public/subjects/ai/data-wrangling

Data wrangling

Data wrangling is one of the crucial tasks in data science and analysis which includes operations like:

• Data Sorting: To rearrange values in ascending or descending order.
• Data Filtration: To create a subset of available data.
• Data Reduction: To eliminate or replace unwanted values.
• Data Access: To read or write data files.
• Data Processing: To perform aggregation, statistical, and similar operations on specific values. Ax explained before, Pandas is an open source library, specifically developed for data science and analysis. It is built upon the Numpy (to handle numeric data in tabular form) package and has inbuilt data structures to ease-up the process of data manipulation, aka data munging/wrangling.

Exercises of the day

• Exercice 0: Environment and libraries
• Exercise 1: Concatenate
• Exercise 2: Merge
• Exercise 3: Merge MultiIndex
• Exercise 4: Groupby Apply
• Exercise 5: Groupby Agg
• Exercise 6: Unstack

Virtual Environment

• Python 3.x
• NumPy
• Pandas
• Matplotlib
• Jupyter or JupyterLab
• Tabulate

Version of Pandas I used to do the exercises: 1.0.1. I suggest to use the most recent one.

Resources

• https://jakevdp.github.io/PythonDataScienceHandbook/

• https://pandas.pydata.org/Pandas_Cheat_Sheet.pdf

• https://www.learndatasci.com/tutorials/python-pandas-tutorial-complete-introduction-for-beginners/

• https://towardsdatascience.com/different-ways-to-iterate-over-rows-in-a-pandas-dataframe-performance-comparison-dc0d5dcef8fe

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Exercise 0: Environment and libraries

The goal of this exercise is to set up the Python work environment with the required libraries.

Note: For each quest, your first exercice will be to set up the virtual environment with the required libraries.

I recommend to use:

• the last stable versions of Python.
• the virtual environment you're the most confortable with. virtualenv and conda are the most used in Data Science.
• one of the most recents versions of the libraries required

1. Create a virtual environment named ex00, with a version of Python >= 3.8, with the following libraries: pandas, numpy ,tabulate and jupyter.

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Exercise 1: Concatenate

The goal of this exercise is to learn to concatenate DataFrames. The logic is the same for the Series.

Here are the two DataFrames to concatenate:

df1 = pd.DataFrame([['a', 1], ['b', 2]],
                   columns=['letter', 'number'])
df2 = pd.DataFrame([['c', 1], ['d', 2]],
                   columns=['letter', 'number'])

1. Concatenate this two DataFrames on index axis and reset the index. The index of the outputted should be RangeIndex(start=0, stop=4, step=1). Do not change the index manually.

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Exercise 2: Merge

The goal of this exercise is to learn to merge DataFrames The logic of merging DataFrames in Pandas is quite similar as the one used in SQL.

Here are the two DataFrames to merge:

#df1

df1_dict = {
        'id': ['1', '2', '3', '4', '5'],
        'Feature1': ['A', 'C', 'E', 'G', 'I'],
        'Feature2': ['B', 'D', 'F', 'H', 'J']}

df1 = pd.DataFrame(df1_dict, columns = ['id', 'Feature1', 'Feature2'])

#df2
df2_dict = {
        'id': ['1', '2', '6', '7', '8'],
        'Feature1': ['K', 'M', 'O', 'Q', 'S'],
        'Feature2': ['L', 'N', 'P', 'R', 'T']}

df2 = pd.DataFrame(df2_dict, columns = ['id', 'Feature1', 'Feature2'])

1. Merge the two DataFrames to get this output:

   	id	Feature1_x	Feature2_x	Feature1_y	Feature2_y
   0	1	A	B	K	L
   1	2	C	D	M	N

2. Merge the two DataFrames to get this output:

   	id	Feature1_df1	Feature2_df1	Feature1_df2	Feature2_df2
   0	1	A	B	K	L
   1	2	C	D	M	N
   2	3	E	F	nan	nan
   3	4	G	H	nan	nan
   4	5	I	J	nan	nan
   5	6	nan	nan	O	P
   6	7	nan	nan	Q	R
   7	8	nan	nan	S	T

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Exercise 3: Merge MultiIndex

The goal of this exercise is to learn to merge DataFrames with MultiIndex. Use the code below to generate the DataFrames. market_data contains fake market data. In finance, the market is available during the trading days (business days). alternative_data contains fake alternative data from social media. This data is available every day. But, for some reasons the Data Engineer lost the last 15 days of alternative data.

