What is pandas in Python?
pandas (all lowercase) is a popular Python-based data analysis toolkit which can be imported using import pandas as pd. It presents a diverse range of utilities, ranging from parsing multiple file formats to converting an entire data table into a NumPy matrix array. This makes pandas a trusted ally in data science and machine learning.
Similar to NumPy, pandas deals primarily with data in 1-D and 2-D arrays; however, pandas handles the two differently.
Series
In pandas, 1-D arrays are referred to a series. A series is created through the pd.Series constructor, which has a lot of optional arguments. The most common argument is data, which specifies the elements of the series.
Similar to the NumPy arrays, a pandas series also uses the dtype keyword for manual casting.
import pandas as pdimport numpy as npser = pd.Series()print('{}\n'.format(ser))ser = pd.Series(10)print('{}\n'.format(ser))ser = pd.Series([0, 10, 20, 30, 40, 50, 60, 70, 80, 90])print('{}\n'.format(ser))ser = pd.Series([20, 15.7, '10'])print('{}\n'.format(ser))# A series from a numpy arrayarr = np.array([0, 10, 20, 30, 40, 50])ser = pd.Series(arr, dtype = np.double)print('{}\n'.format(ser))ser = pd.Series([[15, 4], [89, 23.5], [21, 22]])print('{}\n'.format(ser))
In the code above, there are integers on the left side of the Series elements. These integers are collectively known as the index of the series.
Indexing
A custom index can be defined as long as the index has the same length as the Series.
import pandas as pdser = pd.Series([1, 2, 3], index = ['first', 'second', 'third'])print('{}\n'.format(ser))ser = pd.Series([50.45, 71, 90], index = [-7, 'second', 18.7])print('{}\n'.format(ser))
DataFrame
A DataFrame is simply a 2-D array. It can be created through the pd.DataFrame constructor, which takes in essentially the same arguments as pd.Series. However, while a series could be constructed from a scalar (representing a single value Series), a DataFrame cannot.
The index (row) and column labels of a DataFrame can be defined in the constructor.
import pandas as pddf = pd.DataFrame()print('{}\n'.format(df))df = pd.DataFrame([20, 100, -30])print('{}\n'.format(df))df = pd.DataFrame([18, -12], [54, 72])print('{}\n'.format(df))df = pd.DataFrame([[18, -12], [54, 72]],index=['row1', 'row2'],columns=['column1', 'column2'])print('{}\n'.format(df))df = pd.DataFrame({'a': [18, -12], 'b': [54, 72]},index=['x', 'y'])print('{}\n'.format(df))
Addition and deletion
Rows can be added to a DataFrame using the append function, which takes in either a series or another DataFrame. In the case of a series, we either need to specify the index for the series or use the ignore_index keyword. Setting ignore_index=True will change the row labels to integer indexes.
The append method does not alter the original DataFrame. Instead, a new DataFrame with the appended row is created.
The drop method
The drop method allows us to remove columns and rows in a DataFrame. This can be done one of two ways:
-
By using the
labelsargument to specify the labels of the rows/columns to be dropped.labelsworks alongside theaxiskeyword argument (which has a default value of 0) to drop from the rows or columns axis. -
By directly using the
indexorcolumnskeyword arguments to specify the labels of the rows or columns directly, without needing to use axis.
import pandas as pddf = pd.DataFrame([[10, 20, 30], [40, 50, 60]])ser = pd.Series([70, 80, 90], name='new row')print('{}\n'.format(df))df2 = df.append(ser)print('{}\n'.format(df2))df3 = df.append(ser, ignore_index = True)print('{}\n'.format(df3))df3 = pd.DataFrame([[100, 110, 120],[120, 120, 130]])df4 = df.append(df3)print('{}\n'.format(df4))
Similar to append, drop returns a new DataFrame.
DataFrames can be concatenated using the concat function. merge joins two DataFrames using all their common column labels.
Further utilities
pandas has an extensive library of data analysis utilities which allow us to group, sort, filter or describe a dataset. DataFrames can also be represented visually using the pyplot API.
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