How to sort a pandas dataframe by date

Question

I am importing data into a pandas dataframe from Google BigQuery and I'd like to sort the results by date. My code is as follows:

import sys, getopt
import pandas as pd
from datetime import datetime

# set your BigQuery service account private private key
pkey ='#REMOVED#'
destination_table = 'test.test_table_2'
project_id = '#REMOVED#'

# write your query
query = """
SELECT date, SUM(totals.visits) AS Visits
FROM `#REMOVED#.#REMOVED#.ga_sessions_20*`
WHERE parse_date('%y%m%d', _table_suffix) between
DATE_sub(current_date(), interval 3 day) and
DATE_sub(current_date(), interval 1 day)
GROUP BY Date
    """

data = pd.read_gbq(query, project_id, dialect='standard', private_key=pkey, parse_dates=True, index_col='date')
date = data.sort_index()

data.info()
data.describe()

print(data.head())

My output is shown below, as you can see dates are not sorted.

<class 'pandas.core.frame.DataFrame'>
RangeIndex: 3 entries, 0 to 2
Data columns (total 2 columns):
date      3 non-null object
Visits    3 non-null int32
dtypes: int32(1), object(1)
memory usage: 116.0+ bytes
       date  Visits
0  20180312  207440
1  20180310  178155
2  20180311  207452

I have read several questions and so far tried the below, which resulted in no change to my output:

• Removing index_col='date' and adding date = data.sort_values(by='date')
• Setting the date column as the index, then sorting the index (shown above).
• Setting headers ( headers = ['Date', 'Visits'] ) and dypes ( dtypes = [datetime, int] ) to my read_gbq line ( parse_dates=True, names=headers )

What am I missing?

Answer 1

As most of the work is done on the Google BigQuery side, I'd do sorting there as well:

query = """
SELECT date, SUM(totals.visits) AS Visits
FROM `#REMOVED#.#REMOVED#.ga_sessions_20*`
WHERE parse_date('%y%m%d', _table_suffix) between
DATE_sub(current_date(), interval 3 day) and
DATE_sub(current_date(), interval 1 day)
GROUP BY Date
ORDER BY Date
"""

Answer 2

这应该工作：

data.sort_values('date', inplace=True)

Answer 3

I managed to solve this by transforming my date field into a datetime object, I assumed this would be done automatically by parse_date=True but it seems that will only parse a existing datetime object.

I added the following after my query to create a new datetime column from my date string, then I was able to use data.sort_index() and it worked as expected:

time_format = '%Y-%m-%d'
data = pd.read_gbq(query, project_id, dialect='standard', private_key=pkey)

data['n_date'] = pd.to_datetime(data['date'], format=time_format)  

data.index = data['n_date']

del data['date']
del data['n_date']

data.index.names = ['Date']

data = data.sort_index()