1. Using market_data as the reference, merge alternative_data on market_data

   #generate days
   all_dates = pd.date_range('2021-01-01', '2021-12-15')
   business_dates = pd.bdate_range('2021-01-01', '2021-12-31')
   
   #generate tickers
   tickers = ['AAPL', 'FB', 'GE', 'AMZN', 'DAI']
   
   #create indexs
   index_alt = pd.MultiIndex.from_product([all_dates, tickers], names=['Date', 'Ticker'])
   index = pd.MultiIndex.from_product([business_dates, tickers], names=['Date', 'Ticker'])
   
   # create DFs
   market_data = pd.DataFrame(index=index,
                           data=np.random.randn(len(index), 3),
                           columns=['Open','Close','Close_Adjusted'])
   
   alternative_data = pd.DataFrame(index=index_alt,
                                   data=np.random.randn(len(index_alt), 2),
                                   columns=['Twitter','Reddit'])
   

reset_index is not allowed for this question

2. Fill missing values with 0

• https://medium.com/swlh/merging-dataframes-with-pandas-pd-merge-7764c7e2d46d

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Exercise 4: Groupby Apply

The goal of this exercise is to learn to group the data and apply a function on the groups. The use case we will work on is computing

1. Create a function that uses pandas.DataFrame.clip and that replace extreme values by a given percentile. The values that are greater than the upper percentile 80% are replaced by the percentile 80%. The values that are smaller than the lower percentile 20% are replaced by the percentile 20%. This process that correct outliers is called winsorizing. I recommend to use NumPy to compute the percentiles to make sure we used the same default parameters.

            def winsorize(df, quantiles):
                """
                    df: pd.DataFrame
                    quantiles: list
                        ex: [0.05, 0.95]
                """
                #TODO
                return

Here is what the function should output:

            df = pd.DataFrame(range(1,11), columns=['sequence'])
            print(winsorize(df, [0.20, 0.80]).to_markdown())

    |    |   sequence |
    |---:|-----------:|
    |  0 |        2.8 |
    |  1 |        2.8 |
    |  2 |        3   |
    |  3 |        4   |
    |  4 |        5   |
    |  5 |        6   |
    |  6 |        7   |
    |  7 |        8   |
    |  8 |        8.2 |
    |  9 |        8.2 |

2. Now we consider that each value belongs to a group. The goal is to apply the winsorizing to each group. In this question we use winsorizing values that are common: [0.05,0.95] as percentiles. Here is the new data set:

   groups = np.concatenate([np.ones(10), np.ones(10)+1,  np.ones(10)+2, np.ones(10)+3, np.ones(10)+4])
   
   df = pd.DataFrame(data= zip(groups,
                               range(1,51)),
                   columns=["group", "sequence"])
   

   The expected output (first rows) is:

   	sequence
   0	1.45
   1	2
   2	3
   3	4
   4	5
   5	6
   6	7
   7	8
   8	9
   9	9.55
   10	11.45

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Exercise 5: Groupby Agg

The goal of this exercise is to learn to compute different type of aggregations on the groups. This small DataFrame contains products and prices.

	value	product
0	20.45	table
1	22.89	chair
2	32.12	chair
3	111.22	mobile phone
4	33.22	table
5	100	mobile phone
6	99.99	table

1. Compute the min, max and mean price for each product in one single line of code. The expected output is:

product	('value', 'min')	('value', 'max')	('value', 'mean')
chair	22.89	32.12	27.505
mobile phone	100	111.22	105.61
table	20.45	99.99	51.22

Note: The columns don't have to be MultiIndex

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Exercise 6: Unstack

The goal of this exercise is to learn to unstack a MultiIndex Let's assume we trained a machine learning model that predicts a daily score on the companies (tickers) below. It may be very useful to unstack the MultiIndex: plot the time series, vectorize the backtest, ...

business_dates = pd.bdate_range('2021-01-01', '2021-12-31')

#generate tickers
tickers = ['AAPL', 'FB', 'GE', 'AMZN', 'DAI']

#create indexs
index = pd.MultiIndex.from_product([business_dates, tickers], names=['Date', 'Ticker'])

# create DFs
market_data = pd.DataFrame(index=index,
                        data=np.random.randn(len(index), 1),
                        columns=['Prediction'])

1. Unstack the DataFrame.

The first 3 rows of the DataFrame should like this:

Date	('Prediction', 'AAPL')	('Prediction', 'AMZN')	('Prediction', 'DAI')	('Prediction', 'FB')	('Prediction', 'GE')
2021-01-01 00:00:00	0.382312	-0.072392	-0.551167	-0.0585555	1.05955
2021-01-04 00:00:00	-0.560953	0.503199	-0.79517	-3.23136	1.50271
2021-01-05 00:00:00	0.211489	1.84867	0.287906	-1.81119	1.20321

2. Plot the 5 times series in the same plot using Pandas built-in visualization functions with a title.