pandas group by month

b = pd.read_csv('b.dat')
b.index = pd.to_datetime(b['date'],format='%m/%d/%y %I:%M%p')
b.groupby(by=[b.index.month, b.index.year])
# or
b.groupby(pd.Grouper(freq='M'))  # update for v0.21+
# or
df.groupby(pd.TimeGrouper(freq='M'))

Here is what the above code is Doing:
1. Reads in the data from the CSV file.
2. Sets the index to the date column.
3. Creates a groupby object that groups the data by month and year.
4. Creates a groupby object that groups the data by month.
5. Creates a groupby object that groups the data by month.

The output of the code is a DataFrame that contains the grouped data.

In the first groupby object, the data is grouped by month and year. In the second groupby object, the data is grouped by month. In the third groupby object, the data is grouped by month.

The output of the code is a DataFrame that contains the grouped data.

In the first groupby object, the data is grouped by month and year. In the second groupby object, the data is grouped by month. In the third groupby object, the data is grouped by month.

The output of the code is a DataFrame that contains the grouped data.

In the first groupby object, the data is grouped by month and year. In the second groupby object, the data is grouped by month. In the third groupby object, the data is grouped by month.