward way to construct a multiply indexed Series or DataFrame is to simply pass a list of two or more index arrays to the constructor. For example:. Similarly, if you pass a dictionary with appropriate tuples as keys, Pandas will automatically recognize this and use a MultiIndex by default:. Nevertheless, it is sometimes useful to explicitly create a MultiIndex ; we'll see a couple of these methods here.

For more flexibility in how the index is constructed, you can instead use the class method constructors available in the pd. For example, as we did before, you can construct the MultiIndex from a simple list of arrays giving the index values within each level:.Another way to do this is to reassign df based on a cross section of dfusing the.

This strategy is also useful if you want to combine the names from both levels like in the example below where the bottom level contains two 'y's:. Dropping the top level would leave two columns with the index 'y'. That can be avoided by joining the names with the list comprehension. That's a problem I had after doing a groupby and it took a while to find this other question that solved it.

I adapted that solution to the specific case here.

Pandas: plot the values of a groupby on multiple columns

You can use MultiIndex. This involves a manual step but could be an option especially if you would eventually rename your data frame. Pandas: drop a level from a multi-level column index?

Another way to drop the index is to use a list comprehension: df. You could also achieve that by renaming the columns: df. Selecting multiple columns in a pandas dataframe Renaming columns in pandas Adding new column to existing DataFrame in Python pandas Delete column from pandas DataFrame using del df.

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