What are the 'levels', 'keys', and names arguments for in Pandas' concat function?

The pandas.concat function is a powerful tool for combining multiple Series or DataFrame objects along a specified axis. In addition to the basic arguments for specifying the objects to concatenate and the axis along which to concatenate them, concat also provides several optional arguments that allow you to customize the resulting output. Three of these arguments are levels, keys, and names.

levels

The levels argument is used to specify the levels of the resulting MultiIndex. If levels is not specified, Pandas will infer the levels from the keys. However, you can specify levels to override the inferred levels or to create a MultiIndex with a specific set of levels.

The levels argument takes a list of sequences. Each sequence represents one level of the MultiIndex. The sequences can be any type of object that can be used to create a pandas.Index object.

For example, the following code creates a MultiIndex with two levels:

df = pd.concat([df1, df2], keys=[['a', 'b'], ['c', 'd']], levels=[['A', 'B'], ['C', 'D']])

The resulting MultiIndex will have two levels, 'A' and 'B', and two levels, 'C' and 'D'. The first level of the MultiIndex will be the 'a' and 'b' values, and the second level will be the 'c' and 'd' values.

keys

The keys argument is used to specify the keys for the resulting MultiIndex. If keys is not specified, Pandas will use the names of the objects being concatenated. However, you can specify keys to override the inferred keys or to create a MultiIndex with a specific set of keys.

The keys argument takes a list of objects. Each object represents one key for the resulting MultiIndex. The objects can be any type of object that can be used to create a pandas.Index object.

For example, the following code creates a MultiIndex with two keys:

df = pd.concat([df1, df2], keys=['a', 'b'])

The resulting MultiIndex will have two keys, 'a' and 'b'. The first level of the MultiIndex will be the 'a' value, and the second level will be the 'b' value.

names

The names argument is used to specify the names for the levels of the resulting MultiIndex. If names is not specified, Pandas will use the names of the objects being concatenated. However, you can specify names to override the inferred names or to create a MultiIndex with a specific set of names.

The names argument takes a list of strings. Each string represents the name for one level of the resulting MultiIndex. The strings can be any valid string.

For example, the following code creates a MultiIndex with two names:

df = pd.concat([df1, df2], keys=[['a', 'b'], ['c', 'd']], names=['A', 'B'])

The resulting MultiIndex will have two names, 'A' and 'B'. The first level of the MultiIndex will be the 'a' and 'b' values, and the second level will be the 'c' and 'd' values.